Clouds and Global Warming

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When it comes to global warming from CO2, the toughest question is how are clouds going to respond? The IPCC has about 20 differant climate models, but only one includes a change in cloud cover. The reason is that there hasn't been much evidence (until now) about how clouds would actually respond to CO2 warming. However, what is being found is not encouraging...


The first reliable analysis of cloud behavior over past decades suggests—but falls short of proving—that clouds are strongly amplifying global warming. If that's true, then almost all climate models have got it wrong. On page 460, climate researchers consider the two best, long-term records of cloud behavior over a rectangle of ocean that nearly spans the subtropics between Hawaii and Mexico. In a warming episode that started around 1976, ship-based data showed that cloud cover—especially low-altitude cloud layers—decreased in the study area as ocean temperatures rose and atmospheric pressure fell. One interpretation, the researchers say, is that the warming ocean was transferring heat to the overlying atmosphere, thinning out the low-lying clouds to let in more sunlight that further warmed the ocean. That's a positive or amplifying feedback. During a cooling event in the late 1990s, both data sets recorded just the opposite changes—exactly what would happen if the same amplifying process were operating in reverse.
http://www.sciencemag.org/cgi/content/short/325/5939/376



Scientists create climate models to try to predict how the earth will respond to higher levels of greenhouse-gas emissions, but only one model — created by the Hadley Centre in Britain — includes the possible impact of changing cloud behavior. And the bad news is that the Hadley model contains particularly high temperature increases for the 21st century, in part because it sees dissipating cloud cover as a positive-feedback cycle — meaning the warmer it gets, the less cloud cover there will be, which will further warm the earth. Though it's just one data set over one part of the earth's surface, the Science study indicates that the pessimistic Hadley model may be right. "These low clouds are like the mirrors of the climate system," says Clement. "If they disappear, you might see that positive-feedback cycle."
http://www.time.com/time/health/article/0,8599,1912448,00.html
 

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  • #2
sylas
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When it comes to global warming from CO2, the toughest question is how are clouds going to respond? The IPCC has about 20 differant climate models, but only one includes a change in cloud cover. The reason is that there hasn't been much evidence (until now) about how clouds would actually respond to CO2 warming. However, what is being found is not encouraging...
Thanks for that... interesting!

But I have a couple of quibbles.

First, most climate models do include changes in cloud cover -- and I would have thought this would be in ALL the models compared by the IPCC. I have no idea what the Time article means by saying only one model does this.

Added in edit. Found out. The extract from Time is inacccurate, and mixes up what is reported in science magazine. In fact, all the models do include the effects of cloud cover. The Hadley model stands out for obtaining results that are closest to what has been measured in this research for the effect of low lying cloud.

A caution is that this is not a comprehensive look at all the complexities of cloud, but it is a look at one aspect of cloud models and a comparison of models and observations. The results give some support to a higher end climate sensitivity... which is bad news.

Science magazine actually has two articles. There's the article by Kerr, which is a summary by a science writer for news of the week, and there is the main research he's commenting upon. I'll read it all up a bit more and perhaps add more detail later.


It should be noted that models don't pre-suppose how cloud cover changes. The amount of cloud is a something that depends on local conditions; same as in a weather model. I can't tell from the article what if anything is different about the Hadley climate model.

One of the basic comparisons of climate models is to see the effect of cloud in the model. In studies of feedback within models (section 8.6 of the IPCC report covers this) cloud feedbacks from chances in cover and composition and so on are compared... along with other feedback diagnostic measurements... for a large range of models.

Second... and I am not sure how relevant this quibble really is... one of the biggest problems with cloud in models is (I think) not so much with the amount of cover, but with the impact of cloud on the way energy flows in the system. Clouds reflect, absorb and scatter light in ways that depend on altitude, thickness, droplet size, and so on. Sorting out their effect is hard.

It is certainly the case that cloud modelling is identified as one area on which climate models need to make progress, and the work you are reporting seems to be a useful input on studies of how cloud and climate interact.

Cheers -- sylas
 
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  • #3
Xnn,

The study in question only focuses on a relatively small region of the globe (the Northeast Pacific, roughly 115° to 145°W, 15° to 25°N) and is thus a relatively small sample of what may happen over the globe. It also focuses exclusively on the low clouds which influence the albedo, whereas high clouds may potentially play as big a part in feedback (or bigger) through their influence on the outgoing energy side of the equation (see Lindzen's IRIS or Hartmann's FAT hypothesis for instance). So, I've emphasized in some other comments at RealClimate and ClimateProgress for example that the paper should not be over-hyped.

The claim about "1 model producing cloud changes" is not correct, but the potential confusion is that the authors note in the Science paper that only one model produces the change in NE pacific cloud cover which is consistent with their observations. This may mean that a tendency exists to undersubscribe a positive cloud-albedo feedback in many other models, and evaluating future cloud feedback from one model is not ideal. But I think this is something that needs to be looked at over different regions and the quality of observations must improve as well. Cloud trends and how global cloud feedback will behave in the future is still very much uncertain.
 
  • #4
Xnn
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Chris/Sylas;

Thanks for the good comments.

Earth is called the Blue Planet, but there sure are a lot of clouds too.
 
  • #5
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Another interesting recent paper on cloud formation is:

Title:Potential Biases in Feedback Diagnosis from Observational Data: A Simple Model Demonstration
Authors:Spencer, Roy W.; Braswell, William D.
Publication:Journal of Climate, vol. 21, issue 21, p. 5624
Publication Date:11/2008

You can download the full paper here: http://www.drroyspencer.com/Spencer-and-Braswell-08.pdf" [Broken]

The paper seems to suggest that there are basic causality issues with the interactions between temperature and clouds in climate models.

Based on the Acknowledgments, it appears the paper was significantly revised based on the input from reviewers, Piers Forster and Isaac Held. I checked their CV and found this paper from Dr. Held:

Soden, Held, Colman, Shell, Kiehl, and Shields, 2008: Quantifying climate feedbacks using radiative kernels. Journal of Climate

Download Here:
http://www.gfdl.noaa.gov/bibliography/related_files/bjs0801.pdf" [Broken]

You can access some more of Dr. Held peer-reviewed papers here:

http://www.gfdl.noaa.gov/isaac-held-homepage"
 
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