Over the last 60 years, New England has seen a 61-percent increase in extreme rainstorms, an environmental advocacy group said in a study released yesterday, and unchecked global warming will likely make them worse.

The study, which was conducted by the nonprofit environmental advocacy group Environment America, said greenhouse gas emissions are putting the United States at risk of flooding, crop damage, erosion and pollution of waterways with sewer runoff.

By analyzing data from more than 3,000 weather stations in the 48 contiguous states from 1948 to 2006, the group found the likelihood of severe rainstorms is increasing almost everywhere in the United States.

Described in the report as “extreme precipitation,” these incidents were determined by taking the 59 most severe precipitation events in a region, such as New England or the Pacific Northwest, and plotting them on a timeline over the last 59 years.

The results showed a gradual increase of extreme precipitation in recent years.

“We wrote this report to show how our local climate is already changing,” said Agatha Wein, a global warming associate at Environment Rhode Island.

Other conclusions in the report:

• That an increase in the number of downpours does not necessarily mean more water will be available. Scientists expect that extreme rainstorms will punctuate longer periods of relative dryness, increasing the risk of drought.

• New England and the Mid-Atlantic states experienced the largest increase in extreme precipitation frequency.

New England could be more vulnerable to floods, drought and water-quality problems if the pattern continues, said University of Rhode Island Prof. John Merrill, who specializes in atmospheric chemistry and how its changes have an impact on the environment.

As an example of the possible consequences of heavier rainstorms, Wein cited the Blackstone Valley flood of 2005, which inundated homes, businesses and a sewage treatment facility in Woonsocket, resulting in millions of dollars of damage. More than 15 inches of rain fell that October.

Merrill said torrential downpours, like the one that flooded Blackstone Valley, are happening more as the planet warms.

“It is as if the occurrence of the common events is becoming less common and the occurrence of the extreme events is becoming more common,” he said. “And that is a cause of concern.”

The study attributes the changes to increased greenhouse gas emissions that alter the chemistry of the atmosphere. As the temperature over land and water rises, evaporation increases, putting more water vapor in the atmosphere, creating larger and heavier storm clouds.

Merrill said the science behind the report and its conclusions were sound, but the group is actively supporting policy changes.

“The report is based on science but extends into policy,” Merrill said. “I think it’s a good report but their agenda is apparent.”

Environment Rhode Island planned the release of the report to coincide with the U.S. Senate debate over the Lieberman-Warner bill, the first significant effort by Congress to make polluting industries cut greenhouse gas emissions.

Wein said Environment Rhode Island is part of a coalition of environmental groups in the state pushing for local regulations to reduce pollution by 20 percent by 2020.

“We think what will happen on the federal level will not be as strong as we want it to be,” Wein said.

ngarcia@projo.com