November 18, 1997
The paper, written by scientists Drs. Inez Fung, Anthony Del Genio, and Aiguo Dai, is based on a recalibrated compilation and analysis of data from 1900-1988 and confirms previous speculation that land precipitation is increasing. The new research shows a global land trend of a 2.4 mm per decade increase in annual precipitation amounts. Multiplied by almost nine decades, this means that there is about 22 mm more rain falling now each year than there was at the turn of the century -- rainfall as a global mean has risen by slightly more than two percent.
"Though much speculation remains as to the cause of this increase, further long-term study is needed to help ascertain the reasons for this change. The research does show, however, 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," said Dr. Anthony Del Genio, research scientist at NASA's Goddard Institute for Space Studies (GISS), New York City.
NASA scientists learned of this rise in precipitation from a new data set constructed at GISS. Scientists analyzed the data at face value but, in the process, used mathematical techniques to detect patterns of historical errors. Researchers developed new ways to objectively determine variations and remove ones that are not accurate. Finally, they performed a statistical test to determine the numerical confidence and came up with the revised database, which they believe shows the long term changes more precisely than previous analyses of the data.
"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, a branch of NASA's Goddard Space Flight Center, Greenbelt, MD.
For example, the research analysis confirms the global patterns of the presence of an El Nino and also depicts the well- known Sahelian drought of the past few decades that has been a major influence over parts of Africa.
Through the construction of this historical database, researchers confirmed the occurrences of 24 droughts and five floods world-wide in the 20th century, though most took place in the tropics rather than middle latitudes.
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. Water vapor is the most important greenhouse gas and thus helps to determine Earth's surface temperature. The amount of water vapor in the atmosphere determines when and where clouds form.
"The latent heat released when water precipitates out of the atmosphere is the primary way in which the tropical ocean 'communicates' with the atmosphere and drives the tropical atmospheric circulation," said Del Genio.
Tracking precipitation and its relationship to global climate, however, has been difficult because the data have not been recorded into coordinated databases until recently. In addition, precipitation measurements vary widely across small geographic areas making it difficult to measure accurately. Through further study, NASA and researchers at GISS hope to create more accurate models from which conclusions about global rainfall can be drawn.
NASA also is due to launch this month the Tropical Rainfall Measuring Mission (TRMM), the first mission dedicated to measuring tropical and subtropical rainfall through microwave and visible infrared sensors. The TRMM mission includes the first spaceborne rain radar. Tropical rainfall comprises more than two-thirds of global rainfall. Data from the TRMM mission should greatly enhance researchers' understanding and prediction abilities of global climate change.
The ongoing research at GISS is funded through NASA's Mission to Planet Earth Enterprise, a long-term coordinated research effort to study the total Earth system and the effects of natural and human-induced changes on the global environment.
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