Data Show Water Vapor Amplifies Greenhouse Effect
BOSTON — DATA from Earth-scanning satellites have confirmed a key factor in the so-called atmospheric greenhouse theory - the ability of water vapor to amplify a man-made global warming. Computer climate simulations project a warming of several degrees Celsius over the next century if heat-trapping gases, such as carbon dioxide from burning fossil fuels, continue to accumulate in the air. The underlying theory features a reinforcing feedback by water vapor, which is also an efficient heat trapper. As the sea surface and lower atmosphere warm, the air can hold more water vapor and more of it evaporates from the sea. This, in turn, reinforces the warming, and so forth.
Prof. Veerabhadran Ramanathan and his student, Ameet Raval, of the University of Chicago have looked for this effect in data taken by the National Aeronautics and Space Administration's Earth Radiation Budget Experiment (ERBE). This is a three-satellite system that measures our planet's energy income from the sun and the energy it radiates back into space. These data show that the amplification works as expected.
``The rate of increase gives compelling evidence for the positive feedback between surface temperature, water vapor, and the greenhouse effect: The magnitude of the feedback is consistent with that predicted by [computer] climate models,'' the Chicago scientists say.
They also find hints of what Dr. Ramanathan calls a ``super-greenhouse effect.''
This phenomenon involves a threshold temperature beyond which the feedback process really takes off. Its greenhouse enhancement rises linearly with surface temperature until that temperature reaches about 25 degrees C (77 degrees F). Then the amplification begins to increase more rapidly.
If this is an inherent part of the water vapor feedback process, it would mean that a significant effect has been overlooked in the greenhouse theory. However, the two scientists have so far seen this only in the western tropical Pacific Ocean. They caution that, right now, they do not know whether this effect has general significance or is ``simply a climatological feature of low latitudes, unrelated to temperature variations.''
Whether further study confirms such a threshold phenomenon or not, the present research shows that the water vapor feedback, in general, does work as theory specifies. This is comforting to the modelers whose simulations are troubled with major uncertainties. They cannot, for example, take adequate account of changes in cloudiness or of the ocean's response to greenhouse warming. Commenting on the Chicago work, Robert Cess of State University of New York at Stony Brook, who has compared the simulation abilities of 14 different computer climate models, observes: ``With all the uncertainties in our knowledge of climate processes, ... it is gratifying that at least one such process has a simple explanation.''
Dr. Cess's comment accompanies a paper by Raval and Ramanathan in the current issue of Nature. This follows a report Raval gave at the fall meeting of the American Geophysical Union in San Francisco earlier this month.
Briefly put, the greenhouse effect is Earth's way of balancing its energy budget. In the long run, the planet radiates back to space the solar energy it absorbs.
The surface temperature needed to strike this balance would be too low to support life if there were no atmospheric gases that absorb the outgoing infrared (heat) radiation.
However, naturally occurring water vapor, carbon dioxide, and certain trace gases such as methane are infrared absorbers. They allow the surface to be relatively warm and, thus, to radiate more heat than is needed to balance the energy budget. Meanwhile, the upper air remains cold and radiates into space only the smaller amount of energy the budget requires.
The difference between the energy radiated by the surface and the energy that finally makes it into space is trapped in the atmosphere. This is the greenhouse effect. If man-made pollution adds more carbon dioxide, the theory predicts that the climate system will readjust with a warmer surface temperature and more heat trapped in the air.
The ERBE system keeps track of the energy budget and, hence, of the greenhouse effect. Ramanathan and Raval used ERBE data for April, July, and October of 1985 and January of 1986 to study how that effect varies from the equator to the poles and in several seasons over the ocean. These are the data that show that the theoretical water vapor feedback actually does enhance greenhouse warming.