WASHINGTON — The deadliest tornadoes in decades. Severe flooding on the Mississippi River. Heavy rains in Kentucky and Tennessee.
What's up with the weather?
Scientists say there are connections between many of the severe weather events of the past month and global warming.
"Basically, as we warm the world up, the atmosphere can hold more moisture in it," said Anne Jefferson, an assistant professor in the geography and earth science department at the University of North Carolina at Charlotte.
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"Weather patterns that used to be limited to the South move farther north now," she said. "Both of those things together will increase the frequency with which we see these big rainstorms, and those are likely to increase flooding in the future."
Flooding on the Mississippi has become more frequent and more extensive since about 1950, Jefferson said. This year's huge flood was created by snowmelt and rain-on-snow in the upper Mississippi River basin, and very intense rain in its middle regions.
"Climatically we have a higher frequency of rain-on-snow events, a real recipe for flooding," she said. "Also you're getting more warm moist air from the Gulf of Mexico farther north up the Mississippi. It's both a warming and, more so, the fact that the weather patterns have changed and are projected to continue to change, so the precipitation patterns are changing."
All of these changes are part of the general shift in the world's climate known as global warming — primarily the result of billions of tons of heat-trapping gases released into the atmosphere from burning fossil fuels, as well as deforestation.
"Anything that's happening now is occurring superimposed on very different background conditions than we used to have. We have significantly altered our background conditions," said Katharine Hayhoe, a climate scientist at Texas Tech University.
The study of long-term statistical trends shows an increase in heat waves and heavy rainfalls, Hayhoe said.
But for tornadoes, the jury is still out because the historical record is too patchy.
Climate change might enhance one precursor of tornadoes while diminishing another one, she said. "We will need a lot more data and modeling before we can say for sure which effect will dominate," Hayhoe said.
Kevin Trenberth, a senior scientist in the climate analysis section at the National Center for Atmospheric Research, said that when warmth and moisture at the surface build up, one response by the atmosphere is to transport it upward, a process known as convection. That's what causes intense thunderstorms, tropical storms and hurricanes, he said.
The main driver of super-cell thunderstorms — those with deep rotating updrafts — is warm moist air near the surface, he said. "In terms of climate change, the main sources of warm moist air in the regions that have been hit is out of the Gulf of Mexico or out of the tropical Atlantic. This sets the stage for the atmosphere becoming very unstable."
What happened in the case of the three big tornado outbreaks in April and May was that normal weather systems tapped into the moisture out of the Gulf and became massive super-cell thunderstorms that set the stage for very large tornadoes, Trenberth said.
Tornadoes also depend on wind shear, a weather component that normally occurs at this time of year, he added.
Jeff Masters, a meterologist and co-founder of the Web site Weather Underground, writing on his WunderBlog on Tuesday, said that this year's violent tornado season is "either a fluke, the start of a new trend or an early warning symptom that the climate is growing unstable and is transitioning to a new, higher energy state with the potential to create unprecedented weather and climate events. All are reasonable explanations, but we don't have a long enough history of good tornado data to judge which is most likely to be correct."
During the past 50 years, the average U.S. temperature has risen more than 2 degrees Fahrenheit, precipitation has increased an average of 5 percent, and many types of extreme weather events have increased in frequency and intensity. With hurricanes, only the intensity has increased.