Drawing attention to misinformed pseudoskeptical analyses of peer-reviewed climatology studies is usually counterproductive. But in this case, it’s worth mentioning because the author makes such a common mistake that exploring the error might actually help shed light on the why so many people are easily led astray.
The offender is Anthony Watts, who is arguably (depending on how much weight you assign to blog popularity polls) among the most influential anti-science bloggers out there. His error was to confuse (or conflate, to use a fancier term beloved by social scientists) a direct effect with a feedback.
Most instances of this error involve the argument that because 1) there is so much more water vapor in the atmosphere than there is carbon dioxide (orders of magnitude), and 2) both are “greenhouse gases” whose presence is proportional to the amount of heat the atmosphere can trap, then logically, it is silly to worry about CO2 levels. The whole global warming scare is much abo about nothing, and there’s no reason to stop pouring CO2 into the air.
Both 1 and 2 are correct. Indeed, most of the greenhouse effect is related to water vapor. But the conclusion does not follow, because water vapor is a feedback related to the amount of heat in the system. The hotter it gets, the more water evaporates and ends up in the atmosphere, where it helps trap more heat. But that vapor rapidly returns to a liquid state and falls to the surface in the water cycle. The amount of water available to the entire system is constant, and while changing the share that’s in the air at any given moment can amplify the forces causing the greenhouse effect and therefore global warming, it doesn’t cause the problem. It is a feedback. Water vapor’s net effects only strengthen if the actual forcings strengthen. Remove the real causes and the Earth will stop warming, even though there will still be a lot more water vapor in the air than CO2.
Carbon dioxide, by comparison, is released to the atmosphere when carbon in fossil fuel deposits laid down a couple of hundred million years ago is burned. Unlike water, CO2 levels in the biosphere are changing. And changes in the amount of heat-trapping CO2 in the air changes the strength of the greenhouse effect. It is a forcing — something that causes the problem. It is also part of a feedback loop as a warmer planet can release CO2 in the permafrost, for example, but it’s CO2’s role as a forcing that we can control, and why need need to stop burning so much fossil carbon. Stop burning carbon and the Earth will stop warming (eventually).
Watt’s error is rooted a similar failure to appreciate the difference between what elements of a system do to the climate and how their effects on the climate can change. He applies this variety of muddled understanding to clouds, in discussing the subject.
A new paper in Meteorological Applications by Richard P. Allan of the University of Reading someone who’s securely in the not-very-exclusive club of anthropogenic-climate-change-consensus supporters, by the way tweaks the effect of cloud cover on local climate conditions.
This is not all that remarkable. It’s long been known that “the net effect of clouds is a cooling.” You can read statements like that on old science-based blog posts like this one.
Or you can walk outside and wait for a cloud to pass in front of the sun.
Here’s how the author of the blog post linked above, climatologist Bart Verheggen, explains it in a comment on Watt’s blog:
I think you’re confusing two issues:
– the net effect of clouds on climate
– the net feedback of clouds on a change in climate
The paper, as I read it with a first quick overview, addresses the first, whereas you interpret it as if it addresses the second.
They are two distinctly different issues. The second (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 become more or less negative[?]
[UPDATE: Allan has posted his own response at Watt’s blog:
I was surprised that this paper was mis-interpreted as suggesting negative cloud feedback. This is a basic error by the author of the post that has been highlighted by many contributors including Roy Spencer.*]
We know what will happen with water vapor if the forcings of climate change continue to strengthen. It will create more vapor and consequently will amplify the warming. The question of what will happen with the clouds is proving harder to nail down. It’s a critical question, because it is conceivable that more clouds could cool the Earth by reflecting more sunlight. But it is also conceivable that more clouds could trap more heat.
Allan’s paper deals with the issues involved, but it doesn’t answer the question. All it does is refine a number that climatologists use in their models all the time. (If you’re interested, Allan’s estimate is -21 watts per square meter). The older estimates are negative, too. Allan doesn’t change the fundamental understanding of global warming science in any way whatsoever. And if Watts had any understanding of the basics of climate science he would know that.
Instead he writes a blog post trumpeting Allan’s paper as support for the idea that clouds are putting the brakes on global warming. In a just universe, he would hang his head in shame and admit he just doesn’t have a clue.
* Note that Roy Spencer is usually counted among those very few climatologists who don’t agree with the ACC consensus.
Richard P. Allan (2011). Combining satellite data and models to estimate cloud radiative effect at the surface and in the atmosphere Meteorological Applications, 18 (3), 324-333 : 10.1002/met.285