Recent heavy rains that have lashed Buenos Aires and other parts of Argentina, causing 100-year record flooding that left more than 50 people dead, may be just a preview of the years to come. Storms will bring a lot more rain as global temperatures rise through the 21st century, according to a new study from the National Oceanic and Atmospheric Administration.

The study finds that extreme precipitation events will be more intense in a warming world, with as much as 20 percent to 30 percent more rainfall in most parts of the United States.

Greenhouse gases trap heat that warms the oceans, which, in turn, dispense more moisture into the atmosphere. Scientists from NOAA; the Desert Research Institute; and the University of Wisconsin, Madison, studied climate model simulations to assess the maximum possible water vapor concentrations that such warming could lead to.

"The way we estimate how big the biggest storms could be is [by asking], 'What's the max amount of moisture that can occur in a place or has occurred in a place, and what's the maximum amount the air can be raised up, and if you have a mountain, what's the maximum horizontal air flow that you have?'" said Kenneth Kunkel of the NOAA Cooperative Institute for Climate and Satellites, who is the lead author of the study.

The team of scientists ran climate simulations for two 30-year periods, 2041-2070 and 2071-2100, and compared the results with rainfall measurements for 1971-2000. They found that atmospheric moisture increased a lot more than other factors that influence precipitation -- vertical motion of moisture-laden air and horizontal wind speed.

Increases in atmospheric moisture, or "precipitable water," corresponded to the rise in latent heat in the oceans, both of which were found to be in sync with greenhouse gas projections.

News for dam builders, among others
The study shows that most of the Northern Hemisphere will witness 20 percent to 30 percent more rainfall between 2071 and 2100 compared with the end of the 20th century. The increase in rainfall intensity between 2041 and 2070 will be half that amount.

That gives the United States plenty to worry about. "When you look at it in terms of inches, it does vary across the U.S. The heaviest amounts occur along the Gulf Coast. These heaviest storms, in terms of rain in one day, would be about 40 inches," Kunkel said. The greatest increases in precipitation in the United States will be in the western parts of the country, according to the study.

Rainfall intensity is not expected to rise as much in the Southern Hemisphere, where the larger ocean area moderates rising temperatures.

Kunkel said there are signs of the trend beginning, even if the evidence is only circumstantial. "If you look at the general increases that we see in heavy rains, not any particular event but in general, they are increasing, and there does seem to be some evidence that we are seeing some increase in atmospheric moisture because the globe is warming due to greenhouse gases," he said.

The research team believes its projections of rainfall intensity are crucial for water resource managers and builders to be able to factor in risks of catastrophic events. For example, a dam built for years to come should be able to withstand heavy rain and accumulation of water, Kunkel pointed out.

"The specific uniqueness about this paper is that we focused on heaviest of heavy events. We were looking at the worst storms that would occur," he said.

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC., 202-628-6500