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Earth's Thermal Sensitivity to a Doubling of Atmospheric CO2
Volume 12, Number 48: 2 December 2009

What change in the mean surface air temperature of the planet would be caused by a doubling of the air's CO2 content? In the IPCC's Fourth Assessment Report, the most likely range for this thermal sensitivity parameter, as determined by numerous climate models, is something in the range of 2 to 4.5C; yet even this significant degree of warming has been suggested by many of the world's climate alarmists to be too small. And as a result, a great hue and cry has been raised by folks such as Al Gore, who is hard at work attempting to convince everyone that they have a moral responsibility to "save the planet" by demanding legislative actions designed to drastically reduce anthropogenic CO2 emissions.

In a recent investigation into the question of the magnitude of earth's thermal sensitivity, however, Lindzen and Choi (2009) concluded that just the opposite is true, i.e., that it is the lower limit of the thermal sensitivity range that is too great. And since so much of importance is riding on the answer to this question - like how the world will produce and transport goods in the near future and provide energy for everything else for which energy is required - it is important that all approaches to the determination of the planet's thermal sensitivity be carefully considered.

In the Lindzen and Choi study, the two Massachusetts Institute of Technology scientists used the National Centers for Environmental Prediction's 16-year (1985-1999) monthly record of sea surface temperature (SST), together with corresponding radiation data from the Earth Radiation Budget Experiment, to estimate the sign and magnitude of climate feedback over the oceanic portion of the tropics and thus obtain an empirical evaluation of earth's thermal sensitivity, as opposed to the model-based evaluation employed by the IPCC.

So what did the two researchers learn?

Lindzen and Choi report that all eleven models employed in the IPCC's analysis "agree as to positive feedback," but they find that they all disagree - and disagree "very sharply" - with the real-world observations that they (Lindzen and Choi) utilized, which imply that negative feedback actually prevails. And the presence of that negative feedback reduces the CO2-induced propensity for warming to the extent that their analysis of the real-world observational data only yields a mean SST increase "of ~0.5C for a doubling of CO2."

So how does one decide which of the two results is the more correct? Real-world data would be the obvious standard against which to compare model-derived results; but since Lindzen and Choi's results are based on real-world measurements, the only alternative we have is to seek other real-world results. And, fortunately, there are several such findings, many of which are summarized in the review paper of Idso (1998), who describes eight "natural experiments" that he personally employed in prior studies designed to determine "how earth's near-surface air temperature responds to surface radiative perturbations."

The eight naturally-occurring phenomena employed by Idso were (1) the change in the air's water vapor content that occurs at Phoenix, Arizona, with the advent of the summer monsoon, (2) the naturally-occurring vertical redistribution of dust that occurs at Phoenix between summer and winter, (3) the annual cycle of surface air temperature that is caused by the annual cycle of solar radiation absorption at the earth's surface, (4) the warming effect of the entire atmosphere caused by its mean flux of thermal radiation to the surface of the earth, (5) the annually-averaged equator-to-pole air temperature gradient that is sustained by the annually-averaged equator-to-pole gradient of total surface-absorbed radiant energy, (6) the mean surface temperatures of Earth, Mars and Venus relative to the amounts of CO2 contained in their respective atmospheres, (7) the paradox of the faint early sun and its implications for earth's thermal history, and (8) the greenhouse effect of water vapor over the tropical oceans and its impact on sea surface temperatures.

These eight analyses, in the words of Idso, "suggest that a 300 to 600 ppm doubling of the atmosphere's CO2 concentration could raise the planet's mean surface air temperature by only about 0.4C," which is right in line with Lindzen and Choi's deduced warming of ~0.5C for a nominal doubling of the air's CO2 content. Hence, there would appear to be a goodly amount of real-world data that argue strongly against the over-inflated CO2-induced global warming that is being predicted by state-of-the-art climate models.

Sherwood, Keith and Craig Idso

Idso, S.B. 1998. CO2-induced global warming: a skeptic's view of potential climate change. Climate Research 10: 69-82.

Lindzen, R.S. and Choi, Y.-S. 2009. On the determination of climate feedbacks from ERBE data. Geophysical Research Letters 36: 10.1029/2009GL039628.