“The rainiest year was 2005, followed by 2004,
1998, 2003 and 2002, respectively.”

“The rainfall increase was concentrated over
tropical oceans, with a slight decline over land.”

NASA/Goddard Space Flight Center
Public release date: 27-Aug-2007

Contact: Lynn Chandler

Long-term increase in rainfall seen in tropics

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

Using a 27-year-long global record of rainfall
assembled by the international scientific
community from satellite and ground-based
instruments, the scientists found that the
rainiest years in the tropics between 1979 and
2005 were mainly since 2001. The rainiest year
was 2005, followed by 2004, 1998, 2003 and 2002,

“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 Goddard
Space Flight Center in Greenbelt, Md. The
rainfall increase was concentrated over tropical
oceans, with a slight decline over land.

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

“A warming climate is the most plausible cause of
this observed trend in tropical rainfall,” says
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 further investigate the
possible link. The study appears in the Aug. 1,
2007, issue of the American Meteorological
Society’s Journal of Climate.

Obtaining a global view of our planet’s rainfall
patterns is a challenging work-in-progress. 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. Just recently have the many land- and
space-based data been merged into a single global
record endorsed by the international scientific
community: the Global Precipitation Climatology
Project, sponsored by the World Climate Research
Program. Adler’s research group at NASA produces
the project’s monthly rainfall updates, which are
available to scientists worldwide.

Using this global record, Gu, Adler and their
colleagues identified a small upward trend in
overall tropical rainfall since 1979, but their
confidence was not high that this was an actual
long-term trend rather than natural year-to-year
variability. So they took another look at the
record and removed the effects of the two major
natural phenomena that change rainfall: the El
Niño-Southern Oscillation and large volcanic

El Niño 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, which 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 Niño years,
total tropical rainfall did not change
significantly but more rain fell over oceans than
usual. The two major volcanoes both reduced
overall tropical rainfall by about 5 percent
during the two years following each eruption.
With these effects removed from the rainfall
record, the long-term trend appears more clearly
in both the rainfall data over land and over the

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 during the period.

“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,” says Adler, who is
project scientist of the Tropical Rainfall
Measuring Mission (TRMM), a joint mission between
NASA and the Japan Aerospace Exploration Agency.
The three primary instruments on TRMM are
currently providing the most detailed view of
rainfall ever provided from space. Adler’s group
has been incorporating TRMM rainfall data since
1997 into the global rainfall record.

NASA plans to extend TRMM’s 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 provide measurements
of both rain and snow around the world with
instruments on a constellation of spacecraft
flying in different orbits.


Written By: Steve Cole Goddard Space Flight Center

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