Monday, January 11, 2010

5. Water Vapor Feedback

There is no doubt that the greenhouse effect due to trace gases in the atmosphere is essential to our relatively benign climate. Without them, the average temperature of the Earth would be below freezing. In the troposphere (i.e., the lower part of the atmosphere, up to about 12 kilometers (40,000 feet) at temperate latitudes), most of the warming is due to water vapor, whose concentration is beyond human control because of evaporation from the oceans. Calculations indicate that CO2 contributes about 5% of the overall effect, with smaller contributions from other trace gases (e.g., methane).

Popular discussions of climate change rarely mention that the theoretical direct effect of the increase in CO2 (and other trace gases) is too small to explain the past or projected warming. All of the complex computer models that the Intergovernmental Panel on Climate Change (IPCC ) uses to predict anthropogenic global warming (AGW) depend on positive water vapor feedback – i.e., on the assumption that when an increment in CO2 warms the sea surface, the resulting evaporation of water leads to further warming. The calculated result is an increase in the global warming due to CO2 by a factor that is variously estimated as between 2 and 4.

The uncertainty in this calculation is primarily due to the effects of clouds, which are poorly represented in the models because current understanding is insufficient to predict their behavior. The actual variations in the global cloud cover have however been under detailed observation since NASA launched the Aqua satellite in 2002. According to Roy Spencer, the U.S. Science Team leader for the Advanced Microwave Scanning Radiometer on Aqua, the unsurprising reality is that increasing humidity over the oceans leads to growth of low clouds that reflect sunlight back into space. The result is that water vapor feedback is negative rather than positive, reducing rather than increasing the effect of CO2 alone. This conclusion has been confirmed and quantified in a recent paper in Geophysical Research Letters, using data from the Earth Radiation Budget Experiment that has been flying on several NASA and NOAA satellites since 1984. 

What this means is that clouds are the principal thermostat that maintains a reasonably stable climate. When the temperature increases, water vapor from the oceans forms clouds that reduce the heat input from the sun, offsetting the increase. The set point of the thermostat can of course change -- for example,  when an increase on galactic cosmic rays due to a quiet sun enhances cloud formation (for more information, see #7 in this series of posts, entitled "The Ominously Quiet Sun.")

One implication is that, even if the IPCC's projections about the CO2 rise in the most extreme case are correct (rapid global economic growth and continued reliance on fossil fuels), the temperature increase will be less than 0.4°C, an order of magnitude less than projected. If this is true, global warming driven by CO2 is not a serious problem, whether or not the emissions are anthropogenic.

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