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More Heavy Rainfalls Part of International Climate Trend

Peter Cihelka/Fredericksburg Free Lance-Star

On Sunday evening, another rainstorm drenched the Baltimore area, turning gutters into rivers and streams into muddy torrents.

A day earlier, heavy rainfall caused the worst flooding in two decades along the Rappahanock River in Virginia, inundating several homes.

And on May 27, flash floods devastated historic Ellicott City, Maryland, for the second time in as many years, flipping cars and killing a National Guard sergeant.

May turned out to be the third rainiest in Maryland history, with eight inches falling, more than twice the average for that month, according to National Weather Service monitoring at BWI airport. So far in June, almost 5 more inches of rain have fallen – about a third more than is normal for a June.

Donald Boesch is a professor and former president of the University of Maryland Center for Environmental Science. He said the increase in severe rainfall events is not unique our region or country – and is being driven by global warming, because warmer air holds more moisture.

“If you look around the world, as Ellicott City was being flooded, in international news, there was flooding in, of all places, Oman, in the Middle East, which gets very little rain,” Boesch said.  “There was flooding in parts of Germany.  So this is not just a localized phenomenon, this is global climate change, and it’s affecting places all around the world.”

Andreas Prein is an atmospheric scientist at the National Center for Atmospheric Research in Boulder, Colorado. He said rainfall monitoring data over the last 70 years in the U.S. show varying trends – with 71 percent more heavy rainfall events in Maryland and the Northeast in recent years, compared to in the 1950’s.  There have also been 37 percent more heavy rainfall events in the Midwest and five percent more in the southwest, but 12 percent fewer in Hawaii, compared to the mid 20th century.

“Rainfall is getting more extreme,” said Prein.  “But on the other side, it’s getting less frequent.  So instead of having moderate rainfall events from time to time, in the current system and in the future climate, you can expect that there are long periods of dryness, and then followed by very heavy rainfall events.  For agriculture, it’s very bad. It’s also bad for flooding, of course.”

In the Chesapeake Bay, water quality has improved the last seven years, with underwater grasses expanding to the largest extent in three decades, according to monitoring by the Virginia Institute of Marine Science and the University of Maryland Center for Environmental Science. But an increase in rain storms could deal a blow to this recovery by flushing more fertilizer and pollutants off of farm fields and suburban blacktop.

Karen Rice, a geologist with the U.S. Geological Survey, said the problem of increased runoff pollution into the bay could be more acute in areas like Baltimore and Pennsylvania than in Virginia.

“There is definitely an increase across the entire Chesapeake Bay watershed,” said Rice. “But the really interesting aspect of it is that there is a very strong special gradient, where the northern part of the watershed has seen a larger increase in precipitation than the southern part of the watershed.”

Mike Tidwell, founder of the Chesapeake Climate Action Network, said that one answer is building more stormwater pollution control systems. But the other is attacking the underlying cause of the increase – the burning of fossil fuels.

 “We need to connect the dots between the rainfall that is blowing out our neighborhoods roads and nearby bridges with our use of fossil fuel, and we need to do that,” said Tidwell.  “You know that trends show that carbon emissions are going down worldwide, but they need to go down a lot faster.”

In other words, if find yourself in a hole – the first thing you need to do is to stop digging and making the problem worse by releasing more greenhouse gas pollution.

Tom Pelton, a national award-winning environmental journalist, has hosted "The Environment in Focus" since 2007. He also works as director of communications for the Environmental Integrity Project, a non-profit organization dedicated to holding polluters and governments accountable to protect public health. From 1997 until 2008, he was a journalist for The Baltimore Sun, where he was twice named one of the best environmental reporters in America by the Society of Environmental Journalists.