Knobs

Guest Post by Willis Eschenbach

Andrew Lacis and the good folks at GISS have a new paper, Atmospheric CO2: Principal Control Knob Governing Earth’s Temperature, Andrew A. Lacis, Gavin A. Schmidt, David Rind, Reto A. Ruedy 15 OCTOBER 2010 VOL 330 SCIENCE [hereinafter “Lacis10”]. Although most commenters have dismissed their work as being derivative and not containing anything new, I find that they have actually made a couple of unique and novel errors. I have two main difficulties with their paper. I have a problem with one of their theoretical claims, and I also have large issues with their model results. First, the theoretical claim. Lacis10 says:

Because the solar-thermal energy balance of Earth [at the top of the atmosphere (TOA)] is maintained by radiative processes only, and because all the global net advective energy transports must equal zero, it follows that the global average surface temperature must be determined in full by the radiative fluxes arising from the patterns of temperature and absorption of radiation. This then is the basic underlying physics that explains the close coupling that exists between TOA radiative fluxes, the greenhouse effect, and the global mean surface temperature.

Figure 1. Global Energy Budget from Trenberth et al.

Let me examine this claim one piece at a time.

They start by saying:

Because the solar-thermal energy balance of Earth [at the top of the atmosphere (TOA)] is maintained by radiative processes only …

This is not clear. What does “maintained” mean? I think they mean that on average outgoing radiation must perforce equal incoming solar radiation, which is true. As seen in Fig. 1, 341 W/m2 of incoming solar is balanced by the 102 W/m2 of reflected solar plus 239 W/m2 of outgoing longwave.

Next they say:

… and because all the global net advective energy transports must equal zero, …

“Advection” is defined by the American Meteorological Society as  “the process of transport of an atmospheric property solely by the mass motion (velocity field) of the atmosphere;”

Since advection merely moves energy around, you’d think that advection wouldn’t change the average global temperature. However, while energy is conserved, temperature is not conserved. Suppose we take two equal areas, say the part of the planet from 30N to 30S (average 25°C) and the rest of the planet including the poles (average 4°C).

Advection (also called “atmospheric transport”) moves about 20 W/m2 from within the tropical and subtropical area of 30°N/S to the temperate and polar area outside of 30°N/S http://www.sp.ph.ic.ac.uk/~arnaud/PAPER/Czaja_Marshall_jas06.pdf. Using blackbody calculations for simplicity, from the 20 W/m2 energy transfer the equatorial area cools by three degrees, while the same area at the poles warms by five degrees. And as a result, the average temperature of the two areas warms by a full degree, simply from advection.

So while the authors are entirely correct to say that the net advective energy transports equals zero, the same can not be said about the effect of net advective energy transport on temperature.

However, let’s ignore that. Let’s say that both of those statements are true for the purposes of this analysis. Given those statements, they then say:

… it follows that the global average surface temperature must be determined in full by the radiative fluxes arising from the patterns of temperature and absorption of radiation.

Here’s where we really part company, on two points. First, surface temperature is not “determined in full by the radiative fluxes”. There are also sensible heat fluxes from the surface to and through the atmosphere (conduction/convection, called “Thermals” in Fig. 1) as well as latent heat fluxes (evaporation and transpiration, or “Evapo-transpiration in Fig. 1). Both of these cool the surface without changing the TOA “solar-thermal energy balance of the earth.” Either I don’t understand their conclusion, or I disagree with it. What am I missing?

Second, there is no logical “it follows” path to get from the two statements

“solar in = solar + longwave out”

and

“net advective energy transport = 0”

to their conclusion

“global average surface temperature must be determined in full by the radiative fluxes”.

I cannot think of, and they do not provide, any logical chain of reasoning that connects the third statement to the first two.

So that’s the theoretical problem with the paper. They claim that the surface temperature of the planet is “determined in full by the radiative fluxes”. I say no.

Next, the model problem. They base all of their claims on making very large changes in the variables of the GISSE global climate model. The model problem is that like many other climate models, GISSE has the cloud feedback backwards. The GISSE model says that clouds are a positive feedback. There’s a good study of the question by De-Zheng Sun et al., 2009, Tropical Water Vapor and Cloud Feedbacks in Climate Models: A Further Assessment Using Coupled Simulations, Journal of Climate, 22, 1287–1304 [hereinafter Sun09].

Among other things, Sun09 says:

A more serious concern raised by the study of Sun et al. (2006) is the finding of a common bias in the simulation of the cloud albedo feedback in the leading climate models: with the exception of the GFDL model, all the models they analyzed in that study underestimate the response of cloud albedo to the surface warming.

This finding from Sun 2006 were reconfirmed in Sun09. Here’s an illustration of the problem:

Figure 2. Solar (albedo) cloud feedback (blue bars), cloud longwave (yellow bars), and net cloud feedback (red bars) in models and observations of the equatorial Pacific (5°S-5°N, 150°E-250°E). Net feedback is the sum of the longwave and albedo feedbacks. Period of study 1983-2004. DATA SOURCE Sun09 Table II. See Sun09 notes for Table I and Table II for details on the data.

Note the errors in the modelled albedo feedback (blue bars). In the tropics, solar albedo feedback works as follows. Increasing warmth means increasing clouds. Increasing clouds means more sunlight is reflected into space. This cools the earth, and is a negative feedback.

While most of the of the models at least get the sign of the cloud albedo (solar reflection) feedback correct (more clouds means less sunshine hitting the earth, a negative feedback), the UKMO Hadgem1 and the GISS EH models don’t even get the sign of the albedo feedback correct. The rest of the models underestimate the size of the albedo feedback, with values as low as 16% of the observed cloud albedo feedback.

There are also a very wide range of values for the longwave, some of which are very small compared to the actual observations.

In addition to the albedo and longwave problems, a larger issue is the net cloud feedback (red bars). All but one of the models show positive net cloud feedback. The observations and one model show negative feedback.

Now, the Lacis10 authors are using their model to determine (among other things) what happens in the deep Pacific tropics when the non-condensing GHGs are removed from the atmosphere.

Obviously, the first thing that would happen if GHGs were removed is that the planet would start to cool. The immediate response in the tropics would be that daytime cumulus would decrease. This would allow more sunshine to heat the earth, which would be a negative feedback on the cooling from the lack of GHGs.

In addition, the number of tropical thunderstorms would decrease. This would slow the Equator-to Poles atmospheric transport. Once again, this would warm the earth, and would also be a strong negative feedback on the cooling.

The GISS model, on the other hand, says the opposite. It says that as the Earth cools from the lack of GHGs, the change in clouds would make it cooler yet … and unsurprisingly, it says that the net result would be that the planet would spiral into a permanent snowball. Fig. 3 is a figure from the Lacis10 paper, showing how they think it would evolve:

Figure 3. Lacis10 description (their Fig. 2) of the evolution of GISSE model when non-condensing GHGs (everything but water vapor) is removed.

I find this graph quite odd. Immediately after the GHGs are removed, surface temperature starts to drop. That makes sense. But concurrently, there is a steep increase in clouds, from 59% coverage to 69% coverage in one year. This doesn’t make sense. A warmer world is a wetter world. A warmer world is a world with more moisture in the air, and a world with more rainfall and more clouds. Conversely, a cooler world is a dryer world, with less clouds. What would cause the modelled clouds to increase in coverage as the earth cooled? This may be related to the reversed sign of the GISS albedo feedbacks shown in Fig. 2.

(In addition, the GISS Model E normally shows about 10% less cloud coverage than the real Earth. See Present-Day Atmospheric Simulations Using GISS ModelE, (PDF 2.2 Mb), page 169.)

Finally, Fig. 4 shows the atmospheric transport feedback and the total atmospheric feedback, again from Sun09. This is the net cloud feedback shown in Fig. 2, plus the water vapor feedback and the atmospheric transport feedback. (Water vapor feedback is similar in observations and models, and is not shown.)

Figure 4. As in Fig. 2, for atmospheric transport feedback (blue bars) and total atmospheric feedback (red bars). Total atmospheric feedback is the sum of the feedbacks of water vapor, cloud longwave, cloud shortwave, and atmospheric transport. Fewer models are shown than in Fig. 2, because of lack of data for the remainder. See Sun09 for details.

As with the net cloud and the cloud albedo feedbacks, the atmospheric transport feedback is also underestimated by many models. Atmospheric transport is the movement of energy out of the Equatorial area of the study. This transport of energy out of the area increases as the temperature goes up, so it is a negative feedback. It reduces the size of an expected increase.

And as a result of all of the model underestimations, the net feedback for the observations is much larger than any of the models. And indeed, some of the models go so far as to claim positive feedback in the deep tropics area studied.

So that’s my second problem with the Lacis10 paper. Given the huge variation in the feedbacks of the different models, and given that all but one of them show positive cloud feedback in the tropics, there is absolutely no reason to place the slightest credence in the GISS ModelE results reported in Lacis10. Let me close with this quote from James Hansen, pp 2-3 (bulleting mine):

2.4 Principal Model Deficiencies [of the GISS ModelE climate model]

Model shortcomings include

• ~25% regional deficiency of summer stratus cloud cover off the west coast of the continents with resulting excessive absorption of solar radiation by as much as 50 W/m2

• deficiency in absorbed solar radiation and net radiation over other tropical regions by typically 20 W/m2

• sea level pressure too high by 4-8 hPa in the winter in the Arctic and 2-4 hPa too low in all seasons in the tropics

• ~20% deficiency of rainfall over the Amazon basin

• ~25% deficiency in summer cloud cover in the western United States and central Asia with a corresponding ~5C excessive summer warmth in these regions.

I mean, how could you not trust a model with specs like that?

w.