AIRS

Frequently Asked Questions


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Q.  I understand that water vapor is the largest GHG. Do we understand what
causes the fluctuations in water vapor? Has water vapor content been
increasing for the past 50 years?  

A.  es, water vapor is the major GHG, responsible for at least 80% of the
Greenhouse effect. In fact, without it Earth would be an iceball.

We understand the basic physical processes controlling the amount of
water vapor in our atmosphere, but have trouble modeling the complex
system that is the Earth's atmosphere and its interaction with ocean
and (especially) land. The models get the vertical distribution of
water vapor wrong as well. It is an area of intense ongoing research.

Your question about the trend in water vapor content is right on the
money. We are attempting to determine that trend, and in particular
what the trend is in terms of the vertical distribution.

The action of water vapor as a GHG is more complex than CO2. CO2
remains in a gaseous state, whereas water changes among vapor, liquid
and solid. Clouds are very important and very poorly modeled. If the
trend is increasing water vapor (and thus clouds) in the lower
troposphere, the effect is a reverse Greenhouse-low clouds are
efficient radiators of energy back into space. If the trend is
increasing water (ice crystals) in the upper troposphere (cirrus
clouds), the effect is to enhance the Greenhouse - high (and thus
cold) clouds are inefficient in radiating energy to space but are good
at trapping the IR radiation coming up from below.

So, these are areas in which the AIRS data are contributing to
improving the modeling of the Earth system, and thus forcasting of the
future climate. The coupled climate models have a long way to go yet,
and we are providing the data to test, correct and validate them.

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