More Rain in the Forecast

By Randy Showstack at "In Brief" in EOS (Vol. 78, Num. 48, P550. Dec. 2, 1997)

Global land precipitation has increased during the 20th century, especially at the mid and high latitudes, according to a paper published in the November 1997 issue of the Journal of Climate.

The paper, written by NASA scientists Aiguo Dai, Inez Fung, and Anthony Del Genio, is based on a recalibrated compilation and analysis of data from 1900-1988. The paper shows a global land trend of a 2.4 mm per decade increase in annual precipitation amounts, and that rainfall as a global mean has risen by slightly more than 2%. About 22 mm more rain falls now each year than at the turn of the century, the researchers say.

The research shows that "both the spatial pattern and rate of precipitation increase are reminiscent of global climate model predictions of the atmosphere's response to an increase in greenhouse gas concentrations," says Del Genio, research scientist at NASA's Goddard Institute for Space Studies (GISS) in New York City. He says that further study is needed to ascertain the reasons for the change. "This database represents a potentially valuable resource for understanding the nature of land precipitation variations and their role in climate processes," said Dai, a researcher at GISS.

The research analysis confirms the global patterns of the presence of an El Nino and also Sahelian drought that has been a major influence over parts of Africa for the past several decades.

Through the construction of this historical database, researchers also confirmed the occurrences of 24 droughts and five floods world-wide in the 20th century.

Scientists have long held that precipitation is one of the most important aspects in Earth's climate system because of its impact on the global biosphere. In addition, precipitation limits the amount of water vapor in the atmosphere, which helps to determine Earth's surface temperature, and when and where clouds form.

Tracking precipitation and its relationship to global climate has been difficult because, until recently, the data have not been recorded into coordinated databases. In addition, precipitation measurements vary widely across small geographic areas and are difficult to measure accurately. NASA and researchers at GISS hope to create more accurate models from which conclusions about global rainfall can be drawn.

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