Since it has been a while since I have addressed water vapor feedback, and I am now getting more questions about it, I thought this would be a good time to revisit the issue and my opinions on the subject.

Positive water vapor feedback is probably the most “certain” and important of the feedbacks in the climate system in the minds of mainstream climate researchers. Weak warming caused by more carbon dioxide will lead to more water vapor in the atmosphere, which will then amplify the weak warming through water vapor’s role as the atmosphere’s primary greenhouse gas.

Positive water vapor feedback makes sense intuitively. Warmer air masses, on average, contain more water vapor. Warmer air is associated with greater surface evaporation rates, which is the ultimate source of almost all atmospheric water vapor.

And since water vapor is the atmosphere’s main greenhouse gas, most scientists have reasonably inferred that climate warming will be enhanced by increasing water vapor amounts. After all, water vapor feedback is positive in all of the IPCC climate models, too.


But when one looks at the details objectively, it is not so obvious that water vapor feedback in the context of long-term climate change is positive. Remember, it’s not the difference between warmer tropical air masses and cooler high-latitude air masses that will determine water vapor feedback…its how those air masses will each change over time in response to more carbon dioxide. Anything that alters precipitation processes during that process can cause either positive or negative water vapor feedback.

Here are some of those details.

1) Evaporation versus Precipitation

The average amount of water vapor in the atmosphere represents a balance between two competing processes: (1) surface evaporation (the source), and (2) precipitation (the sink). While we know that evaporation increases with temperature, we don’t know very much about how the efficiency of precipitation systems changes with temperature.

The latter process is much more complex than surface evaporation (see Renno et al., 1994), and it is not at all clear that climate models behave realistically in this regard. In fact, the models just “punt” on this issue because our understanding of precipitation systems is just not good enough to put something explicit into the models.

Even cloud resolving models, which can grow individual clouds, have gross approximations and assumptions regarding the precipitation formation process.

2) Negative Water vapor Feedback Can Occur Even with a Water Vapor Increase
Most atmospheric water vapor resides in the lowest levels, in the ‘turbulent boundary layer’, while the water vapor content of the free troposphere is more closely tied to precipitation processes. But because the outgoing longwave radiation is so much more sensitive to small changes in upper-layer humidity especially at low humidities (e.g. see Spencer & Braswell, 1997), it is possible to have a net increase in total integrated water vapor, but negative water vapor feedback from a small decrease in free-tropospheric humidity. See #4 (below) for observational support for this possibility.

3) Cause Versus Effect
Just because we find that unusually warm years have more water vapor in both the boundary layer and free troposphere does not mean that the warming caused the moistening.

There are a variety of processes (e.g. tropospheric wind shear causing changes in precipitation efficiency) which can in turn alter the balance between evaporation and precipitation, which will then cause warming or cooling as a RESULT OF the humidity change – rather than the other way around.

This cause-versus-effect issue has been almost totally ignored in feedback studies, and is analogous to the situation when estimating cloud feedbacks, the subject of our most recent paper.

Similar to our cloud feedback paper, evidence of causation in the opposite direction is the de-correlation between temperature and humidity in the real world versus in climate models (e.g. Sun et al., 2001).

4) Evidence from Radiosondes
There is some evidence that free tropospheric vapor has decreased in recent decades (e.g. the Paltridge et al., 2009 analysis of the NCEP Reanalysis dataset) despite this being a period of surface warming and humidifying in the boundary layer. Miskolczi (2010) used the radiosonde data which provide the main input to the NCEP reanalysis to show that the resulting cooling effect of a decrease in vapor has approximately counterbalanced the warming influence of increasing CO2 over the same period of time, leading to a fairly constant infrared opacity (greenhouse effect).

Of course, water vapor measurements from radiosondes are notoriously unreliable, but one would think that if there was a spurious drying from a humidity sensor problem that it would show up at all altitudes, not just in the free troposphere. The fact that it switches sign right where the turbulent boundary layer pushes up against the free troposphere (around 850 mb, or 5,000 ft.) seems like too much of a coincidence.

5) The Missing “Hot Spot”
Most people don’t realize that the missing tropospheric “hot spot” in satellite temperature trends is potentially related to water vapor feedback. One of the most robust feedback relationships across the IPCC climate models is that those models with the strongest positive water vapor feedback have the strongest negative lapse rate feedback (which is what the “hot spot” would represent). So, the lack of this negative lapse rate feedback signature in the satellite temperature trends could be an indirect indication of little (or even negative) water vapor feedback in nature.

Conclusion
While it seems rather obvious intuitively that a warmer world will have more atmospheric water vapor, and thus positive water vapor feedback, I’ve just listed the first 5 reasons that come to my mind why this might not be the case.

I am not saying that’s what I necessarily believe. I will admit to having waffled on this issue over the years, but that’s because there is evidence on both sides of the debate.

At a minimum, I believe the water vapor feedback issue is more complicated than most mainstream researchers think it is.

In my previous post I argued, using commonly cited numbers, that the greenhouse effect enhancement of adding carbon dioxide to the atmosphere would be about 3% for a doubling of CO2 (“2XCO2”).

The 3% enhancement is based upon 2 commonly quoted numbers: (1) 33 deg. C global-average surface warming for the natural greenhouse effect, and (2) about 1 deg. C additional surface warming from 2XCO2, without feedbacks. (Interestingly, these numbers can only be computed from theory, which always requires a variety of assumptions.)

The value of 33 deg. C represents the difference between the observed average surface temperature of the Earth, and the estimated surface temperature if there was no atmosphere.


I explained that the 3% statistic is the one we should be dealing with conceptually, rather than what some people seem to be interested in, which is what portion of the Earth’s greenhouse effect is due to CO2. I argued that the answer to that question, which has been recently addressed in a new paper by Schmidt et al., really tells us very little regarding the impact of adding more CO2 to the atmosphere.

But what many people don’t realize is that the 33 deg. C of surface warming is not actually a measure of the greenhouse warming – it represents the balance between TWO competing effects: a greenhouse warming effect of about 60 deg. C (the so-called “pure radiative equilibrium” case), and a convective cooling effect of about 30 deg. C. When these two are combined, we get the real-world observed “radiative-convective equilibrium” case.

This has been known since at least 1964 (Manabe and Strickler, 1964). It was also discussed in Dick Lindzen’s 1990 paper, Some Coolness Regarding Global Warming, which is when I became aware of its significance.

Why is this Important?

When global warming is discussed, the warming effect of greenhouse gases is obviously of prime interest. But it is seldom if ever mentioned that about 50% of the surface warming influence of greenhouse gases has been short-circuited by the cooling effects of weather, as just discussed.

When Danny Braswell and I did similar calculations in 1997 to better understand the physics, we found that 1 deg. C of surface warming was true even for the pure radiative equilibrium case (no convective cooling by weather processes). This would mean that the REAL enhancement of the greenhouse effect with 2XCO2 is really only about 1.5%, not 3%, since the natural greenhouse effect is trying to warm the surface by over 60 deg. C, not by 33 deg. C.

Is this Simple Evidence of Negative Feedback?

These climate basics, which have been known since the 1960s, also raises an intriguing question: If the surface warming effect of 2XCO2 before surface cooling by convection is 1 deg. C, and (as even the IPCC knows) 50% of that natural greenhouse warming is then short-circuited by convection, might this then tell us that negative feedbacks in the climate system can be expected to reduce anthropogenic global warming to only 0.5 deg. C?

I believe this is entirely possible.

How could this happen, since there is so much evidence that water vapor feedback is positive? Because, even if water vapor feedback is positive, an increase in the solar shading effect of clouds (negative cloud feedback) could more than overwhelm the positive water vapor feedback, leading to little net warming.

The IPCC already admits feedbacks due to low clouds are the least understood. Indeed, the evidence presented in Spencer and Braswell (2010), at face value, would suggest this could happen.

We already know that the net effect of clouds is to cool the climate system in response to solar heating. Are we to believe that cloud changes turn into a warming influence when temperatures get a little bit higher? Well, that’s what all of the IPCC coupled climate models do now.

The Importance of Convective Cooling Versus Greenhouse Warming

I sometimes get e-mails asking why I don’t mention convection as a cooling mechanism in the context of global warming. Folks, I used to be virtually the only one speaking out on the subject. For years I harped on this issue.

The reason why I have been recently defending the basic physics of the greenhouse effect is because I think the credibility of those who claim that the greenhouse effect of the atmosphere cannot be increased (or doesn’t even exist) is compromised when they object to something that – as far as I have seen – has no alternative explanation.

I’m always upon open to new theories, but as I have said before, until someone puts their alternative physics into an energy-conserving model of the vertical temperature profile of the atmosphere, which then produces the present-day temperature profile as current models do, it is little more than hand-waving.

[NOTE: What follows assumes the direct (no-feedback) infrared radiative effects of greenhouse gases (water vapor, CO2, methane, etc.) on the Earth’s radiative budget are reasonably well understood. If you want to challenge that assumption, your time might be better spent here.]

I was recently asked by a reader to comment on a new paper by Schmidt et al. which put some numbers behind the common question, What fraction of the Earth’s greenhouse effect is due to carbon dioxide?

There are a wide variety of answers to this question depending on how you define “greenhouse effect”, what your assumed baseline is, etc. Conceptually, in any greenhouse atmosphere, greenhouse gases warm the lower layers and cool the upper layers compared to if those gases were not present. That never changes. It’s the way you compute the relative magnitude (say, in percent) of that warming that depends greatly upon your assumptions.

Note that the greenhouse effect can only be calculated based upon theory. The greenhouse effect isn’t a physical variable like temperature that you can measure. It is a radiative process that affects the atmosphere’s energy budget at all altitudes, warms the surface, and whose components must be calculated based upon radiative transfer theory and the IR absorption characteristics of greenhouse gases (and clouds).


The Wrong Question

I will argue that if what we are REALLY interested in is how much the Earth’s greenhouse effect will be enhanced by adding CO2 to the atmosphere (the only reason we are interested in the CO2 issue anyway, right?), then the above question is not very relevant.

In fact, the answer to it can totally mislead us. This is easy to show with 2 simple examples.

First, assume there was NO naturally occurring carbon dioxide in the atmosphere, and we added 300 ppm. In that case, the natural influence of CO2 on the Earth’s greenhouse effect would be zero, but the influence of adding 300 ppm would be quite significant.

Now, as the second example let’s assume the natural CO2 concentration is high, say 1,000 ppm, and THEN we added 300 ppm. In this second case, the natural role of CO2 in the Earth’s greenhouse effect would be very significant, but our addition of 300 ppm more would have a relatively small direct warming influence.

This is because the more CO2 there is in the atmosphere, the more “saturated” the CO2-portion of the greenhouse effect becomes, a well known feature that has a standard simplified, logarithmic formula for its computation.

Everyone already knows about this mostly saturated condition relative to the radiative effect of carbon dioxide – even the IPCC. Adding more and more CO2 causes incrementally less and less warming (again, assuming no feedback, which is a separate issue)….but the radiative effect of CO2 in the atmosphere is not totally saturated.

And it never can be, for the same reason that you can keep dividing a number by two forever, and the resulting number will get extremely small…but it will never reach zero.

So what do these two examples tell us? If the natural contribution of CO2 to the greenhouse effect was ZERO, then the warming effect of our addition of 300 ppm would be relatively large. But if the natural contribution of CO2 to the greenhouse effect was already large, then the incremental warming effect of adding more will be small.

An extreme example would be Venus, which has 230,000 times as much CO2 in its atmosphere as Earth does. Our addition of CO2 to that atmosphere would have essentially no effect.

The point is that knowing what percentage of the Earth’s natural greenhouse effect comes from carbon dioxide alone tells us little of use in determining how much warming might result from adding more CO2 to the atmosphere.

How Much is the Earth’s Greenhouse Effect Enhanced by Adding More CO2?
This is the question we should be asking, and it can be easily answered with a couple of numbers quoted in the Schmidt et al. article.

Schmidt et al. assumes the commonly quoted 33 deg. C as the amount of surface warming due to the Earth’s greenhouse effect, and for the time being I will assume the same. (In my next blog post, I will explain why this number is NOT a good measure of the Earth’s greenhouse effect.)

Thirteen years ago, Danny Braswell and I did our own calculations to explore the greenhouse effect with a built-from-scratch radiative transfer model, incorporating the IR radiative code developed by Ming Dah Chou at NASA Goddard. The Chou code has also been used in some global climate models.

We calculated, as others have, a direct (no feedback) surface warming of about 1 deg. C as a result of doubling CO2 (“2XCO2”).

So, this immediately gives us numbers we can use to compute a percentage increase in the greenhouse effect: Doubling of atmospheric CO2 (which will probably happen by late in this century) enhances the Earth’s greenhouse effect by about (1/33=) 3%.

This value (3%) for the enhancement of the Earth’s greenhouse effect from our addition of CO2 is much smaller than the 20% value that Schmidt et al. get…but remember that we are addressing two different issues. I claim what we should be interested in is the relative size of our enhancement of the greenhouse effect, rather than how much of the Earth’s natural greenhouse effect is due to CO2. The latter question really proves nothing about how much effect adding MORE CO2 to the atmosphere will have.

Next Time: Why 33 deg. C is a Misleading Number

In my next post, I will discuss why the use of 33 deg. C for surface warming due to the greenhouse effect is very misleading. The issue is not new, as it has been known since the 1960s. I wasn’t aware of its central importance to the global warming debate until Dick Lindzen published his 1990 paper, Some Coolness Concerning Global Warming.

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 0.342 0.420
2010 8 0.511 0.672 0.349 0.362


(NOTE: 9/9/10, fixed 13-month running avg…it was 2 months behind)
While the global-average lower tropospheric temperature remained high, +0.51 deg. C in August, 2010, monitoring of the daily Aqua Ch.5 data at the Discover web site suggests that the cooling of global average sea surface temperatures that started several months ago is now causing the troposphere to cool as well. I will probably provide an update of that plot tomorrow.


As of Julian Day 243 (end of August), 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 0.06 C:

YEAR GL
1998 +0.61
2010 +0.55

As a reminder, six 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.]

I am seeing increasing chatter about one or more papers that will (or already have) debunked my ideas on feedbacks in the climate system.

Yet, I cannot remember a climate issue of which I have ever been so certain.

I understand that most people interested in the climate debate will simply believe what their favorite science pundits at RealClimate tell them to believe, which is fine, and I can’t do anything about that.

But for those who want to investigate for themselves, I recommend reading only our latest and most comprehensive paper in Journal of Geophysical Research. It takes you from the very basics of feedback estimation — which I found I had to include because even the experts in the field apparently did not understand them — and for the first time explains why satellite observations of the climate system behave the way they do.

No one has ever done this before to anywhere near the level of detail we do.


[Unfortunately, our 2008 paper in Journal of Climate, I now realize, had insufficient evidence to make the case we were trying to make in 2008. I believe our claims were correct, but the evidence we presented could not unequivocally support those claims. Only after finishing our most recent 2010 paper did I realize the insufficiency of that previous work on the subject.]

Then, once you think you understand the main points we make in the new JGR paper, read any other critiques or criticisms that catch your fancy.

As a teaser, one of the clear conclusions the new paper supports is this: The only times that there is clear evidence of feedback in global satellite data, that feedback is strongly negative.

All I ask is that you evaluate whether anyone can come up with a better explanation than what we have given for the structures we see in the satellite observations of natural climate variations. Do not settle for others’ vague arm-waving dismissals based upon preconceived notions or what others have told them.

You engineers and scientists from other fields are capable of understanding this, and I am appealing to you to bring fresh eyes to a field where the research establishment has become hopelessly inbred and too beholden to special interests to see that which is staring them in the face.

This is the main reason why I wrote The Great Global Warming Blunder…the evidence is simple enough for the science-savvy public to understand. But the experts do not see the evidence because they refuse to open their eyes.

Now that I have opened the political Pandora’s Box, I might as well continue getting some of this off my chest.

Some people think that I hurt my scientific credibility by revealing my political views from time to time. Well, I don’t like politicians exploiting and ultimately destroying public faith in my scientific discipline (climate science) for their own political and financial gain.

We scientists will be sorry we ever allowed ourselves to be manipulated by powerful people who transformed what was a theoretical possibility for climate scientists, into a near certainty for public consumption.

While I firmly believe that the ultimate motivation behind the IPCC’s existence is not at all what they advertise it to be, I must admit the United Nations still has the upper hand: the theoretical possibility of catastrophic global warming (aka ‘climate change’).


As a scientist, I must admit that warming of 4 deg. C or more this century is theoretically possible. But it’s a little like concerns that the Large Hadron Collider will cause the Earth to be swallowed by a black hole when it is switched on.

Unlike particle physicists, climate researchers currently have no way to objectively determine the probability of dramatic changes like climate tipping points. At least when particle physicists talk probabilities, they are talking about real probabilities, based upon real observable events which are repeatable. The IPCC’s probabilities regarding one-of-a-kind events with uncertain causes (e.g. warming in the last 50 years) are no more than measures of their faith expressed in pseudo-scientific jargon.

And the people who write the Summary for Policymakers for the IPCC reports are masters at wordsmithing their documents to convey maximum alarm without resorting to outright falsehoods. How clever.

The fact that catastrophic warming will remain a possibility indefinitely allows the U.N. to continue its propaganda campaign.

Living is Risky

Modern fears of global warming and other perceived dangers to the consumer support my claim that our society is more risk adverse than any in history.

The very existence of the Precautionary Principle shows that even though every one of us weighs risks against benefits in every decision we make on a daily basis, some people can still dream up totally illogical reasons why humanity should stop doing this or that.

Never in history have so many advocates with so little common sense held so much influence over so selfish a political class who were elected by so gullible a public for such irrelevant reasons.

Living is risky. Get over it.

I’m sure that the IPCC’s embattled Grand Poobah, Rajendra Pachauri, flies in airplanes even though they might crash, crosses streets even though he might get run over, eats food even though he might choke to death, and writes vapid steamy romance novels even though he might be ridiculed.

Obviously, Pachauri weighs risks against benefits in his own life, as do we all. Please don’t insult our intelligence by demanding that we dismantle all coal-fired power plants and replace them with a million bird Cuisinarts (aka windmills).

Political Versus Climatic Tipping Points

We are endlessly fascinated by tipping points. The movie Day After Tomorrow involved a mini-Ice Age forming in a matter of days as a result of your SUV getting together with other SUVs after work and drinking too much at the local Exxon watering hole.

Scientists and engineers think of tipping points in terms of nonlinear relationships. A given forcing results in a certain response, but beyond a certain level of forcing the response grows dramatically. The future behavior of nonlinear systems is notoriously difficult to forecast.

The climate system is a nonlinear system. This doesn’t necessarily mean it has tipping points, but it sure doesn’t exclude the possibility either.

As long as people like James Hansen believe that the Ice Ages were the greatly amplified response to a weak forcing, they will be able to claim that our climate system has experienced tipping points in the past. No matter how long we go without significant warming, influential people like Hansen will still claim that it is only a matter of time before Mother Nature decides she has had enough, and turns the oven up from “warm” to “broil”.

As a result, there will always remain some science that can be used to justify the work of the IPCC.

The Future of the IPCC

The IPCC is now at a tipping point. Will its self-destruct? Probably not. Dramatic organizational changes will be instituted, and at some point success will be declared. The IPCC will be refocused, leaner, meaner, more transparent, more open to the views of the deniers…er…I mean skeptics.

But the ultimate purpose of the IPCC will remain unchanged: to cherry pick and misuse climate science in order to eventually control humanity’s access to energy.

So, in order to put the IPCC out of its misery, it will take more than to just point out its selective use of facts and its biased science. Its demise will have to be the result of political pressure related to its biased political agenda.

And at least in the U.S., the current indications are that the citizens have had just about as much as they can take from those whose (arguable) good intentions force others to pay for paving that proverbial road to hell.