Watts Up With That (WUWT) has a new post by Willis Eschenbach called the Cloud Radiative Effect (CRE). The CRE refers to the influence that clouds have on the Earth’s albedo. Clouds act to reflect incoming radiation and hence have a negative forcing.
Willis appears to have downloaded CERES data and produced a very nice figure showing the forcing due to clouds. The figure is below and if you average over the whole globe you find that the clouds produce a negative forcing of -20.6 Wm-2. I believe this was discussed in an earlier WUWT post in which they got very excited because they thought this could completely cancel any anthropogenic forcings. This was nicely rebutted by Bart Verheggen in a post called confusing the net cloud effect with a cloud feedback: Very different beasts. Basically, the figure shows the net radiative forcing of clouds (measured by CERES) relative to having no clouds at all. So, this illustrates the role that clouds play in overall greenhouse warming (making the Earth about 30oC warmer than it would be with no greenhouse warming) and does not indicate how their influence has changed since pre-industrial times.
Willis has, however, gone further and also determined the rate of change of CRE with respect to temperature. This is shown in the figure below. The values can vary quite substantially from -26 Wm-2K-1 to 36 Wm-2K-1. Willis then determines an average of -2.9 Wm-2K-1.
This result may seem quite interesting because how clouds influence anthropogenic warming is quite important. They tend to increase albedo and hence reduce the overall change in forcing since pre-industrial times. According to the IPCC, however, the change in radiative forcing due to clouds since 1750 is between 0 and -1.2 Wm-2 with a best estimate of about -0.4 Wm-2 (unless I’ve read the diagram wrong – correction added : These are the wrong numbers. This is the aerosol-cloud effect which is one of the anthropogenic forcings. In fact, as Karsten points out, the likely cloud feedback is positive with a best estimate of +0.6Wm-2 and with a range of -0.2Wm-2 to 2.0Wm-2). Willis’s estimate would suggest that it should be more like -2.9 Wm-2. This is large and would actually cancel a large fraction of the anthropogenic forcings if true. Willis actually says
Finally, there’s one more oddity. This is the fact that overall, as an area-weighted average trend, for every degree the globe warms, the warming is strongly opposed by the cloud radiation effect. The action of the clouds reduces the downwelling radiation by 3 W/m2 for every degree the planet warms … in IPCC terminology, this is not only a negative feedback, but a strong negative feedback.
And the cooling effect of the clouds is even stronger in the ITCZ. There, for every degree it warms, the downwelling radiation drops by ten W/m2 or so …
I think, although I’m by no means sure, that this is the first global observational analysis of the size of the so-called “cloud feedback”. It shows that the cloud feedback is strongly negative overall, -3 W/m2 for each degree of warming. In addition, in the critical control areas such as the ITCZ, the cooling effect is much larger, 10 W/m2 or so. Finally, it shows a very strong negative cloud feedback, 20 W/m2 or more, in the area of the Southern Ocean
So, is this some amazing analysis that suggests clouds have a much bigger influence than current mainstream climate science suggests? No, I don’t think it is. I think it’s an illustration (sadly maybe) of why some basic training in science/mathematics is actually quite important (and I hope I don’t have to eat my words here :-)). Let’s see if I can explain why. To determine the average CRE, one could simply sum over the fluxes for each grid cell, Fi, multiplied by the area of each grid cell, Ai and then divide by the surface area of the Earth, A.
Willis then, I believe, calculates the gradient with respect to temperature, for each grid cell, using something like
To get his average gradient, he then simply uses (unless I misunderstand what he’s done)
This, however, isn’t the same as what is referred to in the IPCC radiative forcing estimates. This is simply an average of the gradient of the forcing with respect to temperature (in a sense, this is probably also a weather effect, rather than a climate effect). What you really want to know is how does the overall forcing change as the average global temperature changes. As far as I can tell, to get this you’d need to do something more like
In other words, you’d need to know the difference between the average cloud forcing when global surface temperatures are T and when global surface temperatures are T ’.
So, in a sense I’m writing this to illustrate two basic points. Having some basic training is important. Unless, I’ve made some silly blunder, Willis has just illustrated that without some basic scientific/mathematical training, you can easily make a silly mistake without realising. If I want to determine the overall gradient of a function, I don’t take an average of the gradients at different points along the function. You need to consider how the function has changed over the whole interval. The other point I would make is that if you think you’ve discovered something important that other really bright people have missed, consider the possibility that you’re wrong (I’ve done this myself a number of time – see yesterday’s post on climate sensitivities for example). This is especially true if what you’ve done is quite straightforward. If your calculation is right, why would those who work in the field not have done it already themselves? I’ll even accept that maybe I’m wrong about Willis’s calculation, so – if I am – feel free to let me know. But, as far as I can tell, Willis hasn’t really discovered a new way of determining the change in cloud forcing. He’s really just illustrated why one should be cautious when you think you’ve just determined something that many other bright people have missed (something I should possibly consider myself sometimes, to be honest).