Posts Tagged ‘cloud effect’

Confusing the net cloud effect with a cloud feedback: Very different beasts

September 20, 2011

I stumbled upon a new post at WUWT today:

New peer reviewed paper: clouds have large negative-feedback cooling effect on Earth’s radiation budget

Note that Anthony has since changed the title to leave out the word “feedback”, which was the source of his confusion. It starts out as follows:

Oh dear, now we have three peer reviewed papers (Lindzen and Choi, Spencer and Braswell, and now Richard P. Allan) based on observations that show a net negative feedback for clouds, and a strong one at that. (…) The key paragraph from the new paper:

…the cloud radiative cooling effect through reflection of short wave radiation is found to dominate over the long wave heating effect, resulting in a net cooling of the climate system of −21 Wm−2.

The attentive reader will immediately spot the problem here. Watts is confusing two issues:

– the net radiative effect of clouds on climate (i.e. in comparison with having no clouds at all)

– the net feedback of clouds in response to a change in climate

The paper addresses the first, whereas Anthony interpreted it as if it addresses the second.

These are two distinctly different issues. The latter (clouds as feedback) is about how cloud cover and properties might change in response to a warming or cooling of the climate: Will the net cloud radiative effect (i.e. the former) become more or less negative.

The net radiative effect of clouds on Earth’ climate has long been known to be negative (i.e. cooling). See e.g this quote from the paper:

The overall global net cloud radiative effect is one of cooling as documented previously (Ramanathan et al., 1989).

That can be verified in any textbook on the subject and most introductions of papers on this topic. Or in my introductory post on aerosols, clouds and climate.

I pointed this error out in the thread, as did more than a few others after me (including Roy Spencer). Only after the author of the paper, Richard Allan, came in to say that this post mis-interpreted the paper, did Anthony change the title and added an update. The mistaken interpretations are still in the body of the text though.

Richard Allan wrote to me in email (reproduced with permission):

I was surprised that this paper was linked to cloud feedback since, as you mention, it attempts to quantify the well known influence of cloud on Earth’s radiation budget (at the top of the atmosphere, at the surface and within the atmosphere and also during day and night) and does not attempt to diagnose cloud feedback.

Watts goes on to say (bold in original):

The cooling effect is found to be -21 Watts per meter squared, more than 17 times the posited warming effect from a doubling of CO2 concentrations which is calculated to be ~ 1.2 Watts per meter squared.

He’s comparing apples and oranges. The 21 W/m2 is the top of the atmosphere (TOA) cloud forcing in reference to having no clouds at all (see table 1 in the paper); the 1.2 W/m2 is the surface forcing due to a doubling in CO2 concentrations. The TOA forcing of a doubling in CO2 is closer to 4 W/m2. But that’s not the “zero” point. The total greenhouse effect (due to water vapor, clouds, CO2 and other GHG) is about 150 W/m2.                                                                                                                   

In other words, this paper falls squarely within the mainstream; it further quantifies a (previously known) net cooling effect of clouds on the Earth’ climate; it does not quantify how clouds may change in response to warmer climate (cloud feedback), though it does provide a carrot stick in saying that these types of analyses are important “in assessing cloud climate feedbacks which contribute substantially to uncertainty in climate prediction.”. That may very well be, but it hasn’t been done in this paper (as confirmed by its author). 

Judging by the comments, many at WUWT took this, in combination with the whopping -21 W/m2, to mean that they discovered a gigantic negative feedback. Nope.

Tallbloke (from Lisbon fame) still insists that

if [cloud forcing] becomes slightly less negative, it’s still very negative, and overwhelms the effect of changes in co2.

… being very confused. Comments vary over a very wide range though. Many are confused (e.g. stating that as specific humidity goes up in a warmer world, so should cloud cover, whereas cloud formation depends on relative humidity rather than on specific humidity), there’s lots of laughing-at-the-scientists going on, but there are also sensible comments that either offer insight or good questions.

Mosher makes the following observation:

it is also fascinating because of what we dont see. usually you will see a whole crew of commeters pounce on the word “model”. This time they didnt.

They didnt because they thought the paper supported spencer. But it was on an entirely different topic. That misunderstanding kinda silenced the usual “models are bad” crew.

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