Increased Tropical Rainfall Caused By Global Warming

WASHINGTON - 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," said 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," said co-author Robert F. 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 Ni?o-Southern Oscillation and large
volcanic eruptions.

El Ni?o is a cyclical warming of the ocean waters in the central and
eastern tropical Pacific that generally occurs every three to seven
years and alters weather patterns worldwide. Volcanoes that loft
debris into the upper troposphere and stratosphere create
globe-circling bands of aerosol particles that slow the formation of
precipitation by increasing the number of small cloud drops and
temporarily shielding the planet from sunlight. The result lowers
surface temperatures and evaporation that fuels rainfall. Two such
eruptions - El Chicon in Mexico and Mount Pinatubo in the Philippines
- occurred during the 27-year period.

The scientists found that during El Ni?o years, total tropical
rainfall did not change significantly, but more rain fell over oceans
than usual. During the two years following each volcanic eruption,
overall tropical rainfall was reduced by about 5 percent. With these
effects removed from the rainfall record, the long-term trend appears
more clearly in the rainfall data both over land and over the ocean.

According to Adler, evidence for the rainfall trend is holding as more
data come in. The latest numbers for 2006 show another record-high
year for tropical rainfall, tying 2005 as the rainiest year. Adler's
research group at NASA produces the Global Precipitation Climatology
Project's monthly rainfall updates.

"The next step toward firmly establishing this initial indication of a
long-term tropical rainfall trend is to continue to lengthen and
improve our data record," said Adler, who is project scientist of the
Tropical Rainfall Measuring Mission (TRMM), a joint effort between
NASA and the Japan Aerospace Exploration Agency. The satellite's
three primary instruments are providing the most detailed view of
rainfall ever provided from space. Since 1997, Adler's group has been
incorporating the mission's rainfall data into the global rainfall
record.

NASA plans to extend the success of monitoring rainfall over the
tropics to the entire globe with the Global Precipitation Measurement
mission, scheduled for launch in 2013. This international project
will measure both rain and snow around the world.

For related images and more information about this story, visit:

http://www.nasa.gov/centers/goddard/news/topstory/2007/rainfall_increase.html