I’ve been wading through James Hansen’s recent 52-page unpublished paper explaining why he thinks the cooling effect of manmade sulfate aerosols has been underestimated by climate modelers.

This is the same theme as the “cooling from Chinese pollution is canceling out carbon dioxide warming” you might have heard about recently.

As I read Hansen’s paper, I stumbled upon a sentence on page 23 that sounds like one I just wrote in a new paper we are preparing. I’m going to use Hansen’s statement — which I agree with — because it provides a good introduction to understanding the basics of climate change theory:

“…surface temperature change depends upon three factors: (1) the net climate forcing, (2) the equilibrium climate sensitivity, and (3)…..the rate at which heat is transported into the deeper ocean (beneath the mixed layer).”

To better understand those 3 factors, consider what controls how much a pot of water on the stove will warm over a short period of time. The temperature rise depends upon (1) how much you turn up the stove, or cover up the pot with a lid (“forcing”); (2) how fast the pot can lose extra heat to its surroundings as it warms, thus limiting the temperature rise (“climate sensitivity”); and (3) the depth of the water in the pot.

Most people working in this business (including the IPCC) agree that probably the biggest uncertainty in determining the extent to which manmade global warming is something we need to worry about is #2, climate sensitivity.

But not Hansen.

Hansen believes he knows climate sensitivity very accurately, based upon paleoclimate theories of what caused temperature changes hundreds of thousands to millions of years ago, and how large those temperature changes were. Most of Hansen’s climate sensitivity claims are based upon the Ice Ages and the Interglacial periods.

I must admit, it astounds me how some scientists can be so sure of theories which involve events in the distant past that we cannot measure directly. Yet we measure the entire Earth every day with a variety of satellite instruments, and we are still trying to figure out from that abundance of data how today’s climate system works!

In Hansen’s case, in order to explain the amount of warming in recent decades, he thinks he knows #2 (the climate sensitivity) is quite high, and so he has been experimenting with various realistic values for #3 (the assumed rate of heat diffusion into the deep ocean) and has decided that factor #1 (the radiative forcing of the climate system) has not been as large as everyone has been assuming. Again, this is in order to explain why surface warming has not been as strong as expected.

Now, I tend to agree with Hansen that the main portion of that forcing, from increasing CO2 (a warming effect), is known pretty well. (Please, no flaming from the sky dragon slayers out there…I already know about your arguments). So in his view there MUST be some cooling influence canceling it out. That’s where the extra dose of aerosol cooling comes in.

All modelers have already fudged in various amounts of cooling from sulfate aerosols in order to prevent their climate models from warming more than has been observed. But Hansen ALSO thinks that the real climate system does not mix heat into the deep ocean as fast as the IPCC climate models do.

Unfortunately, correcting this error (if it exists, which I think it does) would push the models in the direction of too much surface warming. Therefore, Hansen thinks this must then mean that the IPCC models are assuming too much forcing (the only remaining possibility of the 3 factors).

Of course, as Hansen correctly points out, accurate global measurements of the cooling effect of aerosols are essentially non-existent. How convenient. This means that modelers can continue to use increasing amounts of sulfate aerosols as an “excuse” for a lack of recent warming, despite the lack of quantitative evidence that this is actually occurring.

As I sometimes point out, this line of reasoning verges on blaming NO climate change on humans, too.

It is unfortunately that so few of us (me, Lindzen, Douglass, and a few others) are actively researching the OTHER possibility: that climate sensitivity has been greatly overestimated. Lindzen and Choi have a new paper in press on the subject, and Braswell and I have another that I expect to be accepted for publication in the next few days.

I sort of understand the reluctance to research the possibility, though. If climate sensitivity is low, then global warming, climate disruption, tipping points, and carbon footprints all suddenly lose their interest. And we can’t have that.

I agree with Hansen that our best line of observational evidence is how fast the oceans warm — at all depths. Our recent work on estimating climate sensitivity from the rate of warming between 1955-2010 at different depths has been very encouraging, something which I hope to provide an update on soon.