April 15, 2009

Climate Models: Better Clouds=Less Warming?

While all the attention has been focused on a paper by Warren Washington and colleagues that has just been accepted by the scientific journal Geophysical Research Letters (GRL), whose conclusions can perhaps best be summarized by “Yes, Virginia, there is something you (acting together with 7 billion of your closest friends) can do to lessen climate change,” another paper has been published by GRL that seems to argue that if climate models had a better handle on the true behavior of clouds, that they may project less warming than they do now.

So, perhaps Virginia, if you wait for the scientists to get things right, there may be less that you actually have to do in the first place (which is a good thing, because according to the Washington et al. results, you are way behind already).

Researcher Kevin Trenberth and John Fasullo examined how climate models actually calculate why the earth warms as greenhouse gases increase (and aerosol emissions decrease). They found that initially, the enhanced greenhouse gas concentrations (and the accompanying positive feedback from water vapor increases) warm the planet. But then, as the planet warms, it emits more radiation to space, and that at some point in the second half of this century, the extra radiation emitted to space by a warmer planet about equals the radiation that is trapped by the enhancing greenhouse effect and so the two effectively cancel each other out. Still though, the planet continues to warm. Why? Because of the way climate models handle clouds.

All models, by and large, have the total cloud amount of the earth (or at least between the latitudes 60ºS and 60ºN) decreasing as greenhouse gases build-up. This allows more direct solar radiation to reach the earth’s surface and thus things continue to warm up.

However, there is some indication, that this is not quite correct. Instead of a decreasing cloud cover in the future, some processes may serve to increase cloud cover, at least in the lower levels of the atmosphere. And, low level clouds turn out to be particularly good at reflecting away incoming solar radiation—that is, they act to cool the earth’s surface.

In one climate model (the NCAR model) that actually has a different cloud parameterization scheme than most of the other climate models, total cloud cover actually increases a bit as the greenhouse effect enhances. And, it turns out, that the temperature rise (and climate sensitivity) produced by the NCAR model is less than the overall average of all models—although, admittedly, not all that much less.

Trenberth and Fasullo describe this result:

A key question is how large the role of changing cloud is in the overall climate change? The NCAR model is the only one with increasing cloud between 60ºN and 60ºS, but has a climate sensitivity of 2.7 K for doubling of carbon dioxide, only slightly lower than the overall mean of all models which average 3.2 ± 0.7 K. The transient climate response at time of doubling of carbon dioxide of 1.6 K is typical of many models which average 1.7 ± 0.4 K. Hence without the change in model clouds, warming would still occur and, while it may not be quite as large, it would be manifested somewhat differently owing to the change in solar radiation at the surface.

The biggest point that Trenberth and Fasullo make is to reaffirm the fact that clouds play an important role in climate and climate change, and that climate models are still all over the place when it comes in how they simulate them. And, for the most part, the models don’t even include some cloud-influencing processes that are likely very important in developing an accurate overall picture of cloud and cloud changes in the future.

Bottom line: There is still much work to be done on the cloud issue before we can be confident that we have things right.

Reference:

Trenberth, K.E.., and J.T. Fasullo., 2009. Global warming due to increasing absorbed solar radiation. Geophysical Research Letters, 36, L07706, doi:10.1029/2009GL037527.




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