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Global Warming and Soil Moisture Trends
Robock, A., Vinnikov, K.Y., Srinivasan, G., Entin, J.K., Hollinger, S.E., Speranskaya, N.A., Liu, S. and Namkhai, A.  2000.  The global soil moisture data bank.  Bulletin of the American Meteorological Society 81: 1281-1299.

What was done
The authors developed a massive collection of soil moisture data sets for over 600 stations from a large variety of climatic regimes, including the former Soviet Union, China, Mongolia, India and the United States.  They describe the datasets and how they were obtained, display examples of the data, and present trends of summer soil moisture from the stations with the longest records.

What was learned
As an introduction to their work, the authors state an important "ground rule."  Sometimes, they say, "the word 'data' is used to describe output from theoretical model calculations, or values derived from theoretical analysis of radiances from remote sensing."  However, as they put it, "we prefer to reserve this word for actual physical observations," noting that "all the data in our data bank are actual in situ observations."

This distinction is important, for one of the illuminating analyses the authors perform with their data is to check the summer soil moisture trends simulated by the Geophysical Fluid Dynamics Laboratory's general circulation model of the earth's climate as forced by transient CO2 and tropospheric sulfate aerosols for specific periods and regions for which the authors have actual soil moisture data.  What they found was that "although this model predicts summer desiccation in the next century, it does not in general reproduce the observed upward trends in soil moisture very well," which is a mammoth understatement, considering that the predictions and observations in this case go in opposite directions!

What it means
Sometimes the predictions of sophisticated global climate models are treated with a reverence as great as - or even greater than! - actual real-world data.  This paper is a classic in demonstrating the dangers inherent in such behavior, which we all know can have such impact as to even lead to the creation of international treaties of wrenching economic and social implications.  Specifically, the authors note that "in contrast to predictions of summer desiccation with increasing temperatures, for the stations with the longest records, summer soil moisture in the top 1 m has increased while temperatures have risen."

The moral of this story is that when model predictions and actual measurements fail to coincide, or actually diverge, as in this study, the data must rule!  Indeed, it is the authors' fond hope that the real-world data they have assembled in their data bank will help "improve simulations of the recent past so we may have more confidence in predictions for the next century," which is our hope too.

Reviewed 15 June 2000