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Feature: How do they get a lot of warming from a little gas?
July 28, 2009

Saturation chamber experiments with carbon dioxide tell us that, all other things being equal, doubling pre-Industrial Revolution CO2 levels should yield about 1.2 °C warming -- not radical physics and not usually an argument starter.

Worries about catastrophic global warming assume a positive feedback from water vapor. In skeptic circles these positive feedback numbers are known as "marvelous magical multipliers". Bear in mind positive feedback from water vapor is not entirely implausible since a warmer atmosphere can certainly support more water vapor and generally warmer temperatures could increase both evaporation and plant transpiration leading to more available water vapor.

The situation is complicated by how any additional water vapor might form clouds and the efficiency with which the atmosphere limits water vapor levels through precipitation -- what goes up eventually comes down as rain, snow, etc.. Moreover, the type of clouds formed, their latitude and their altitude determine whether these clouds primarily affect incoming solar shortwave radiation or outgoing earth longwave radiation or some combination thereof. We've shown you before how a simple 1% change in albedo (Earth's reflectivity) can yield over 1 °C change in expected mean temperature.

The IPCC gives a range of warming estimates. You will regularly see the "median estimate" of 3 °C for a doubling of atmospheric carbon dioxide (2xCO2). So how do they get that 3 °C from an assayed doubling value of 1.2 °C? Easy, they do it with the water vapor feedback we mentioned earlier and it works like this:

Feedback (F) = 1/(1-f) where f is the fraction of the initial input fed back as an additional input in the first round of a recursive process.


OK, positive feedback means that if we put 1 unit of (in this case) heat into something (the atmosphere) and we have 60% feedback (0.6 = 60%) then we get 1/(1-0.6) = 2.5 units' effect.

Using the agreed 1.2 °C for 2xCO2 and the IPCC's favorite guess of 60% positive water vapor feedback yields 1.2/(1-0.6) = 3 °C for 2xCO2. Don't worry, we'll put a form in for you to play with various feedback levels, both positive and negative, so you can see their effect.

With climate models returning a range from 1.5 °C to 6 °C warming for the varied scenarios, the choice of feedback factor is obviously important and, as you get adventurous with the form below, you'll see just how important. That range is equivalent to 2xCO2 with anything from 20% to 80% positive feedback, with the IPCC's favorite figure equal to 60%. There are no real-world observations to support this or any other positive numbers.

Go ahead, try a few numbers, we'll continue below the form when you are ready.

Response from 2xCO2 with varied feedback

To use the form below simply alter the estimated feedback factor and click "Recalculate". Valid range is from -99.99% to 99.99%. The results may surprise you.

Estimated feedback factor (%)
Resulting mean global surface temperature increase (C)
Resulting mean global surface temperature increase (F)

What is really happening? Is Earth responding to enhanced greenhouse with positive feedback, no feedback or negative feedback? How do we tell whether any of these guesses are more reasonable than others?

Actually there are a few ways.

Firstly, by checking the IPCC's own forcing estimates we see that from all guesses of anthropogenic influences (land clearance, soot, various greenhouse gases...), coupled with a small increase in solar output, variously attributed but thought to account for 20%-40% of 20th Century warming, we should already have experienced warming at least the equivalent of doubling atmospheric carbon dioxide. The IPCC suggests net warming to 2005 of about 0.75 °C but we have not seen the 1.2 °C warming equivalent of 2xCO2 in the absence of any feedbacks at all. This suggests that if there are any feedbacks then they are negative in sign since the warming is smaller than expected without feedback.

Secondly, Earth warms almost 4 °C from January to July each year (due to continental configuration and axial tilt) so we should see water vapor feedback and enhanced greenhouse sufficient to delay the equivalent cooling from July to January with net year on year warming. Eventually this should eliminate freezing winters altogether, even if over a few thousand years (obviously this has not occurred in the current interglacial). Nor does a "super El Niño event" superimposed over this annual effect cause any such sustained feedback (we've looked at this before).

Greenhouse hysterics and wannabe world governors claim water vapor feedback is strongly positive while empirical measures of the world around us demonstrate any feedback effect is actually negative. With currently available measures the 60% proportion appears to be correct but the IPCC has the sign of the feedback wrong. Go ahead, try the negative figure (-60.0) in the form above. Surprise! Suddenly the books balance and we have a representation of how the world is actually working, complete with the numbers we believe we have actually measured.

Let's try another form, this time with an added variable to adjust atmospheric carbon dioxide levels. Valid range is from 280 to 2800 ppmv (from pre-Industrial to 10 times that level of atmospheric CO2 and more than 5 times current levels). Professor Roger Pielke Sr. has previously examined What Fraction of Global Warming is Due to the Radiative Forcing of Increased Atmospheric Concentrations of CO2? and we certainly have no reason to argue with his breakdown of IPCC's numbers. We believe a negative feedback estimate of 60% (-60%) or more yields a fair facsimile of warming attributable to increased atmospheric carbon dioxide and is in general agreement with Pielke's calculations. We see no reason to assume it will not continue to do so in the foreseeable future.

Try this form and see how little effect eliminating carbon dioxide emissions can really have.

Possible response from varied CO2 levels only.

Atmospheric CO2 (ppmv)
Estimated feedback factor (%)
Resulting mean global surface temperature increase (C)
Resulting mean global surface temperature increase (F)

We can succumb to superstition, race to abandon modernity, ration energy and limit human endeavor or we can stop shouting to try to frighten away the thunder storm, continue human development and lift billions more out of poverty and privation.

Not so long ago we shot for the moon and knew we could do anything. Now some would have us shoot ourselves in the energy supply and do nothing.

Are we really prepared to flush the global economy and throw the world's poorest under a bus on the strength of marvelous magical multipliers whose existence remain hypothetical and whose effect cannot even be observed in the real world? Or should we rather concentrate on providing affordable electricity to the developing world?

That's the choice the Western world is struggling with now. How would you like your Senators to vote?


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