Atmospheric CO2: Climate's 'Control Knob'.

If water vapor and clouds account for roughly 75 percent of the greenhouse effect in the atmosphere, then why all of the fuss about carbon dioxide? New experiments by researchers at NASA's Goddard Institute for Space Studies in New York produce a dramatic answer to this question.

Andrew Lacis and colleagues added and subtracted each of the greenhouse gases from the atmosphere of their best climate model -- the water vapor and clouds that condense, evaporate and precipitate from the atmosphere with the rise and fall of temperatures, as well as the "noncondensing" greenhouse gases, mainly CO2, which seems to stay up there practically forever.

Sure enough, they report in the journal Science, the water vapor accounts for 50 percent and clouds about 25 percent of the warming feedback that keeps things cozy for life on Earth -- but look what happens when the non-condensing gases are taken away:

"The results…show unequivocally that the radiative forcing by noncondensing GHGs is essential to sustain the atmospheric temperatures that are needed for significant levels of water vapor and cloud feedback. Without this noncondensable GHG forcing, the physics of this model send the climate of Earth plunging rapidly and irrevocably to an icebound state, though perhaps not to total ocean freezeover."

This illustration, courtesy of Science, of zonally averaged surface temperature change after removing CO2 and other noncondensing greenhouse gases, shows how, within 10 years, polar cold engulfs the planet, leaving only a diminishing sliver of relative warmth near the equator.

Of all the greenhouse gases, they write, CO2 is the "control knob" of climate change. In the vernacular, CO2 is the "forcing" that keeps the atmosphere in a tolerable temperature range.

To these climate scientists, the implications of these experiments to a man-made environment of increasing carbon dioxide concentrations are obvious:

"The continuing high rate of atmospheric CO2 increase is particularly worrisome, because the present CO2 level of 390 ppm (parts per million) is far in excess of the 280 ppm that is more typical for the interglacial maximum, and still the atmospheric CO2 knob is now being turned faster than at any time in the geological record."

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