Scientists See First Signs of Long-Term Changes in Tropical Rainfall

NASA scientists have detected the first signs that tropical rainfall is on the rise, using the
longest and most complete data record available.

The international scientific community assembled a 27-year global record of rainfall from satellite and ground-based instruments. The researchers found the rainiest years between 1979 and 2005 occurred primarily after 2001. The wettest year was 2005, followed by 2004, 2003,
2002 and 1998. The study appeared in the August 1 issue of the American Meteorological Society's Journal of Climate. The rainfall increase was concentrated over tropical oceans, with a slight decline over land.

"When we look at the whole planet over almost three decades, the total amount of rain falling has changed very little. But in the tropics, where nearly two-thirds of all rain falls, there has been an increase of 5 percent," says lead author Guojun Gu, a research scientist at NASA's
Goddard Space Flight Center in Greenbelt, Md.

Climate scientists predict that a warming trend in Earth's atmosphere and surface temperatures would produce an accelerated recycling of water between land, sea and air. Warmer temperatures increase the evaporation of water from the ocean and land and allow air to hold more moisture. Eventually, clouds form that produce rain and snow.

"A warming climate is the most plausible cause of this observed trend in tropical rainfall," says co-author Robert Adler, senior scientist at Goddard's Laboratory for Atmospheres. Adler and Gu are now working on a detailed study of the relationship between surface temperatures and rainfall patterns to investigate the possible link further.

Obtaining a global view of our planet's rainfall patterns is a challenge. Only since the satellite era have regular estimates of rainfall over oceans been available to supplement the long-term, but land-limited record from rain gauges. Recently, the many different land- and space-based
data have been merged into a global record: the Global Precipitation Climatology Project, organized under the World Climate Research Program.

Using this global record, the scientists identified a small upward trend in overall tropical rainfall since 1979. To assess whether this pattern was a long-term trend rather than natural year-to-year variability, they removed the effects of the two natural phenomena that change rainfall:
the El Nino-Southern Oscillation and large volcanic eruptions.

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