Clouds Cool the Climate System…But Amplify Global Warming?

April 14th, 2009 by Roy W. Spencer, Ph. D.

One of the basic tenets of the IPCC view on global warming is that cloud feedbacks are positive. That is, clouds react to a warming influence by further amplifying the warming.

This makes all the difference in the world for forecasts of global warming because the existence of negative cloud feedbacks could limit manmade global warming to less than 0.5 deg. C by late in this century, while positive feedbacks could result in ten times that amount of warming: 5 deg. C.

What is peculiar about all of the IPCC climate models now producing positive cloud feedbacks is that it is well known in the climate business that the average effect of clouds on the climate system is one of cooling…not warming. In the presence of radiative heating by the sun, clouds provide a stronger solar shading effect than their greenhouse warming effect, leading to a net reduction in average global temperatures by about 5 deg. C.

Another way of looking at this is, as the sun warms the Earth, a point is reached where the clouds in effect say “OK, that’s enough sunlight. We’ve got the temperature we want now.”

So, isn’t it peculiar that clouds would be claimed to do just the opposite in response to the small radiative warming effect from more carbon dioxide in the atmosphere? I think it’s more than peculiar…it verges on being logically inconsistent.

I believe there has been a self-delusion of sorts in the climate research community regarding cloud feedbacks. As I have discussed elsewhere, the observation that specific years with warmer-than-average global temperatures have less cloud cover, thus letting more sunlight in (and so suggesting positive feedback) ignores the fact that the warmth was probably the result of less cloud cover – not the other way around.

Another observation that has led to confusion over cloud feedbacks is the fact that the tropics – the geographic region where the greatest amount of sunlight is absorbed – show a distribution of low and high clouds that lead to an approximate cancellation between the low clouds’ solar shading effect, and the high clouds’ greenhouse warming effect.

This apparent ‘zero feedback’ state suggests to some researchers that feedbacks in response to warming from more CO2 could go either way. Indeed, the original paper discussing the net cooling effect of clouds, contained in its abstract the following statement regarding this cancellation, “…which indicates the delicately balanced state of the tropics.

(And where did THAT come from? Since when does a balance between two opposing forces constitute a ‘delicate’ balance? This published statement was an early foretaste of today’s religious obsession among scientists that nature is precariously balanced).

But what is ignored is the fact that the tropics can not be studied in isolation. The tropics are continuously exporting heat to higher latitudes. The higher latitudes, in contrast, are where the net effect of clouds is confidently known to be one of cooling. This equator-to-pole difference in the radiative effect of clouds then leads to an enhancement of the equator-to-pole temperature difference, which then helps drive the transport of heat from the tropics to high latitudes.

I know this sounds complicated…but that’s my point. One can’t look at clouds in just the tropics and make any deductions about the net effect of clouds on climate…even on just tropical climate…because the tropics are not a “closed system”.

From the standpoint of energy being moved from one region to another, only the whole Earth is a closed system. So, we are forced to return to the fact that the net effect of clouds is to cool the whole climate system.

Finally, I think another mistake that has been made when trying to determine the warming versus cooling influence of clouds is the way in which those effects have been defined. This has been done with satellite observations by comparing cloudy regions to surrounding clear regions. The difference between cloudy and clear regions has been assumed to be just due to the clouds.

The trouble with this assumption is that the clear regions have also been cooled by clouds. The air in the clear regions traveled there from somewhere else, where the air was also influenced by the cooling effects of clouds. This mistake in interpretation has probably led to an underestimate of the net radiative cooling effect of clouds.

Everyone agrees clouds are complicated beasts. So, one can expect that handling of clouds is probably the single biggest uncertainty in climate model predictions of global warming. Even the IPCC has admitted this in their latest (2007) report: “Cloud feedbacks are the primary source of inter-model differences in equilibrium climate sensitivity, with low cloud being the largest contributor”.

I know the IPCC would disagree, but I think what Robert Cess said 12 years ago remains true today:

the [models] may be agreeing now simply because they’re all tending to do the same thing wrong. It’s not clear to me that we have clouds right by any stretch of the imagination.

In a court of law, you would never be able to convict clouds as accomplices in the ‘crime’ of global warming. Indeed, the ‘balance of evidence’ suggests they have been acting to reduce the small amount of warming being caused by more greenhouse gases in the atmosphere.

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