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Raining on the Global Warming Parade
By Roy W. Spencer


There are many remaining scientific uncertainties that limit our ability to predict how much global warming can be expected due to mankind's use of fossil fuels. The largest uncertainties are related to feedbacks. Feedbacks describe how various elements of the climate system respond to an initial warming tendency, and possibly change it. This warming tendency is caused by increased trapping of infrared radiation in the atmosphere from increasing concentrations of carbon dioxide.

Feedbacks can either be positive (amplifying the warming), or negative (reducing the warming), and exert a potentially huge influence. In theory at least, they make the difference between benign warming (say, 1 deg F), or strong warming (say, 7 deg. F) over the next 100 years. Most feedbacks involve water in one form or another. Water vapor, clouds, snow cover, sea ice, and ocean circulations are the major players usually researched.

I believe the largest uncertainty, though, is one that receives little attention -- maybe because we know so little about it. That is precipitation, the only process that removes water vapor (Earth's major greenhouse gas) from the atmosphere. Since water vapor accounts for 80%-90% of the Earth's natural greenhouse effect, it is critical to understand what processes determine its equilibrium value in the atmosphere and how they might change with warming.

All of the water vapor that is being continuously evaporated from the Earth's surface must eventually return to the surface as precipitation. The climate system strikes a balance, allowing only so much water vapor to accumulate before it is depleted by either rain or snow. The term used to describe this self-limiting process is "precipitation efficiency," which is a measure of how readily precipitation processes in clouds convert cloud water into droplets large enough to fall to the surface. Theoretical research has shown that for a given amount of sunlight, high precipitation efficiency leads to cool, dry climates and low precipitation efficiency leads to warm, moist climates [1].

All of the dozen or so leading computerized climate models increase the amount of water vapor in the atmosphere, significantly resulting in a doubling of the warming that might be expected due to increased carbon dioxide alone. The absolute water vapor increase is such that the relative humidity remains about constant with warming. This is strongly positive water vapor feedback. But how could these models have any credibility on this issue unless they contain knowledge of how precipitation might change with warming? I believe that they can't.

What does the current climate system tell us about this issue? In the tropics, where more sunlight causes warmer conditions, there is indeed more water vapor in the lowest part of the atmosphere (the "boundary layer") than there is outside of the tropics. By itself, this suggests a positive water vapor feedback. But above the boundary layer, the tropical free troposphere has only slightly more water vapor, and much lower relative humidity, than at high latitudes. This wasn't widely realized until Earth-orbiting satellites gave us a global view of the relative humidity field, revealing large regions with RH below 10% in the tropics [2]. And it is this dry, free-tropospheric air that allows huge amounts of Earth-cooling infrared radiation to escape more readily to space. Deep tropical rain systems are apparently more efficient at keeping the free-tropospheric vapor at very low levels, even though the relative humidity near the surface remains nearly the same as that at high latitudes.

The processes which control precipitation efficiency are not well understood. On a theoretical level, we still don't even understand what initiates rain formation in clouds. We do know that precipitation proceeds by larger drops falling faster and growing by collecting smaller, more slowly-falling cloud droplets. But we don't know how the small and large droplets got close together in the first place. Turbulence within clouds probably plays some role.

Unlike the old adage that two wrongs don't make a right, climate models contain many processes that are probably wrong (or non-existent), but are tuned to get the right average climate conditions. So models can be tuned to reproduce the very dry tropical free-tropospheric seen in nature. But this doesn't mean that they contain information on how precipitation efficiency changes with temperature. We need to know how these precipitation processes in clouds change with warming, not just their average values, before we can have much confidence in global warming predictions. And as Joni Mitchell sang in her hit "Both Sides Now", we still "really don't know clouds at all."
 

  1. Renno, N.O., K.A. Emanuel, and P.H. Stone, 1994: Radiative-convective model with an explicit hydrologic cycle, 1: Formulation and sensitivity to model parameters. J. Geophys. Res., 99, 14,429-14,441.
  2. Spencer, R.W., and W.D. Braswell, 1997: How dry is the tropical free troposphere? Implications for global warming theory. Bull. Amer. Meteor. Soc., 78, 1097-1106.
Roy Spencer is a principal research scientist for University of Alabama in Huntsville. In the past, he was served as Senior Scientist for Climate Studies at NASA's Marshall Space Flight Center in Huntsville, Alabama. Dr. Spencer is the recipient of NASA's Medal for Exceptional Scientific Achievement and the American Meteorological Society's Special Award for his satellite-based temperature monitoring work. He is the author of numerous scientific articles that have appea-red in Science, Nature, Journal of Climate, Monthly Weather Review, Journal of Atmospheric and Oceanic Technology, Journal of Climate and Applied Meteorology, Remote Sensing Reviews, Advances in Space Research, and Climatic Change.



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