Clouds Cool the Climate System…But Amplify Global Warming?
James
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 radiation is absorbed by the sun – 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.
marcodbeast
> 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.
"Water vapour changes represent the largest
feedback affecting climate sensitivity and are now
better understood than in the TAR. Cloud feedbacks
remain the largest source of uncertainty. {8.6, 9.6, Box
10.2}"
http://www.ipcc.ch/pdf/assessment-report/ar4/wg1/ar4-wg1-spm.pdf
As usual, Roy Spencer simply lies.
Tom P
The problem with the negative feedback theory is quite simple.
If clouds or whatever other mechanism provide a negative feedback to
regulate temperature, how can it be that relatively small changes in the
energy balance could have provoked the ice ages? Surely the negative
feedback mechanisms should have stepped in?
T.
Flaps_50!
Good point, but it ignores the other parts of the equation: e.g. ocean
conveyors and ice.
Cheers