Why 33 deg. C for the Earth’s Greenhouse Effect is Misleading

September 13th, 2010 by Roy W. Spencer, Ph. D.

In my previous post I argued, using commonly cited numbers, that the greenhouse effect enhancement of adding carbon dioxide to the atmosphere would be about 3% for a doubling of CO2 (“2XCO2”).

The 3% enhancement is based upon 2 commonly quoted numbers: (1) 33 deg. C global-average surface warming for the natural greenhouse effect, and (2) about 1 deg. C additional surface warming from 2XCO2, without feedbacks. (Interestingly, these numbers can only be computed from theory, which always requires a variety of assumptions.)

The value of 33 deg. C represents the difference between the observed average surface temperature of the Earth, and the estimated surface temperature if there was no atmosphere.

I explained that the 3% statistic is the one we should be dealing with conceptually, rather than what some people seem to be interested in, which is what portion of the Earth’s greenhouse effect is due to CO2. I argued that the answer to that question, which has been recently addressed in a new paper by Schmidt et al., really tells us very little regarding the impact of adding more CO2 to the atmosphere.

But what many people don’t realize is that the 33 deg. C of surface warming is not actually a measure of the greenhouse warming – it represents the balance between TWO competing effects: a greenhouse warming effect of about 60 deg. C (the so-called “pure radiative equilibrium” case), and a convective cooling effect of about 30 deg. C. When these two are combined, we get the real-world observed “radiative-convective equilibrium” case.

This has been known since at least 1964 (Manabe and Strickler, 1964). It was also discussed in Dick Lindzen’s 1990 paper, Some Coolness Regarding Global Warming, which is when I became aware of its significance.

Why is this Important?

When global warming is discussed, the warming effect of greenhouse gases is obviously of prime interest. But it is seldom if ever mentioned that about 50% of the surface warming influence of greenhouse gases has been short-circuited by the cooling effects of weather, as just discussed.

When Danny Braswell and I did similar calculations in 1997 to better understand the physics, we found that 1 deg. C of surface warming was true even for the pure radiative equilibrium case (no convective cooling by weather processes). This would mean that the REAL enhancement of the greenhouse effect with 2XCO2 is really only about 1.5%, not 3%, since the natural greenhouse effect is trying to warm the surface by over 60 deg. C, not by 33 deg. C.

Is this Simple Evidence of Negative Feedback?

These climate basics, which have been known since the 1960s, also raises an intriguing question: If the surface warming effect of 2XCO2 before surface cooling by convection is 1 deg. C, and (as even the IPCC knows) 50% of that natural greenhouse warming is then short-circuited by convection, might this then tell us that negative feedbacks in the climate system can be expected to reduce anthropogenic global warming to only 0.5 deg. C?

I believe this is entirely possible.

How could this happen, since there is so much evidence that water vapor feedback is positive? Because, even if water vapor feedback is positive, an increase in the solar shading effect of clouds (negative cloud feedback) could more than overwhelm the positive water vapor feedback, leading to little net warming.

The IPCC already admits feedbacks due to low clouds are the least understood. Indeed, the evidence presented in Spencer and Braswell (2010), at face value, would suggest this could happen.

We already know that the net effect of clouds is to cool the climate system in response to solar heating. Are we to believe that cloud changes turn into a warming influence when temperatures get a little bit higher? Well, that’s what all of the IPCC coupled climate models do now.

The Importance of Convective Cooling Versus Greenhouse Warming

I sometimes get e-mails asking why I don’t mention convection as a cooling mechanism in the context of global warming. Folks, I used to be virtually the only one speaking out on the subject. For years I harped on this issue.

The reason why I have been recently defending the basic physics of the greenhouse effect is because I think the credibility of those who claim that the greenhouse effect of the atmosphere cannot be increased (or doesn’t even exist) is compromised when they object to something that – as far as I have seen – has no alternative explanation.

I’m always upon open to new theories, but as I have said before, until someone puts their alternative physics into an energy-conserving model of the vertical temperature profile of the atmosphere, which then produces the present-day temperature profile as current models do, it is little more than hand-waving.

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