In a groundbreaking study that fits global warming models, NASA says that its scientists have detected the first signs that rainfall in the tropics is on the rise.

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

Using data from satellite and ground-based instruments, the researchers found that most of the rainiest years between 1979 and 2005 were after 2001. The wettest 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.

"A warming climate is the most plausible cause of this observed trend in tropical rainfall," said co-author Robert Adler, senior scientist at Goddard's Laboratory for Atmospheres.

The latest numbers for 2006 show another record-high year for tropical rainfall, tying 2005 as the rainiest year, Adler said.

Climate models show that a warming trend would increase tropical rain via evaporation of water from the ocean and land.

Adler and Gu are now studying the relationship between surface temperatures and rainfall patterns to investigate the possible link further.

The research team noted that because they weren't certain that the rise was a long-term trend rather than natural year-to-year variability, they separated from the data the effects of two well-known factors: the El Nino ocean pattern and large volcanic eruptions.

El Nino is a cyclical warming of Pacific Ocean waters, while volcanoes loft debris that slow rainfall formation.

With these effects removed from the rainfall record, the long-term trend appeared more clearly in the rainfall data both over land and over the ocean, the researchers found.