I’m getting more and more questions about the daily global temperature updates we provide at the NASA Discover website. I suppose this is because 2010 is still in the running to beat 1998 as the warmest year in our satellite data record (since 1979).

But also we have made a couple of significant changes recently, and there continue to be some misunderstandings of the data that are posted there.

The bottom line is this: You can rely ONLY upon two channels at the Discover “Temperature Trends” page:

(1) the “Aqua ch.5 v2” channel for global-average mid-tropospheric temperatures, from the AMSU on NASA’s Aqua satellite, and

(2) the “Sea Surface” temperatures, which are averaged over the global ice-free oceans (60N to 60S), from the AMSR-E instrument on Aqua.

Do not trust any of the other channels for temperature trend monitoring. This is because, while the Aqua satellite equatorial crossing time is kept very near 1:30 am and pm with periodic orbit maneuvers, the rest of the channels come from the NOAA-15 satellite whose equatorial crossing time has now drifted from its original 7:30 am/pm value in late 1998 to about 4:30 am/pm now.

This orbital drift makes the NOAA-15 channels (4 and 6) unusually warm, and is why those of you who have been monitoring channel 4 and 6 at the Discover site are seeing such warm temperatures.


Tropospheric Temperature Monitoring
The following AMSU channel 5 image comes from the Discover “Recent Global Temperatures” page, and illustrates the kinds of signals present in this channel used in the construction of our UAH MT (mid-tropospheric) and LT (lower tropospheric) temperature products:

Note that even though NOAA-15 should not be used for trend monitoring, all of our global imagery at the “Recent Global Temperatures” page come from that satellite since the spatial patterns are not substantially affected by diurnal drift of the satellite orbit. If you scan through the global images for channels 1, 2, 3, 4, and 5 at the web site you will see how the surface and oceanic cloud water signatures change as you progress from the window channels (1, 2), to those channels more sensitive to oxygen emission at higher altitudes (3, 4, 5, etc.)

The next image is a screenshot of the Aqua AMSU ch.5 portion of our “Temperature Trends” page. In order to plot daily values that can be compared to previous years before the Aqua satellite was launched, we have intercalibrated the Aqua ch. 5 average annual cycle in daily global-average temperatures to the official UAH MT product during their overlap period (June 2002 through December 2009). This also allows us to compute curves for daily maximum, minimum, and 1979-1999 daily averages:

Most of the daily record high temperatures were set in 1998. As can be seen, 2010 has also been quite warm. For those who are wondering, the main reason why 1998 was warmer in the satellite record than the surface thermometer record is due to strong warming of the troposphere over the tropical east Pacific during the El Nino conditions in early 1998. These regions are not well represented in the surface thermometer data.

Sea Surface Temperature Monitoring
The following SST image comes from the Remote Sensing Systems website. It is based upon the most recent 3 days of SST retrievals from the AMSR-E instrument on Aqua. These measurements are made through most cloud conditions; areas of precipitation contamination are blacked out.

Because of AMSR-E’s through-cloud sensing, it provides a more accurate global average SSTs on short time scales compared to the traditional infrared measurements. We download the binary gridded SST data from the RSS website once a day and compute global area averages, which are labeled “Sea Surface” in the channel list on the Discover Temperature Trends page:

(Processing of the data is not trivial, and requires some programming skills.)

Since the AMSR-E data are available only since mid-2002, our SST record only extends back that far. There are no Max, Min, or Avg traces provided for this web page.

Why the Tropospheric Temperature Variations Don’t Match the Sea Surface Temperature Variations
Many people have noticed that the up- and down-ticks in these two temperature measures (troposphere versus sea surface) often diverge from each other. This is partly because the tropospheric temperatures include global land areas, whereas the SST data are (obviously) only over the ice-free oceans, approximately between 60N and 60S latitudes.

But another reason they diverge is because there are slight variations in the heat loss by the ocean to the atmosphere. These “intraseasonal oscillations” are usually in the tropics, and are only about +/- 1% variations in the average heat flux from the ocean to the atmosphere. Nevertheless, they can cause substantial temperature swings, especially in the troposphere.

This is why they produce opposing temperature signals. When there is above-normal ocean heat loss, the ocean surface cools below normal. Most of that heat loss is through evaporation. Meanwhile, the extra moisture in the atmosphere leads to above-normal rainfall, and so causes excess latent heating of the troposphere. The result is that SST cooling is accompanied by tropospheric warming, while SST warming is accompanied by tropospheric cooling.

These events occur on time scales of around 1 month, and so there is usually no long-term climate change significance to them. These high-frequency signals are always riding upon a more slowly varying background of temperature variability, which I believe are mostly caused by natural variations in cloud cover changing the solar energy input into the ocean.

Sea Surface Temperatures (SSTs) measured by the AMSR-E instrument on NASA’s Aqua satellite continue the fall which began several months ago. The following plot, updated through yesterday (August 18, 2010) reveals the global average SSTs continue to cool, while the Nino34 region of the tropical east Pacific remains well below normal, consistent with La Nina conditions. (click on it for the large, undistorted version; note the global SST values have been multiplied by 10):



Anomalously High Oceanic Cloud Cover
The following plot shows an AMSR-E estimate of anomalies in reflected shortwave (SW, sunlight) corresponding to the blue (Global) SST curve in the previous figure. I have estimated the reflected SW anomaly from AMSR-E vertically integrated cloud water contents, based upon regressions against Aqua CERES data. The high values in recent months (shown by the circle) suggests either (1) the ocean cooling is being driven by decreased sunlight, or (2) negative feedback in response to anomalously warm conditions, or (3) some combination of (1) and (2). Note that negative low-cloud feedback would conflict with all of the IPCC climate models, which exhibit various levels of positive cloud feedback.

Measuring The (Nonexistent) Greenhouse Effect in My Backyard with a Handheld IR Thermometer and The Box

Laypersons are no doubt confused by all of our recent esoteric discussions regarding radiative transfer, and whether global warming is even possible from a theoretical standpoint.

So, let’s take a break and return to the real world, and the experiments you can do yourself to see evidence of the “greenhouse effect”.


One of the claims of greenhouse and global warming theory that many people find hard to grasp is that there is a large flow of infrared radiation downward from the sky which keeps the surface warmer than it would otherwise be.

Particularly difficult to grasp is the concept of adding a greenhouse gas to a COLD atmosphere, and that causing a temperature increase at the surface of the Earth, which is already WARM. This, of course, is what is expected to happen from adding more carbon dioixde to the atmosphere: “global warming”.

Well, it is one of the marvels of our electronic age that you can buy a very sensitive handheld IR thermometer for only $50 and observe the effect for yourself.

These devices use a thermopile, which is an electronic component that measures a voltage which is proportional to the temperature difference across the thermopile.

If you point the device at something hot, the higher-intensity IR radiation heats up the hot-viewing side of the thermopile, and the IR thermometer displays the temperature it is radiating at (assuming some emissivity…my inexpensive unit is fixed at e=0.95).

If you instead point it at the cold sky, the sky-viewing side of the thermopile loses IR radiation, cooling it to a lower temperature than the inside of the thermopile.

For instance, last night I drove around pointing this thing straight up though my sunroof at a cloud-free sky. I live in hilly territory, the ambient air temperature was about 81 F, and at my house (an elevation of 1,000 feet), I was reading about 34 deg. F for an effective sky temperature.

If the device was perfectly calibrated, and there was NO greenhouse effect, it would measure an effective sky temperature near absolute zero (-460 deg. F) rather than +34 deg. F, and nighttime cooling of the surface would have been so strong that everything would be frozen by morning. Not very likely in Alabama in August.

What was amazing was that driving down in elevation from my house caused the sky temperature reading to increase by about 3 deg. F for a 300 foot drop in elevation. My car thermometer was showing virtually no change. This pattern was repeated as I went up and down hills.

The IR thermometer was measuring different strengths of the greenhouse effect, by definition the warming of a surface by downward IR emission by greenhouse gases in the sky. This reduces the rate of cooling of the Earth’s surface (and lower atmosphere) to space, and makes the surface warmer than it otherwise would be.

If you have a day where there are patches of blue and clouds, you can point the thermometer at the clouds and pick up a warmer reading than the surrounding blue sky.

I did it this morning (see photo, above). When I moved from a view of the blue sky to the patch of clouds, the sky-viewing side of the thermopile became warmer…even though the thermopile is already at a higher temperature than the sky. The display would read a few degrees warmer than the reading looking at blue sky.

If you perform this experiment yourself, you need to be careful about the elevation angle above the horizon you are pointing being about the same. Even in a clear sky, as you move from the zenith (overhead), down toward the horizon the path length of sky the IR thermometer sees increases, and so you measure radiation from lower altitudes, which are warmer. This makes the effective sky temperature goes up. (This is ALSO evidence of the greenhouse effect, since looking at the sky above the horizon is like adding greenhouse gases to the atmosphere overhead. The (apparent) concentration of greenhouse gases in the lower atmosphere goes up, and so does the intensity of the back radiation.)

Even earlier in the morning, about 5:30, the middle-level clouds were thicker, and I measured a sky temperature in the 50’s F. We will see more evidence of that using air temperatures, below.

This shows that the addition of an IR absorber/emitter, even at a cold temperature (the middle level clouds were probably somewhere around 30 deg. F), causes a warm object (the thermopile) to warm even more! This is the effect that some people claim is impossible.

Remember, the IR thermometer calibrated temperature output is based upon real temperatures, the temperatures on either side of the thermopile.

And if you think this is just an effect of some sunlight reflecting off the cloud….read on.

Evidence from The Box

I have been seeing the same effect in “The Box”, which is my attempt to use the greenhouse effect to warm and cool a thin aluminum plate coated with high-emissivity paint, that is heavily insulated from its surroundings in order to isolate just the radiative transfers of energy between the sky and the plate. This can be considered a clumsy, inefficient version of the IR thermometer. But now, *I* am making actual temperature measurements.

The following plot (click on it for the full-size version) shows data from the last 2 days, up through this morning’s events. The plate gets colder at night than the ambient temperature because it “sees” the cold sky, and is insulated from heat flow from the surrounding air and ground.

In the lower right, I have also circled where thin middle-level clouds came over, emitting more IR radiation downward than the clear sky, and causing a warming of the plate. Since the plate is mostly isolated from heat exchanges with the surrounding air and warm ground, it responds faster than the ambient air temperature to the intensity of “back radiation” downwelling from the sky.

When I woke this morning before sunrise, around 5:30, I saw these mid-level clouds (I used to be a certified aviation weather observer), I measured about 50 deg. F from the handheld IR thermometer.

This supports what people already experience…cloudy nights are, on average, warmer than clear nights. The main reason is that clouds emit more IR downward, change the (im)balance between upwelling and downwelling IR, and if you change the balance between energy flows in and out of an object, its temperature will change. Conservation of Energy, they call it.

(WARNING: a technical detail about the above measurements and their importance to greenhouse theory follows.)
What this Means for the Miskolczi “Aa=Ed” Controversy

Except for relatively rare special cases, the total amount of IR energy downwelling from the sky (Ed) will ALWAYS remain less than the amount upwelling from below and absorbed by the sky (Aa). As long as (1) the atmosphere has some transparency to IR radiation (which it does), and (2) the atmosphere is colder than the surface (which it is), then Ed will be less than Aa…even though they are usually close to one another, since temperatures are always adjusting to minimize IR flux divergences and convergences.

But it is those small differences that continuously “drive” the greenhouse effect.

SPECIAL MESSAGE: For those following Miskolczi’s work, and his claims regarding “Aa=Ed”, if those two radiative fluxes (Aa and Ed) are not EXACTLY equal, then Miskolczi has found nothing that disagrees with current greenhouse theory. That they are NEARLY equal has been known for a long time (e.g. Kiehl & Trenberth, 1997). Their near-equality is due to the fact that IR radiative flows are continuously “trying” to achieve radiative equilibrium between layers of the atmosphere, and between the atmosphere and the Earth’s surface. If those two quantities were more “un-equal” then they are in nature, then radiation-induced temperature changes in the atmosphere, and at the surface, would be much larger than we observe.

Again…if Aa does not EXACTLY balance Ed, then Miskolczi has found NOTHING that departs from the fundamental mechanism of the greenhouse effect.

ADDENDUM…his additional finding of a relatively constant greenhouse effect from 60 years of radiosonde data (because humidity decreases have offset CO2 increases) is indeed tantalizing. But few people believe long-term trends in radiosonde humidities. His result depends upon the reality of unusually high humidities in the 1950s and 1960s. Without those, there is no cancellation between decreasing humidity and increasing CO2 as he claims.

Executive Summary

Using both radiative transfer theory and radiosonde (weather balloon) observations to support his views, Miskolczi (2010) builds a case that the Earth’s total greenhouse effect remains constant over time.

While this might well be true, I do not believe he has demonstrated from theory why this should be the case.

His computation of a relatively constant greenhouse effect with 60 years of radiosonde observations is tantalizing, but depends upon the reality of high humidities measured by these sensors before the mid-1960s, data which are widely considered to be suspect. Even with today’s radiosonde humidity sensors, the humidity accuracy is not very high.

On the theory side, much of what he claims depends upon the validity of his statement,

“for..two regions (or bodies) A and B, the rate of flow of radiation emitted by A and absorbed by B is equal to the rate of flow the other way, regardless of other forms of (energy) transport that may be occurring.”

If this statement was true, then IR radiative transfers cannot change the temperature of anything, and Earth’s natural greenhouse effect cannot exist. Yet, elsewhere he implies that the greenhouse effect IS important to temperature by claiming that the greenhouse effect stays constant with time. The reader is left confused.

His italicized statement, above, is an extreme generalization of Kirchoffs Law of Radiation, where he has allowed the 2 bodies to have different temperatures, and also allow any amount of extra energy of any type to enter or leave the 2-body system. No matter what else is going on, Miskolczi claims there is no net radiative energy exchanges between two objects, because those 2 flows in opposite directions are always equal.

This appears to fly in the face of people’s real world experiences.

Nevertheless, Miskolczi’s (and previous investigators’) calculations of a NEAR-equality of these IR flows are quite correct, and are indeed consistent with current greenhouse theory. Others trying to understand this issue need to understand that greenhouse theory already “knows” these flows are almost equal. If the imbalance between them was not small, then the temperature changes we see in nature would be much larger than what we do see.

But it is their small departure from equality that makes all the difference.

Introduction

For the last couple of years I have been getting requests for my opinion on papers published by Ferenc Miskolczi, the latest of which recently appeared in Energy & Environment.

Since his latest work builds upon earlier work, here I will comment on his most recent paper.

I have been reluctant to comment (and still am) because the material is rather slow going, and I do not understand a couple of the claims he makes.

I glad to see his most recent paper has dropped discussion of the Virial Theorem (VT). From what I’ve read, I suspect the VT does not preclude the Earth’s average surface temperature from changing as greenhouse gas (GHG) concentrations change. After all, since GHGs cause temperature falls in the upper atmosphere at the same time they are causing temperature rises at the surface and lower atmosphere, catastrophic global warming could theoretically occur without much change in the average temperature of the atmosphere, anyway.

Nevertheless, the fact that one of his claims would undermine the theory of anthropogenic global warming makes it unusually important for us to understand his work, and so I will provide what I think I understand at this point in time. I have spent many hours examining it and thinking about it, since I think scientists always need to remain open to radical new ideas.

Some of what he reports is indeed useful. For instance, his idea that nature might keep the Earth’s total greenhouse effect relatively constant is a valid hypothesis…one which I have advanced before. The observational evidence he finds to support it is certainly tantalizing, but entirely depends on the reality of relatively high humidities measured by radiosondes way back in the 1950s and early 1960s.

But I disagree with his explanation of why the atmosphere’s total greenhouse effect should remain the same, particularly his use of Kirchoff’s Law of Radiation.

Different amounts of IR being absorbed and re-emitted by greenhouse gases at different altitudes in the atmosphere are fundamental to the explanation of Earth’s natural greenhouse effect. But Miskolczi claims that there is no net exchange of infrared radiation between different layers of the atmosphere, or between the atmosphere and surface of the Earth.

If this were true, then (as far as I can tell) there is no way for IR radiation to affect the temperature of anything. I know of no one else who believes this, and it seems to fly in the face of common sense.

But then, understanding the greenhouse effect requires more than an average amount of common sense, anyway. So I will spend a fair amount of time explaining how the greenhouse effect works…partly to convince you, the reader, and partly to convince myself that it still makes sense to me.

Of course, my opinions are always open for revision given new understanding. If I have misinterpreted or misrepresented something Miskolczi believes or has published, then I apologize.

If after reading this, he would like to respond to my criticisms, I would be glad to post that response here, unedited by me.

The Importance of an Outside Energy Source to the “Greenhouse Effect”

There is a recurring theme to the arguments from those who say adding greenhouse gases (GHGs) to the atmosphere cannot change its temperature. I’ve been trying to understand where this idea comes from, and I think I know one major source of confusion. I want to mention it up front, because it impacts people preconceived notions when they approach the issue.

If the Earth’s atmosphere was isolated, with a constant amount of total energy contained within it, and you added more CO2 at the same temperatures as the surrounding air, then it is indeed true that the average temperature of the atmosphere would not change.

In other words, simply adding CO2 cannot increase the heat content of an energetically isolated atmosphere.

But that is not what happens in the real world, because the real world is not energetically isolated. In the real world, there is an outside energy source available to the climate system — the sun.

Since temperature is, in some sense, a measure of accumulated thermal energy in an object, any change which alters the rates at which energy flows into, or out of, the object can change how much heat accumulates in the object, and thus its temperature. Greenhouse gases change the rate at which an object loses energy.

I think this might be one source of confusion on the part of those who claim that increasing the Earth’s greenhouse effect cannot change its temperature. Hopefully, this will make sense to you, because it is a key point.

Miskolczi’s Global Infrared Energy Budget

One of the useful things Miskolczi did was to make detailed calculations of the infrared (IR) radiative energy flows within the atmosphere, and between the atmosphere and the Earth’s surface, from many years of radiosonde (weather balloon) data.

I have no serious problems with how he has done those calculations; but I do have a problem with what infers about how IR radiation impacts (or doesn’t impact) the temperatures we observe in the climate system. As I’ve often said, making the measurements is usually the easy part of research; determining what they mean in terms of causation is the difficult part.

Curiously, Miskolczi claims some of these radiative flow rates (fluxes) have never been calculated before, when in fact people have calculated them. Maybe not in exactly the same way, and maybe not in as detailed a manner as he does, but different researchers usually use somewhat different procedures when doing radiative calculations anyway.

But even if his calculations are the most accurate ever performed, their differences from what is already known about infrared energy flows in the atmosphere are not sufficient to require a new explanation of greenhouse theory. There is no new information here that would make us believe that the IR flows in and out of the atmosphere and surface of the Earth are exactly equal.

For instance, let’s examine the same IR energy flows computed by Kiehl & Trenberth (1997, hereafter K&T). I stole the following chart from another website and artistically enhanced it with Miskolczi’s values in parentheses…the green lines separate the three major classes of energy flow: solar, infrared, and convective.

Note that the two studies get similar numbers for the individual components of the Earth’s infrared energy budget (the tan-colored arrows on the right side):

We will examine these numbers in a little more detail, below, but first let’s briefly review what the consensus view of how the “greenhouse effect” operates, and how it is believed to affect temperatures in the climate system.

Radiation, Temperature, and the Greenhouse Effect
Central to the theory of the Earth’s natural greenhouse effect is the fact that greenhouse gases in the atmosphere absorb and emit infrared energy.

In the usual explanation of the greenhouse effect, greenhouse gases warm the lower atmosphere and Earth’s surface above what their temperatures would have been without those greenhouse gases. (Seldom mentioned is that they also make upper atmospheric temperatures lower than they would otherwise be.)

Without greenhouse gases, the observed global average surface temperature of around 59 deg. F would be more like 0 deg. F. (Also seldom mentioned is that without convective heat transfer from the surface to atmosphere, that temperature would be more like 140 deg. F…but that’s another blog post).

Understanding the greenhouse effect can be confusing because of the seemingly contradictory roles of greenhouse gases in the climate system. Without GHGs, the atmosphere would have no way of losing the heat it accumulates from convective heat transfer caused by solar heating of the surface.

So, one major role of GHGs is to allow the atmosphere to COOL, to lose excess energy to space in the face of continual solar heating of the climate system.

But, at the same time GHGs allow the atmosphere to cool, they also WARM the surface temperature above what it would be without those gases.

But how can this be? How can something that allows the atmosphere to lose energy to space also make the surface warmer?

Because, when an IR absorbing atmosphere is placed between the solar-heated Earth’s surface and the cold depths of outer space, it not only absorbs some of the upwelling IR radiation from the Earth’s surface, it also emits some IR energy back toward the surface.

If you find this difficult to believe, then consider this…

Lost In Space

Imagine you find yourself lost in outer space, floating aimlessly, with your warm skin exposed to the cold background of the cosmos.

Sure, keep your clothes on.

There is no sun or nearby stars to add much energy to your body. Your skin would gradually cool by losing IR radiation. (Of course, if the lack of air didn’t kill you first, you would freeze to death. Bear with me here…)

But now imagine you then surround yourself with a blanket. We won’t even use a fancy, NASA-invented, IR-reflective “space blanket”…just a woolen one. And let’s even assume the temperature of the woolen blanket was extremely low — just above absolute zero.

Some of the IR radiation you emit, instead of being lost to the depths of space, would then be intercepted by the blanket. This would raise the temperature of the blanket. As that happened, the inside of the blanket would begin to emit some IR energy back toward your body, while the outside of the blanket would emit energy to outer space.

As a result, the temperature of your skin would remain higher than it would without the blanket — even though the blanket would remain at a lower temperature than your skin.

So, contrary to what some would intuitively expect, the introduction of a cold object has made a warm object warmer than it would have otherwise been.

But it didn’t actually RAISE the temperature of your skin. In this example, all we have done is slow the rate of cooling of your body, and you would eventually freeze to death anyway.

But if you had a continuous supply of energy available (like the Earth does with the sun), and had reached a steady state of shivering and discomfort and THEN added the blanket, your skin would indeed increase its temperature, compared to if the (colder) blanket was not there.

Of course, this example is just an analog to the Earth in space.

The Earth has an energy source (the sun), and it has a “radiative blanket” (greenhouse gases) enveloping it.

The greenhouse effect has to do with the rate of energy flow OUT of the climate system. It reduces that rate of energy loss.

And since temperature represents the amount of energy accumulated by one object, a second object entering the picture and reducing the first object’s ability to lose energy can cause the first object’s temperature to rise – IF – like the Earth, the first object has some external source of energy being continuously pumped in.

Miskolczi’s Computed Infrared Flows in and Out of the Earth’s Surface

So now let’s return to the above energy budget illustration, and look first at the IR flows at the Earth’s surface which I have circled in the lower right portion of the diagram. I’ll reproduce the figure, below, for your convenience.

Note that the average intensity of IR radiation emitted by the sky down to the surface (with the somewhat misleading name, “back radiation”) is nearly as large as the IR flow in the opposite direction.

As can be seen, both investigators find these two flows to be very nearly equal. Miskolczi states,

“the total flux of IR energy emitted by the atmosphere downward toward the Earth’s surface (E_D) very nearly equals the upward flux from the surface and absorbed by the atmosphere (A_A).”

That these two quantities are NEARLY equal has been known for a long time. It is partly a reflection of the fact that the entire depth of the atmosphere is mostly opaque to the transfer of IR radiation all the way through it. Miskolczi computes a global average infrared “optical thickness” of 1.87 for the entire depth of the atmosphere. I doubt that others would strenuously object to this value.

It is also partly due to something Miskolczi does not believe: that IR flows of energy from greenhouse gases have changed temperatures in the system to MINIMIZE the imbalances in IR energy flows between different components of the system.

But, just like any continuous heat transfer process (conduction, convection), the net heat flow of thermally emitted radiation from higher to lower temperatures can never quite “catch up”. After all, without some energy imbalance, the heat flow would end completely. Yet, we know that it is going on day after day.

Now, let’s discuss just how close these radiative flows are to each other in magnitude. Due partly to the large infrared opacity of the atmosphere, K&T calculated that the downwelling IR emitted by the atmosphere (324 Watts per sq. meter) is about 93% of the upwelling IR absorbed by the atmosphere (350 Watts per sq. meter).

Miskolczi gets a somewhat higher proportion, about 96%. If you see people discussing “E_D=A_A“, it is this (near-) equality they are talking about.

So, at face value, both studies have computed that the surface of the Earth, on average, loses somewhat more IR energy to the atmosphere than it absorbs from the atmosphere.

This makes physical sense since (1) the Earth’s surface is totally opaque to IR radiation, while the atmosphere isn’t; and (2) the Earth’s surface is warmer than the average temperature of the atmosphere.

But Miskolczi’s startling claim is that these two flows must be EQUAL — not only between the surface and the atmosphere as a whole, but between any two layers within the atmosphere.

He further claims that computations anyone makes that suggest otherwise are in error, due to neglect of other effects. He removes the small observed difference between the flows in opposite directions with an “empirical hemispheric emissivity factor” to force them to be equal, consistent with his assumption that they are equal.

I believe this claim regarding the equality of IR energy flows is the most fundamental issue that others would disagree with.

Radiative Exchange Equilibrium: A Consequence of Kirchoff’s Radiation Law?

Miskolczi makes the following statement regarding this supposed equality, which he calls “radiative exchange equilibrium”:

“for..two regions (or bodies) A and B, the rate of flow of radiation emitted by A and absorbed by B is equal to the rate of flow the other way, regardless of other forms of (energy) transport that may be occurring.”

This is the most surprising claim I have ever seen in this business, and I am quite certain it is false. (I’m not TOTALLY certain, because I could be dreaming right now, and you know how dreams can fool you).

He appears to attribute this to Kirchoff’s Law of Radiation (which he notes was actually discovered before Kirchoff).

But Kirchoff originally demonstrated his law with two plates in isolation, in a vacuum, with no other sources of energy from their surroundings.

Let’s look at how Kirchoff’s Law is stated by several different sources:

“At thermal equilibrium, the emissivity of a body (or surface) equals its absorptivity.”

“The ratio of emitted radiation to absorbed radiation is the same for all blackbodies at the same temperature.”

“The emissivity of a body is equal to its absorbance at the same temperature.”

“At equilibrium, the radiation emitted must equal the radiation absorbed.”

Note Miskolczi has done away with two caveats regarding his 2 bodies, A and B, that Kirchoff included: (1) energy equilibrium between two bodies, and (2) the bodies are isolated (no energy exchanges) from their environment. These conditions are not satisfied either at the Earth’s surface or in the atmosphere.

If Miskolczi is correct that the amount of thermal radiation emitted by an object (or layer of the atmosphere) ALWAYS equals the amount absorbed, this necessarily implies something that no one else I know of believes: that INFRARED RADIATIVE FLOWS BETWEEN IR ABSORBERS AND EMITTERS CANNOT CHANGE THEIR TEMPERATURE.

Let’s think about that. For IR energy flows to change the temperature of something, you need either a “convergence” of IR energy (absorption greater than emission) to cause the temperature to rise, or “divergence” of IR energy (emission greater than absorption) to cause temperature to fall.

But if the IR fluxes emitted and absorbed by an atmospheric layer are always the same, as Miskolczi claims, then the temperature of that layer cannot be changed through IR energy flows at all. Period.

And if THAT is true, then the greenhouse effect does not exist. Or, at a minimum, it is not caused by infrared radiation.

Another Thought Experiment

Obviously, if IR-absorbing layers A and B are identical in every way, including their temperatures, then the rate of IR flows between them will indeed be equal.

But let’s say layers A and B don’t touch (no conduction), and they do not interact with their surroundings. This would be like Kirchoff’s original experiment with the two plates.

Now, let’s take a blowtorch and heat layer A by 100 degrees. Layer A will now emit IR at a greater intensity than before, since its emission is proportional to the 4th power of its absolute temperature. Since the amount emitted is now greater that the rate of IR it is absorbing from layer B, layer A’s temperature will fall.

Meanwhile, over at layer B, since IR opacity is defined based upon the fraction of incident radiation it absorbs as that radiation is passing through, and layer A is now emitting IR at a greater rate than before (due to the blowtorch), Layer B now absorbs more than it is emitting.

This process – which Miskolczi claims does not exist – will eventually cause both layers to reach a new state of equilibrium, with equal temperatures, where both are emitting and absorbing IR energy at the same rate.

But Miskolczi’s theory says that the hotter layer will still emit radiation with the same intensity as it absorbs it. There would be no way for the hotter layer to transfer energy to the cooler layer. Presumably, the two layers’ temperatures would stay 100 deg. different.

Unless I am missing something important, this is a necessary consequence of Miskolczi’s claim. Maybe he thinks that since two atmospheric layers are already in a “quasi-steady state”, that their IR absorption equals their IR emission.

But this ignores other energy flows that we know are happening…most importantly, the convective transport of heat from the surface to the atmosphere. The surface is continuously dumping more energy into the atmosphere through convection. The atmosphere must emit more than it absorbs in order to cool itself.

The Hypothesis of a Constant Greenhouse Effect

Miskolczi additionally shows from 61 years of radiosonde data that a long-term decrease in the Earth’s greenhouse effect from humidity decreases in the middle and upper atmosphere have approximately counterbalanced the increase in the greenhouse effect from rising CO2 levels.

At face value, this might suggest that nature has mechanisms in place so that the total infrared opacity of the atmosphere remains about constant, consistent with the absorbed solar energy, and so the Earth’s temperature is naturally stabilized.

This might well be true.

But his conclusion from the radiosonde data depends upon the reality of relatively high humidity values in the very early years of radiosonde measurements, the 1950s and early 1960s. If you remove those years from his Fig. 9, then the drying trend that cancels the warming from increasing CO2 turns into a moistening trend.

Global “reanalysis” datasets extending back that far in time would have also the same problem, because those early radiosondes provide the most important source of information for the reanalysis.

Now, it might well be that nature has such a greenhouse effect-stabilizing mechanism in place, and that the total greenhouse effect stays at a relatively constant value for a given amount of absorbed solar energy. I have sometimes advanced the same possibility myself.

But I do not believe that Miskolczi has demonstrated either that it is the case, or why it should be the case.

In fact, the very nature of his claim that there are natural counterbalancing mechanisms at work keeping the greenhouse effect at a constant value implies that he thinks that the greenhouse effect DOES impact global average temperatures.

This seems to conflict with his claim that, by “law”, anything that absorbs IR at a certain rate must also emit IR at the same rate.

Since this law would remove the greenhouse effect entirely from the discussion of temperature change, why talk about compensating influences on the greenhouse effect? This does not make sense to me.

If Miskolczi is correct that the surface of the Earth does not lose any more IR energy than it gains from the overlying atmosphere, how is the surface cooled? Through 2 other mechanisms: (1) convective heat transfer from the surface to the atmosphere, and (2) loss of IR directly from the surface to outer space.

That convective heat transport is the dominant mechanism for moving heat from the surface to the atmosphere is not in dispute. I get angry e-mails from people who ask, “Why do you always talk about radiation? Convection is where it’s at!”

Yes, we all know that. For years I have talked and written about the cooling effects of weather are stronger than the warming effects of greenhouse gases. Lindzen in 1990 also emphasized this. We meteorologists were taught much more about convection than about solar and infrared radiation.

Even the (controversial and often maligned) K&T energy diagram shows the convective heat loss by the surface to the atmosphere (102 Watts per sq. meter) is about 4 times larger that the rate of IR loss by the surface to the atmosphere (26 Watts per sq. meter).

Thus, even in the “scientific consensus” view of global warming, convection is by far the primary mechanism by which the surface transfers heat to the atmosphere in the face of solar heating.

Yet, most of the computerized climate models still predict substantial global warming. So, obviously, they think a small change in radiation from more CO2 is pretty important.

IR Absorption and Emission Between Atmospheric Layers

So far, we have discussed the IR fluxes between the Earth’s surface and the atmosphere as a whole. What about the interaction between different layers in the atmosphere?

As I mentioned above, Miskolczi claims that the rates of IR exchange between atmospheric layers must be equal. He presents as evidence the fact that at any given level in the atmosphere, the rate of IR absorption by greenhouse gases is *nearly* the same as the rate of emission. This is shown in Fig. 3 of Miskolczi’s paper.

But the fact that these two flows are *nearly* the same is also consistent with standard greenhouse gas theory. It’s the tiny imbalance in them that makes all the difference. The greenhouse effect only becomes significant as we add up the cumulative effect of all the layers of the atmosphere.

Temperature changes have already minimized the imbalances between these IR flows, but a small imbalance still remains. This keeps the NET flow of IR energy through the climate system going “downhill”, from higher temperatures to lower temperatures.

To illustrate how tiny these IR imbalances in nature are, let’s examine what happens when we look at IR absorption and emission in 1 meter thick atmospheric layers, as Miskolczi presents in his Fig. 3.

The heat capacity of air is somewhat over 1,000 Joules per kilogram per degree C, which means it takes 1,000 Joules of energy to raise the temperature of 1 kilogram of air by 1 deg. C.

Conveniently, in the lower atmosphere 1 kg of air corresponds to about 1 cubic meter (1 m3) of air. So, for a 1 meter thick layer of air, 1,000 Watts per sq. meter (W m-2) heating applied for 1 sec would raise the temperature by 1 deg. C.

Or, since there are 86,000 seconds in a day, it would take (1000/86,000) = 0.01 Watts per sq. meter to get 1 deg. C per day warming rate.

Finally, if we double this, it takes about 0.02 Watts per sq. meter imbalance between IR absorption and emission to get 2 deg. C per day of temperature change, a very small number, indeed. And since the 1960s, investigators have been publishing atmospheric cooling rates of about 2 deg. C per day, which are caused by these tiny imbalances.

So, we see that it only takes a tiny imbalance between absorbed and emitted IR energy to accomplish realistic rates of cooling – or heating.

It’s the great depths over which these tiny numbers add up that matters. If we scale up to a layer 1,000 m thick in the lower atmosphere, then we need around 20 W/m2 more IR lost than gained by that layer for a cooling rate of 2 deg. C per day. (The required radiative flux imbalances go down dramatically with height, though, since air density drops rapidly with height…I have not added this effect in).

Once we reach the TOP of the atmosphere, the flow if IR from outer space into the atmosphere (essentially 0 Watts per sq. meter) is WAY out of balance with that upwelling from below: 235 Watts per sq. meter if you believe K&T; 250 Watts per sq. meter if you believe Miskolczi.

So, we see that for very thin layers of the atmosphere the IR emitted is very close to the IR absorbed. At the Earth’s surface, the flows exchanged between the surface and the atmosphere are very nearly equal. But not quite.

All of this has been known for a long time, and is totally consistent with greenhouse theory.

The Big Picture

With few exceptions, no two layers of the atmosphere ever reach a state of radiative equilibrium with one another, as Miskolczi claims. The same is true for the Earth’s surface and the overlying atmosphere as a whole.

All components are usually at different temperatures, with external sources of energy being absorbed, released, and flowing through them. As a result of these complexities, there is no requirement through Kirchoff’s Law that they emit and absorb radiation at the same rates.

Now, it IS true that those flows are “trying” to equalize, by exchanging IR energy in a direction that reduces temperature differences between layers. As a result, the differences in IR flows in opposite directions are indeed small – but they are not zero. Temperature changes have already relieved much of the imbalance.

Despite that fact that a major function of greenhouse gases is to provide a way for an atmosphere to cool to outer space, their presence at the same time warms the surface and lower atmosphere. While this seems counterintuitive, upon some reflection and thought we realize that this does make sense after all.

The currently ‘accepted’ theory suggests that adding more CO2 to the atmosphere has a small, but not totally negligible additional warming influence. Yes, the atmosphere is already mostly opaque at those IR wavelengths where CO2 absorption is significant. But not totally. Everyone knows that, including those scientists who work on climate models that produce catastrophic global warming.

The big question is, how much will that warming be? That’s where feedbacks come in…the warming magnification (positive feedback) or reduction (negative feedback) of the relatively weak CO2-induced warming by changes in clouds and other elements of the climate system. And that’s what I spend most of my research time on.

I have not yet seen any compelling evidence that there exists a major flaw in the theory explaining the basic operation of the Earth’s natural Greenhouse Effect.

I would love for there to be one. But I don’t see it yet.

And, again, if I have mangled what Miskolczi has said, I apologize. He is free to respond here if he wants to.

Reference
Miskolczi, F., 2010: The stable stationary Value of the Earth’s global average atmospheric Planck-weighted greenhouse gas optical thickness. Energy and Environment, 21, No.4, 243-272.

YR MON GLOBE NH SH TROPICS
2009 1 0.251 0.472 0.030 -0.068
2009 2 0.247 0.565 -0.071 -0.045
2009 3 0.191 0.324 0.058 -0.159
2009 4 0.162 0.315 0.008 0.012
2009 5 0.139 0.161 0.118 -0.059
2009 6 0.041 -0.021 0.103 0.105
2009 7 0.429 0.190 0.668 0.506
2009 8 0.242 0.236 0.248 0.406
2009 9 0.505 0.597 0.413 0.594
2009 10 0.362 0.332 0.393 0.383
2009 11 0.498 0.453 0.543 0.479
2009 12 0.284 0.358 0.211 0.506
2010 1 0.648 0.860 0.436 0.681
2010 2 0.603 0.720 0.486 0.791
2010 3 0.653 0.850 0.455 0.726
2010 4 0.501 0.799 0.203 0.633
2010 5 0.534 0.775 0.292 0.708
2010 6 0.436 0.550 0.323 0.476
2010 7 0.489 0.635 0.344 0.422


The global-average lower tropospheric temperature remained high, +0.49 deg. C in July, 2010, although the tropics continued to cool as La Nina approaches.

As of Julian Day 212 (end of July), the race for warmest year in the 32-year satellite period of record is still too close to call with 1998 continuing its lead by only 0.07 C:

YEAR GL NH SH TRPCS
1998 +0.62 +0.73 +0.51 +0.90
2010 +0.55 +0.74 +0.36 +0.63

To exceed 1998 as the warmest year, the daily global average temperature for the remainder of this year (1 Aug to 31 Dec, 2010) will need to average above +0.466 deg. C.

As a reminder, five months ago we changed to Version 5.3 of our dataset, which accounts for the mismatch between the average seasonal cycle produced by the older MSU and the newer AMSU instruments. This affects the value of the individual monthly departures, but does not affect the year to year variations, and thus the overall trend remains the same as in Version 5.2. ALSO…we have added the NOAA-18 AMSU to the data processing in v5.3, which provides data since June of 2005. The local observation time of NOAA-18 (now close to 2 p.m., ascending node) is similar to that of NASA’s Aqua satellite (about 1:30 p.m.). The temperature anomalies listed above have changed somewhat as a result of adding NOAA-18.

[NOTE: These satellite measurements are not calibrated to surface thermometer data in any way, but instead use on-board redundant precision platinum resistance thermometers (PRTs) carried on the satellite radiometers. The PRT’s are individually calibrated in a laboratory before being installed in the instruments.]

Sea Surface Temperatures (SSTs) measured by the AMSR-E instrument on NASA’s Aqua satellite continue the fall which began several months ago. The following plot, updated through yesterday (July 29, 2010) shows that the cooling in the Nino34 region in the tropical east Pacific continue to be well ahead of the cooling in the global average SST, something we did not see during the 2007-08 La Nina event (click on it for the large, undistorted version; note the global SST values have been multiplied by 10):

(UPDATED: 3:10 p.m. July 29, 2010 with temperature difference plot)

As promised, here are the first results from my little backyard experiment to investigate the role of downwelling infrared (IR) sky radiation on air temperature. (High school students looking for a science experiment, pay attention).

It’s a heavily insulated box that — theoretically — should chill air at night to a temperature below that of the outside air. The following is a conceptual design of The Box before I built it, along with the key components:

This all came about because I got tired of being asked about the theory behind global warming, specifically, how can downwelling infrared sky radiation from greenhouse gases (mostly water vapor, to a lesser extent CO2) cause global warming of the Earth surface, when the emitting temperature of the sky is colder than the surface?

Some people are convinced that this cannot happen, since the 2nd Law of Thermodynamics says energy naturally flows from higher temperature to lower temperature. In contrast, the mainstream science community, while agreeing the NET energy flow is from warm to cold, you can still cause warming by adding more greenhouse gas to the colder atmosphere. This happens even though the IR emitting temperature of the sky “causing” that warming is 10’s of degrees colder than the surface.

[NOTE: the direct warming effect of more atmospheric CO2 is small; its the resulting indirect warming (positive feedbacks) from clouds and water vapor that has most scientists worried. But not me…I think the net feedbacks are negative.]

The Box

So, since I have two automated weather stations in my backyard, I decided to build a heavily insulated box that would contain a small amount of air, and try to reduce all the other kinds of energy exchange between that air sample and the environment to a minimum EXCEPT for the influence of the downwelling sky radiation.

The air sample and the sky would be allowed to exchange IR radiation, and the colder the infrared emitting temperature of the sky is, the colder the air in the box should become compared to the air outside of the box. More about that later.

While we might not put the debate to rest with such an experiment, we can build some intuition about the energy flows that cause day and night air temperatures to be what they are. Of course, one could simply buy a hand-held infrared radiometer and take the sky’s “temperature” directly. But since everyone (myself included) has at least some trouble conceptualizing the role of infrared radiation in weather and climate (after all, we can’t see IR radiation), I thought that letting the IR effect be measured through its influence on temperature would make a bigger impact.

So, here’s a picture of the real thing that I took this morning, after collecting data since about noon yesterday:

The wireless data processor for the cavity temperature data is the little unit on the top. It sends a new temperature measurement every 5 minutes to my desktop computer in the house.

Here’s a close-up of the cavity. There is an insulating layer of air trapped between the two thin sheets of polyethylene, which are nearly transparent to infrared energy. The temperature sensor itself can be seen below that, in the cavity, the walls of which are painted with high emissivity paint (Krylon 1502 Flat White, IR emissivity = 0.99; Note that in the infrared, black is not necessarily more emissive than white…it depends on what the paint is made of, and whether the surface is rough or smooth).

Meanwhile, my regular weather station is about 20 feet away, and it is collecting air temperature and dewpoint data on the same schedule as The Box cavity temperatures are taken:

First Data from The Box
The first 17 hours of data, from midday yesterday until 8:05 a.m. this morning, are plotted below:

When I first closed up the box with the thermometer placed in the cavity, I was surprised how hot the cavity became. The maximum temperature recorded yesterday afternoon was 158 deg. F, and that must have been the limit for the sensor, because the temperature then flatlined for about an hour.

The reason for the high temperature was some direct sunlight reflecting off of one wall of the airspace, above the cavity. Even though the cavity was painted white, it still absorbed enough energy to make the air very hot. From what I have been able to gather, it is very difficult to get the solar reflectance of white paint above about 0.9.

It is interesting to calculate what rate of energy input would be required to cause this rapid rate of warming, which was about 3 deg. F per minute. If the cavity is initially in energy equilibrium, and we start reflecting 20 Watts per sq. meter more onto the cavity walls, about 10% of that (2 Watts per sq. meter) would be heating the paint, and so the air in the cavity.

According to my calculations, that would be more than enough to explain the initial rapid rise of temperature in the cavity on its way to 158+ deg. F. My calculations are only approximate, though, since I did not take into account the heat capacity of the cavity walls (painted aluminum foil), or the increased loss of IR as the cavity warmed, or conductive losses to the styrofoam and air space above the cavity.

But what we are really interested in is what happens when the overwhelming influence of solar radiation subsides. In the above plot, look at what happens as sunset approaches. Despite diffuse solar radiation still entering The Box from the blue sky, the cavity air cools to a couple of degrees below the ambient air temperature by sunset. Then, during the night, the cavity air averages about 4 deg. F colder than the outside air. This is easier to see in the next plot of the temperature difference between the cavity and the outside air, which we see remains pretty constant during the night:

To see how even a little diffuse sunlight from the sky can cause warming of the cavity, note what happened just after sunrise this morning…even though our yard does not see direct sunlight till close to 11 a.m. (very tall trees in the way), the blue sky started warming the cavity almost immediately after sunrise.

Then, after a short while, I put a white cover from a plastic cooler over the cavity to minimize the daytime heating of the cavity. At the end of the data plot you can see this solar cover caused the cavity to cool back down to the same temperature as the ambient air.

So, we already can see the cooling effect of infrared radiation in the data…in the form of cavity temperatures colder than the air. This happens from just before sunset, until sunrise — the period when there is little or no sunlight, either direct, or diffuse from the sky. But what, exactly, is the reason for this chilling effect?

Why Was the Cavity Colder than the Outside Air Temperature?
The temperature of virtually anything is the result of a balance between (1) energy gained and (2) energy lost. As long as the energy gained exceeds that lost, the temperature will rise. This was clearly seen when I closed up The Box, and the rate of sunlight absorption in the cavity exceeded the rate of energy lost by infrared emission (and any — hopefully small — conductive losses). The temperature skyrocketed.

But once the rate of energy loss exceeds that gained, then the temperature will fall, as was seen when The Box entered the shade. Then, then rate of IR energy lost (which increases rapidly with temperature) exceeded that gained from diffuse solar radiation, and the cavity temperature fell.

So, at night when there is no solar energy available, what is to prevent the cavity from getting very cold? Outer space is supposed to emit near absolute zero, 3 K. The Box’s cavity enters the hours of darkness at something like 300 K temperature. At 300 K, and assuming an IR emissivity of 0.99, the cavity is emitting IR at a rate of just over 400 Watts per sq. meter. Assuming the box is very well insulated, and is not leaking air, what is to prevent the cavity temperature from dropping well below freezing (273 K)?

The answer is downwelling IR from the sky. During the day in the summer, the broadband infrared sky temperatures viewed from the ground generally runs about 10 – 20 deg. F cooler than the near-surface air temperatures. This source of energy must exist, because without it the temperature of a cavity in a well insulated box at night would plunge even faster than we saw it heat up when exposed to indirect sunlight. And that rapid rate of temperature rise was due to only about 2 Watts per sq. meter! Imagine what in imbalance of 400 Watts per sq. meter would do.

Instead, the sky emits at only a slightly lower temperature than the surface, so the cavity cools only a little at night: about 4 deg. F cooling out of a “potential cooling” of 15 deg. F, assuming the IR emissivity of the cavity is 1.0.

By the way, I calculate that, if the cavity emissivity was only 0.90 rather than the advertised 0.99 (we really don’t know), we could explain the entire 4 deg. F drop based upon the cavity coming to a radiating temperature equal to that of the sky.

Presumably, once drier air arrives here in Alabama in another couple months, I should see larger temperature falls in the cavity, since water vapor is the Earth’s main greenhouse gas. In the meantime, I’m open to suggestions regarding simple ways to make The Box more efficient at rejecting all sources of energy except downwelling infrared radiation from the sky.

…a radiation source which some say, does not exist. 😉

As a follow-up to my controversial post on the effect of infrared “back radiation” downwelling from the colder sky to the warmer surface, the existence of which some dispute (despite the real-time availability of such data), I’ve come up with an experimental setup to see how IR radiation from the sky influences air temperature near the ground. (Yes, I know some of you think there is no such thing, but please indulge my fantasy as if it was true, ok?)

The design is pretty simple and inexpensive, and looks a little like the blackbody radiators that are used as calibration sources. The following cartoon shows the main components:

The idea is to isolate a sample of air and control its environment so that it’s main source of energy gain or loss is through an opening that looks at the sky. You have probably noticed that on a clear evening, dew forms first on the tops of cars and other surfaces. This is because these surface are losing IR energy faster than the air and other surfaces are, so their temperature falls below the dewpoint temperature first.

If we can isolate that effect sufficiently from other sources and sinks of energy, we should be able to get air temperature drops within the cavity in the direction of the colder, effective sky temperature. (We use air since it is very hard to measure the temperature of a cold surface accurately, so we let the cold surface inside the cavity chill the air in contact with it).

The cavity will be lined with aluminum foil, which has very high reflectivity in the infrared, painted on the inside only with high-emissivity paint (Krylon flat white, #1502 if I can find it…apparently, black paint isn’t as good an emitter in the IR.)

The 2 thin polyethylene sheets are in the upward-looking cavity opening to trap a layer of air for thermal conductive insulation, while at the same time passing most IR radiation (something polyethylene is apparently quite good at). The thermal conductivity of the trapped air is a little better (less) than that of Styrofoam, but since convection can occur in an air cavity, I’m sure the actual rate of heat transfer will be more than that for Styrofoam.

SO WHAT KIND OF SIGNALS CAN WE EXPECT?

(…assuming the experiment isn’t a complete failure because of something important I haven’t thought of…)

If you search around on the internet you will find that those who have made such broadband IR measurements of the sky (from what I can tell, usually with instruments that measure between 8 and 14 microns wavelength) report that the effective sky temperature in the infrared is usually 10 to 30 deg. C lower than the near-surface air temperature. Ten deg. C is more typical during humid conditions or cirrus cloud cover, while 30 deg. C would be during clear, low humidity conditions.

Low clouds produce a downwelling sky temperature nearly the same as the upwelling temperature. The sky temperature increases as you scan from the zenith down in elevation, due to the greater path length through the atmosphere.

As an example of the theoretically-expected difference in IR energy flows in and out of the cavity, at an emissivity of 1, a cavity at 300 K temperature should emit a broadband IR flux of 459 Watts/m2, while a downwelling apparent temperature of 290 K (10 deg. lower than the cavity) would produce 401 Watts/m2, the difference being 58 Watts per sq. meter.

In a perfect setup with a cavity emissivity of 1 and no other losses of energy under these conditions, the inside of the cavity would then cool to 10 deg. C less than the surrounding air temperature as the insulated cavity comes into radiative equilibrium with the sky. (I am currently monitoring 2 temperatures in my back yard, with the data sent to my computer by wireless. My first design failed due to large conductive energy loses, which led to the 2nd design, above).

Of course, a “perfect” experimental setup is not possible. I’ve run some numbers based upon the thermal conductivity of Styrofoam and I think I can keep the energy loses to about 20% of the signal being sought, but this is uncharted territory for me.

OK, TIME FOR YOUR PREDICTIONS

So, for all of you who think you know what will happen in this experiment, come on and tell the rest of us. Will the temperature of the air in the cavity stay the same? Will it cool? By how much?

I especially want to hear an answer to 2 questions:

(1) If you think the cavity will be the only source of IR radiation, and there is no downwelling IR radiation from the sky, then what will keep the air temperature inside from falling dramatically lower than the air temperature outside of the box?

(2) If you think the temperature in the cavity will not change, then what is keeping the IR radiation flowing out of the cavity toward the sky from causing a temperature fall? Wouldn’t want to violate the 1st Law of Thermodynamics, ya know.

Let the thinking begin.

Probably as the result of my recent post explaining in simple terms my “skepticism” about global warming being mostly caused by carbon dioxide emissions, I’m getting a lot of e-mail traffic from some nice folks who are trying to convince me that the physics of the so-called Greenhouse Effect are not physically possible.

More specifically, that adding CO2 to the atmosphere is not physically capable of causing warming.

These arguments usually involve claims that “back radiation” can not flow from the cooler upper layers of the atmosphere to the warmer lower layers. This back radiation is a critical component of the theoretical explanation for the Greenhouse Effect.

Sometimes the Second Law of Thermodynamics, or Kirchoff’s Law of Thermal Radiation, are invoked in these arguments against back radiation and the greenhouse effect.

One of the more common statements is, “How can a cooler atmospheric layer possibly heat a warmer atmospheric layer below it?” The person asking the question obviously thinks the hypothetical case represented by their question is so ridiculous that no one could disagree with them.

Well, I’m going to go ahead and say it: THE PRESENCE OF COOLER OBJECTS CAN, AND DO, CAUSE WARMER OBJECTS TO GET EVEN HOTTER.

In fact, this is happening all around us, all the time. The reason why we might be confused by the apparent incongruity of the statement is that we don’t spend enough time thinking about why the temperature of something is what it is.

How Cooler Objects Make Warmer Objects Even Hotter

One way to demonstrate the concept is with the following thought experiment, which I will model roughly after the Earth suspended in the cold of outer space. Even my oldest daughter, a realtor who has an aversion to things scientific, got the right answer when I used this example on her.

Imagine a heated plate in a cooled vacuum chamber, as in the first illustration, below. These chambers are used to test instruments and satellites that will be flown in space. Let’s heat the plate continuously with electricity. The plate can lose energy only through infrared (heat) radiation emitted toward the colder walls of the chamber, since there is no air in the vacuum chamber to conduct the heat away from the plate. (Similarly, there is no air in outer space to conduct heat away from the Earth in the face of solar heating.)

The plate will eventually reach a constant temperature (let’s say 150 deg. F.) where the rate of energy gain by the plate from electricity equals the rate of energy loss by infrared radiation to the cooled chamber walls.

Now, let’s put a second plate next to the first plate. The second plate will begin to warm in response to the infrared energy being emitted by the heated plate. Eventually the second plate will also reach a state of equilibrium, where its average temperature (let’s say 100 deg. F) stays constant with time. This is shown in the next illustration:

But what will happen to the temperature of the heated plate in the process? It will end up even hotter than it was before the cooler plate was placed next to it. This is because the second plate reduced the rate at which the first plate was losing energy.

(If you are unconvinced of this, then imagine that the second plate completely surrounds the heated plate. Will the heated plate remain at 150 deg., and not warm at all?)

Since the temperature of an object is a function of both energy gain AND energy loss, the temperature of the plate (or anything else) can be raised in 2 basic ways: (1) increase the rate of energy gain, or (2) decrease the rate of energy loss. The temperature of everything is determined by energy flows in and out, and one needs to know both to determine whether the temperature will go up or down. This is a consequence of the 1st Law of Thermodynamics involving conservation of energy.

Note that the above example involving 2 plates, one hotter than the other, is apparently where the greenhouse effect deniers (sorry, I couldn’t help myself) would claim the “physically impossible” has occurred: The presence of a colder object has caused a warmer object to become even hotter. Again, the reason the heated plate became even hotter is that the second plate has, in effect, “insulated” the first plate from its cold surroundings, keeping it warmer than if the second plate was not there.

The only way I know of to explain this is that it isn’t just the heated plate that is emitting IR energy, but also the second plate….as well as the cold walls of the vacuum chamber. The following illustration zooms in on the plates from our previous illustration:

What happens is that the second plate is heated by IR radiation being emitted by the first plate, raising its temperature. The second plate, in turn, cannot cool to the temperature of the vacuum chamber walls (0 deg. F) because it is not in direct contact with the refrigerant being used…it can only lose IR at a rate which increases with temperature, so it achieves some intermediate temperature.

Meanwhile, the cooler plate is emitting more radiation toward the hot plate than the cold walls of the vacuum chamber would have emitted. This changes the energy budget of the hot plate: despite a constant flow of energy into the plate from the electric heater, it has now lost some of its ability to cool through IR radiation. Its temperature then rises until it, once again, is emitting IR radiation at the same rate as it is receiving energy from its surroundings (and the electric heater).

As we will see, below, in the case of the Earth being heated by the sun, the vacuum chamber “wall” (outer space) is close to absolute zero in temperature. Putting anything between that (essentially infinite) heat sink and the Earth’s surface will cause the surface to warm.

Examples are All Around Us

Examples of objects with lower temperatures causing objects with higher temperatures to become even higher still are all around us.

For instance, in terms of these most basic heating and cooling concepts (energy gain and energy loss), the same thing happens when you put a blanket over yourself when it is cold. The blanket stays cooler than your skin, but it nevertheless makes your skin warmer than if the cooler blanket was not there. Even though the direction of flow of heat never changes (it is always from warmer to cooler objects), a cooler object can still make a warm object even hotter.

It doesn’t matter what the mechanisms of energy transfer are….if the presence of a cooler object keeps a warmer object from losing energy as rapidly as before, the warm object will become even hotter.

But if you insist on another real-world example involving infrared radiation, rather than heat conduction, let’s use clouds at night. Almost everyone has experienced the fact that cloudy nights tend to be warmer than clear nights.

The most dramatic effect I’ve seen of this is in the winter, on a cold clear night with snow cover. The temperature will drop rapidly. But if a cloud layer moves in, the temperature will either stop dropping, or even warm dramatically.

This warming occurs because the cloud radiates much more IR energy downward than does a clear, dry atmosphere. This changes the energy budget of the surface dramatically, often causing warming — even though the cloud is usually at a lower temperature than the ground is. Even high altitude cirrus clouds at a temperature well below than of the surface, can cause warming.

So, once again, we see that the presence of a colder object can cause a warmer object to become warmer still.

Extending the Concept to the Atmosphere

As mentioned above, in the case of the cold depths of outer space surrounding the Earth’s solar-heated surface, ANY infrared absorber that gets between the Earth’s surface and space will cause the surface to warm.

This radiative insulating function occurs in the atmosphere because of the presence of greenhouse gases, that is, gases that absorb and emit significant amounts of infrared energy…(mostly water vapor, CO2, and methane). Clouds also contribute to the Greenhouse Effect.

Kirchoff’s Law of thermal radiation says (roughly), that a good infrared absorber is an equally good infrared emitter. So, each layer of the atmosphere is continuously absorbing IR, as well as emitting it. This is what makes the Greenhouse Effect so much more difficult to understand conceptually than solar heating of the Earth. While the sun is a single source, and most of the energy absorbed by the Earth is at a single level (the surface of the ground), in the case of infrared energy, every layer becomes both as source of energy and an absorber of energy.

It also helps that our eyes are much more sensitive to solar radiation than they (or even our skin) are to infrared radiation. It’s more difficult to conceptualize that which you can’t see.

Our intuition begins to fail us when presented with this complexity. The following illustration shows some of these energy flows: just the IR being emitted upward and downward by different atmospheric layers. If I included arrows representing the IR energy being absorbed by those layers, too, it would become hopelessly indecipherable.

As a result of the atmosphere’s ability to radiatively insulate the Earth’s surface from losing infrared energy directly to the “cold” depths of outer space, the surface warms to a higher average temperature than it would have if the atmosphere was not there. The no-atmosphere, global average surface temperature has been theoretically calculated to be around 0 deg. F.

This, then, constitutes the basic mechanism of the Greenhouse Effect. Greenhouse gases represent a “radiative blanket” that keeps the Earth’s surface warmer than it would otherwise be without those gases present.

In fact, research published in the 1960s showed that, if the current atmosphere suddenly became still – with no wind, evaporation, and convective overturning transporting excess energy from the surface to the upper atmosphere – the average surface temperature of the Earth would warm dramatically, from 0 deg. F with no greenhouse gases, to about 140 deg. F. That the real world temperature is much lower, around 59 deg. F, is due to the cooling effects of weather transporting heat from the surface to the upper atmosphere through convective air currents.

Weather as we know it would not even exist without the greenhouse effect continuously destabilizing the vertical temperature profile of the atmosphere. Vertical air currents associated with weather act to stabilize the atmospheric temperature profile, but it is the greenhouse effect that keeps the process going by warming the lower atmosphere, and cooling the upper atmosphere, to the point where convection must occur.

What About Kirchoff’s Law?
One of the statements of Kirchoff’s Law is:

At thermal equilibrium, the emissivity of a body (or surface) equals its absorptivity.

Many well-meaning people think that one of the consequences of Kirchoffs Law of radiation is that an individual layer of the atmosphere that absorbs infrared energy at a certain rate must also emit energy at the same rate. This is NOT true.

The rate of emission becoming the same as the rate of absorption occurs in the very special case where (1) the temperature has reached thermal equilibrium, and (2) that equilibrium is the result of only those two radiative flows, in and out of the object.

Interestingly, this condition of a layer emitting the same amount of IR as it is absorbing is virtually never met anywhere in the atmosphere. This is because of the vertical, convective flows which are also transporting energy between layers.

In the global average, air below about 5,000 feet in altitude is absorbing more infrared energy than it emits, while air above that altitude (up to the top of the troposphere, the 80% of the atmosphere where weather occurs) is losing infrared energy faster than it is gained.

The reason why these two regions stay at roughly a constant temperature, despite very different rates of infrared loss and gain, is convective heat transport by weather: air heated by sunlight absorbed at the Earth’s surface has its excess energy transported to the upper troposphere, where a lack of water vapor (Earth’s main greenhouse gas) allows that energy to escape more rapidly to space.

The 2nd Law of Thermodynamics: Can Energy “Flow Uphill”?
In the case of radiation, the answer to that question is, “yes”. While heat conduction by an object always flows from hotter to colder, in the case of thermal radiation a cooler object does not check what the temperature of its surroundings is before sending out infrared energy. It sends it out anyway, no matter whether its surroundings are cooler or hotter.

Yes, thermal conduction involves energy flow in only one direction. But radiation flow involves energy flow in both directions.

Of course, in the context of the 2nd Law of Thermodynamics, both radiation and conduction processes are the same in the sense at the NET flow of energy is always “downhill”, from warmer temperatures to cooler temperatures.

But, if ANY flow of energy “uphill” is totally repulsive to you, maybe you can just think of the flow of IR energy being in only one direction, but with it’s magnitude being related to the relative temperature difference between the two objects. The result will still be the same: The presence of a cooler object can STILL cause a warmer object to become even hotter.

Anyway, that’s my story, and I’m sticking to it. Until someone convinces me otherwise.

So, let the flaming begin! No, really, have fun…but if you want your comments to remain available for others to read, please keep it civil.

SUGGESTION FROM ROY (7:50 a.m. Monday, July 26): If you want to add intelligently to this discussion, you need to actually read (1) what I have said, and (2) what others have said. Chances are, your point has already been made and discussed.

There is a new paper in press at the Journal of Climate that we were made aware of only a few days ago (July 14, 2010). It specifically addresses our (Spencer & Braswell, 2008, hereafter SB08) claim that previous satellite-based diagnoses of feedback have substantial low biases, due to natural variations in cloud cover of the Earth.

This is an important issue. If SB08 are correct, then the climate system could be substantially more resistant to forcings – such as increasing atmospheric CO2 concentrations — than the IPCC “consensus” claims it is. This would mean that manmade global warming could be much weaker than currently projected. This is an issue that Dick Lindzen (MIT) has been working on, too.

But if the new paper (MF10) is correct, then current satellite estimates of feedbacks – despite being noisy – still bracket the true feedbacks operating in the climate system…at least on the relatively short (~10 years) time scales of the satellite datasets. Forster and Gregory (2006) present some of these feedback estimates, based upon older ERBE satellite data.

As we will see, and as is usually the case, some of the MF10 criticism of SB08 is deserved, and some is not.

First, a Comment on Peer Review at the Journal of Climate

It is unfortunate that the authors and/or an editor at Journal of Climate decided that MF10 would be published without asking me or Danny Braswell to be reviewers.

Their paper is quite brief, and is obviously in the class of a “Comments on…” paper, yet it will appear as a full “Article”. But a “Comments on…” classification would then have required the Journal of Climate to give us a chance to review MF10 and to respond. So, it appears that one or more people wanted to avoid any inconvenient truths.

Thus, since it will be at least a year before a response study by us could be published – and J. Climate seems to be trying to avoid us – I must now respond here, to help avoid some of the endless questions I will have to endure once MF10 is in print.

On the positive side, though, MF10 have forced us to go back and reexamine the methodology and conclusions in SB08. As a result, we are now well on the way to new results which will better optimize the matching of satellite-observed climate variability to the simple climate model, including a range of feedback estimates consistent with the satellite data. It is now apparent to us that we did not do a good enough job of that in SB08.

I want to emphasize, though, that our most recent paper now in press at JGR (Spencer & Braswell, 2010: “On the Diagnosis of Radiative Feedback in the Presence of Unknown Radiative Forcing”, hereafter SB10), should be referenced by anyone interested in the latest published evidence supporting our claims. It does not have the main shortcomings I will address below.

But for those who want to get some idea of how we view the specific MF10 criticisms of SB08, I present the following. Keep in mind this is after only three days of analysis.

There are 2 Big Picture Questions Addressed by SB08 & MF10

There are two overarching scientific questions addressed by our SB08 paper, and MF10’s criticisms of it:

(1) Do significant low biases exist in current, satellite-based estimates of radiative feedbacks in the climate system (which could suggest high biases in inferred climate sensitivity)?

(2) Assuming that low biases do exist, did we (SB08) do an adequate job of demonstrating their probable origin, and how large those biases might be?

I will address question 1 first.

Big Picture Question #1: Does a Problem Even Exist in Diagnosing Feedbacks from Satellite Data?

MF10 conclude their paper with the claim that standard regression techniques can be applied to satellite data to get useful estimates of climate feedback, an opinion we strongly disagree with.

Fortunately, it is easy to demonstrate that a serious problem does exist. I will do this using MF10’s own method: analysis of output from coupled climate models. But rather than merely referencing a previous publication which does not even apply to the problem at hand, I will show actual evidence from 18 of the IPCC’s coupled climate models.

The following plot shows the final 10 years of data from the 20th Century run of the FGOALS model, output from which is archived at PCMDI (Meehl et al., 2007). The plot is global and 3-month averaged net radiative flux anomalies (reflected solar plus emitted infrared) versus the corresponding surface temperature anomalies produced by the model.

This represents the kind of data which are used to diagnose feedbacks from satellite data. The basic question we are trying to answer with such a plot is: “How much more radiant energy does the Earth lose in response to warming?” The answer to that question would help determine how strongly (or weakly) the climate system might respond to increasing CO2 levels.

It is the slope of the red regression fit to the 3-month data points in the above figure that is the question: Is that slope an estimate of the net radiative feedback operating in the climate model, or not?

MF10 would presumably claim it is. We claim it is not, and furthermore that it will usually be biased low compared to the true feedback operating in the climate system. SB08 was our first attempt to demonstrate this with a simple climate model.

Well, the slope of 0.77 W m^-2 K^-1 in the above plot would correspond to a climate sensitivity in response to a doubling of atmospheric carbon dioxide (2XCO2) of (3.8/0.77=) 4.9 deg. C of global warming. [This assumes the widely accepted value near 3.8 W m^-2 K^-1 for the radiative energy imbalance of the Earth in response to 2XCO2].

But 4.9 deg. C of warming is more than DOUBLE the known sensitivity of this model, which is 2.0 to 2.2 deg. C (Forster & Taylor, J. Climate, 2006, hereafter FT06). This is clearly a large error in the diagnosed feedback.

As a statistician will quickly ask, though, does this error represent a bias common to most models, or is it just due to statistical noise?

To demonstrate this is no statistical outlier, the following plot shows regression-diagnosed versus “true” feedbacks diagnosed for 18 IPCC AR4 coupled climate models. We analyzed the output from the last 50 years of the 20th Century runs archived at PCMDI, computing average regression slopes in ten 5-year subsets of each model’s run, with 3-month average anomalies, then averaging those ten regression slopes for each model. Resulting climate sensitivities based upon those average regression slopes are shown separately for the 18 models in the next figure:

As can be seen, most models exhibit large biases – as much as 50 deg. C! — in feedback-inferred climate sensitivity, the result of low biases in the regression-diagnosed feedback parameters. Only 5 of the 18 IPCC AR4 models have errors in regression-inferred sensitivity less than 1 deg. C, and that is after beating down some noise with ten 5-year periods from each model! We can’t do that with only 10 years of satellite data.

Now, note that as long as such large inferred climate sensitivities (50+ deg.!?) can be claimed to be supported by the satellite data, the IPCC can continue to warn that catastrophic global warming is a real possibility.

The real reason why such biases exist, however, is addressed in greater depth in our new paper, (Spencer and Braswell, 2010). The low bias in diagnosed feedback (and thus high bias in climate sensitivity) is related to the extent to which time-varying radiative forcing, mostly due to clouds, contaminates the radiative feedback signal responding to temperature changes.

It is easy to get confused on the issue of using regression to estimate feedbacks because linear regression was ALSO used to get the “true” feedbacks in the previous figure. The difference is that, in order to do so, Forster and Taylor removed the large, transient CO2 radiative forcing imposed on the models in order to better isolate the radiative feedback signal. Over many decades of model run time, this radiative feedback signal then beats down the noise from non-feedback natural cloud variations.

Thus, diagnosing feedback accurately is fundamentally a signal-to-noise problem. Either any time-varying radiative forcing in the data must be relatively small to begin with, or it must be somehow estimated and then removed from the data.

It would be difficult to over-emphasize the importance of understanding the last paragraph.

So, Why Does the Murphy & Forster Example with the HadSM2 Model Give Accurate Feedbacks?

To support their case that there is no serious problem in diagnosing feedbacks from satellite data, MF10 use the example of Gregory et al. (2004 GRL, “A New Method for Diagnosing Radiative Forcing and Climate Sensitivity”). Gregory et al. analyzed the output of a climate model, HadSM3, and found that an accurate feedback could be diagnosed from the model output at just about any point during the model integration.

But the reason why Gregory et al. could do this, and why it has no consequence for the real world, is so obvious that I continue to be frustrated that so many climate experts still do not understand it.

The Gregory et al. HadSM3 model experiment used an instantaneous quadrupling (!) of the CO2 content of the model atmosphere. In such a hypothetical situation, there will be rapid warming, and thus a strong radiative feedback signal in response to that warming.

But this hypothetical situation has no analog in the real world. The only reason why one could accurately diagnose feedback in such a case is because the 4XCO2 radiative forcing is kept absolutely constant over time, and so the radiative feedback signal is not contaminated by it.

Again I emphasize, instantaneous and then constant radiative forcing has no analog in the real world. Experts using such unrealistic cases has led to much confusion regarding the diagnosis of feedbacks from satellite data. In nature, ever-evolving time-varying radiative forcings (what some call “unforced natural variability”) are almost always overpowering radiative feedback.

But does that mean that Spencer & Braswell (2008) did a convincing job of demonstrating how large the resulting errors in feedback diagnosis could be in response to such time-varying radiative forcing? Probably not.

Big Picture Question #2: Did Spencer & Braswell (2008) Do An Adequate Job of Demonstrating Why Feedback Biases Occur?

MF10 made two changes in our simple climate model which had large consequences: (1) they change the averaging time of the model output to be consistent with the relatively short satellite datasets we have to compare to, and (2) they increase the assumed depth of the tropical ocean mixed layer from 50 meters to 100 meters in the simple model.

The first change, we agree, is warranted, and it indeed results in less dramatic biases in feedbacks diagnosed from the simple model. We have independently checked this with the simple model by comparing our new results to those of MF10.

The second change, we believe, is not warranted, and it pushes the errors to even smaller values. If anything, we think we can show that even 50 meters is probably too deep a mixed layer for the tropical ocean (what we addressed) on these time scales.

Remember, we are exploring why feedbacks diagnosed from satellite-observed, year-to-year climate variability are biased low, and on those short time scales, the equivalent mixing depths are pretty shallow. As one extends the time to many decades, the depth of ocean responding to a persistent warming mechanism increases to 100’s of meters, consistent with MF10’s claim. But for diagnosing feedbacks from satellite data, the time scales of variability affecting the data are 1 to only a few years.

But we have also discovered a significant additional shortcoming in SB08 (and MF10) that has a huge impact on the answer to Question #2: In addition to just the monthly standard deviations of the satellite-measured radiative fluxes and sea surface temperatures, we should have included (at least) one more important satellite statistic: the level of decorrelation of the data.

Our SB10 paper actually does this (which is why it should be referenced for the latest evidence supporting our claims). After accounting for the decorrelation in the data (which exists in ALL of the IPCC models, see the first figure, above, for an example) the MF10 conclusion that the ratio of the noise to signal (N/S) in the satellite data is only around 15% can not be supported.

Unfortunately, SB08 did not adequately demonstrate this with the satellite data. SB10 does…but does not optimize the model parameters that best match the satellite data. That is now the focus of our new work on the subject.

Since this next step was not obvious to us until MF10 caused us to go back and reexamine the simple model and its assumptions, this shows the value of other researchers getting involved in this line of research. For that we are grateful.

Final Comments

While the above comments deal with the “big picture” issues and implications of SB08, and MF10’s criticism of it, there are also a couple of errors and misrepresentations in MF10 that should be addressed, things that could have been caught had we been allowed to review their manuscript.

1) MF10 claim to derive a “more correct” analytical expression for the error in feedback error than SB08 provided. If anything, it is ours that is more correct. Their expression (the derivation of which we admit is impressive) is only correct for an infinite time period, which is irrelevant to the issue at hand, and will have errors for finite time periods. In contrast, our expression is exactly correct for a finite time series of data, which is what we are concerned with in the real world.

2) MF10 remove “seasonal cycles” from the randomly forced model data time series. Why would this be necessary for a model that has only random daily forcing? Very strange.

Despite the shortcomings, MF10 do provide some valuable insight, and some of what they present is indeed useful for advancing our understanding of what causes variations in the radiative energy budget of the Earth.

References
Forster, P. M., and J. M. Gregory (2006), The climate sensitivity and its components diagnosed from Earth Radiation Budget data, J. Climate, 19, 39-52.

Forster, P.M., and K.E. Taylor (2006), Climate forcings and climate sensitivities diagnosed from coupled climate model integrations, J. Climate, 19, 6181-6194.

Gregory, J.M., W. J. Ingram, M.A. Palmer, G.S. Jones, P.A. Stott, R.B. Thorpe, J.A. Lowe, T.C Johns, and K.D. Williams (2004), A new method for diagnosing radiative forcing and climate sensitivity, Geophys. Res. Lett., 31, L03205, doi:10.1029/2003GL018747.

Meehl, G. A., C. Covey, T. Delworth, M. Latif, B. McAvaney, J. F. B. Mitchell, R. J. Stouffer, and K. E. Taylor (2007), The WCRP CMIP3 multi-model dataset: A new era in climate change research, Bull. Am. Meteorol. Soc., 88, 1383-1394.

Murphy, D.M., and P. M. Forster (2010), On the Accuracy of Deriving Climate Feedback Parameters from Correlations Between Surface Temperature and Outgoing Radiation. J. Climate, in press. [PDF currently available to AMS members].

Spencer, R.W., and W.D. Braswell (2008), Potential biases in cloud feedback diagnosis: A simple model demonstration, J. Climate, 21, 5624-5628. PDF.

Spencer, R. W., and W. D. Braswell (2010), On the Diagnosis of Radiative Feedback in the Presence of Unknown Radiative Forcing, J. Geophys. Res., doi:10.1029/2009JD013371, in press. [PDF currently available to AGU members] (accepted 12 April 2010)