An aerial view of the flooded city of Brcko, Bosnia and Herzegovina, on May 18. Russian cargo planes and rescue teams from around Europe joined huge volunteer aid efforts in swaths of Serbia and Bosnia, where at least 24 people have died in the worst floods in over a century.


Dado Ruvic/Reuters

The worst flooding in southeastern Europe in more than a century has killed at least 35 people, destroyed as many as 100,000 homes and left entire towns and villages underwater in Serbia and Bosnia.

Five days of torrential rain added up to the heaviest rainfall total in 120 years of records in the region.

It was the latest in a rapid series of record rainfalls and deluge-related disasters around the world in recent weeks, all of which have led some to wonder whether the torrents are related to climate change.

The answer, according to one of the country's leading climate researchers, is almost certainly “yes.”

A key bottom line in all weather these days, says Kevin Trenberth of the National Center for Atmospheric Research in Boulder, Colorado, is that as the world’s oceans and air warm up, the seas are transferring more water into the atmosphere.

The reason for this trend is basic physics: warmer water leads to more evaporation, and warmer air can hold more water.

Trenberth says the oceans are roughly .6 degrees Celsius warmer than they were in the 1970s. Air temperatures have increased as well, and he says that, as a rule, air holds about 7 percent more moisture for every 1 degree Celsius increase in temperature.

And once it’s up there, he says, atmospheric currents and terrestrial topography help concentrate that larger amount of water into larger precipitation events.

Moisture gets concentrated through fronts, Trenberth explains, “and what meteorologist are increasingly calling ‘atmospheric rivers,’” which then dump much of their water when they encounter a disturbance in the air or on the ground, such as land or a mountain range.

The result, he says, is that small differences in atmospheric moisture can be naturally concentrated into intense, local weather events like the deluge in the Balkans, recent torrential rains in Florida, a mudslide that buried thousands of villagers after flooding in Afghanistan, and major flooding in March and April in Trenberth’s hometown of Christchurch, New Zealand.

“So it's happening in different places, but the common element is that the air that's flowing into these weather systems is moister than it used to be,” Trenberth says. “Climate change just makes all of the weather events a little more extreme than they otherwise would be.”

Trenberth says predicting this general trend is easy. The hard part, he says, is forecasting what parts of the world will be hardest hit.

“The past may be a good guide just to where the places are more vulnerable,” he says, “but increasing heavy rains are fairly universal around the world. ... We’re just seeing that when it rains it pours, increasingly.”

And he says planners everywhere need to take that into account.

A previous version of this story misspelled Kevin Trenberth's name.

One expected effect of climate change will be an increase in precipitation intensity: a larger proportion of rain will fall in a shorter amount of time than it has historically. Blue represents areas where climate models predict an increase in intensity by the end of the 21st century, brown represents a predicted decrease.


NASA Earth Observatory, adapted from IPCC Fourth Assessment Report

Map showing areas of alert in Croatia, Bosnia and Serbia affected by the current floods.



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