Global cloud height fluctuations measured by MISR on Terra from 2000 to 2010 (Geophysical Research Letters, abstract)A satellite uses stereo images in the radio spectrum to measure the distance – and therefore height – of the clouds. For the first time, we may study its dynamics quantitatively. Does it change?
Well, it does. It goes up in some regions, it goes down in other regions, it shows some remote relationships with the El Niño and La Niña conditions, but there's still an overall trend one can see between 2000 and 2010 which is 44 ± 22 meters per decade. It's a lot but the error is large, too.
So the claim that the "trend is nonzero" is only demonstrated at a 2-sigma level i.e. 95 percent confidence level. And it still depends on the assumption that the measured height follows a trend superposed with a white noise, a model that is surely inaccurate but that may be vastly inaccurate and misleading, too.
Much of the media argue – and most likely, so do the authors – that the "falling sky" may induce some cooling. I don't know whether it's the case.
Richard Lindzen was specifically telling me (in the context of discussions on cosmoclimatology) that a 50-meter change of the cloud cover height doesn't visibly impact the surface temperatures. If I cared much more than I do, I would surely try to get some more detailed explanation from him. Well, maybe I will later today...
There are lots of unknowns and lots of possible terms contributing with both signs. Some of them sound sensible, others don't, and no one knows what is the most important one and whether the sum of all is really aligned with the most important single one or not.
For example, one could claim that it's very surprising that the clouds are dropping in a world with a rising CO2 concentration. You could say that the troposphere (the lower part of the atmosphere where the temperature decreases with the altitude, and circulates as a result) is getting thicker – because there are more greenhouse gases in it – so the tropopause should go up and the clouds may be expected to go up as well. This could potentially contradict the observation revealed in the paper. But all this intuition may be totally naive, neglecting other effects that may revert the very sign of the answer, too, and CO2 could be just totally irrelevant for all these questions.
It's also being said that the motion of the clouds may be the cause of the recent (mild) cooling – or the somewhat strong cooling relatively to the predictions of global warming, if you recall the fearmongers' discussions about the dog who miraculously ate their heat. ;-) That would mean that the response of the cloud height provides the atmosphere with a negative feedback that reduces the magnitude of the original perturbation, e.g. of the warming caused by CO2 or something else. Such a claim seems very plausible to me.
Nature is dominated by negative feedbacks – a reason to expect that the climate sensitivity is smaller than, not larger than, the no-feedback value of 1.2 °C per CO2 doubling. Why are negative feedbacks more common? It's simple. They're a property of stable points in the landscape of possibilities, in the configuration spaces of complicated physical systems. There can also be unstable points in the landscape where unstable feedbacks may be the kings but because they are unstable, the system usually finds its reason to evolve away from them and end in a more stable situation where the negative feedbacks prevail.
(Particle physicists should think about the stabilization of a tachyon, or the Higgs boson, for that matter.)
At any rate, the cloud height is yet another wild card that may (and probably is actually enough to) substantially invalidate almost all the predictions by the available climate models: all of them are pretty much totally ignorant about the right behavior of quantities such as the cloud height and about the impact of these quantities on others.
These are things that atmospheric physicists of course have to study, both theoretically and experimentally. Claiming that we know everything – which is totally preposterous – or that none of these things matters would be dishonest and counterproductive for the progress in science.
And that's the memo.