Global warming likely to intensify tropical rainfall

 Global warming is likely to intensify tropical precipitation such as thunderstorms, flood-inducing monsoon and wide-sweeping cyclones like the recent Hurricane Isaac, warns a new study.

The MIT study has given an estimate based on model simulations and observations: With every one degree Celsius rise, tropical regions will see 10 percent heavier rainfall extremes, with possible impact on flooding in populous regions. But the rate at which it does so in the tropics has remained unclear.

"The study includes some populous countries that are vulnerable to climate change and impacts of changes in rainfall could be important there," Paul O'Gorman, assistant professor of atmospheric science at MIT, was quoted as saying in the journal Nature Geoscience.

O'Gorman found that, compared to other regions of the world, extreme rainfall in the tropics responds differently to climate change.

"It seems rainfall extremes in tropical regions are more sensitive to global warming," O'Gorman said.

"We have yet to understand the mechanism for this higher sensitivity," he said, according to an MIT statement.

As carbon dioxide and other greenhouse gases enter the atmosphere, they increase the temperature, which in turn leads to increases in the amount of water vapour in the atmosphere.

When storm systems develop, the increased humidity prompts heavier rain events that become more extreme as the climate warms.

Scientists have been developing models and simulations of Earth's climate that can be used to help understand the impact of global warming on extreme rainfall around the world.

For the most part, O'Gorman says, existing models do a decent job of simulating rainfall outside the tropics - for instance, in mid-latitude regions such as the US and Europe. In those regions, the models agree on the rate at which heavy rains intensify with global warming.

To better understand global warming's effect on tropical precipitation, O'Gorman studied satellite observations of extreme rainfall between the latitudes of 30 degrees north and 30 degrees south - just above and below the Equator.

The observations spanned the last 20 years, the extent of the satellite record. He then compared the observations to results from 18 different climate models over a similar 20-year period.

"That's not long enough to get a trend in extreme rainfall, but there are variations from year to year, some years are warmer than others, and it's known to rain more overall in those years," O'Gorman said.

This year-to-year variability is mostly due to El Nino - a tropical weather phenomenon that warms the surface of the Eastern Pacific Ocean.

El Nino causes localized warming and changes in rainfall patterns and occurs independent of global warming.