Strange weather and climate change

Living on Earth

Photo of a 2005 flood (Image by Wikimedia user Ildar Sagdejev (cc:by-sa))

This story was originally covered by PRI's Living on Earth. For more, listen to the audio above.

Relentless rainfall - a deluge that dumped five months worth of rain in just 14 days - and a massive spring snowmelt have had a devastating effect along the Mississippi and Ohio Rivers, forcing the Army Corps of Engineers to take extreme measures. Demolition experts blew up levees along the Mississippi in a daring effort to protect cities and towns from record-breaking floodwaters. Meanwhile, states in the South and Southeast have been experiencing an unprecedented number of intense tornadoes. In April, nearly 900 tornadoes tore through the region - four times as many as usual.

Before the tornadoes, the upper-Midwest was hit by near-record snow pack. When that melted, it sent a huge amount of floodwater down the Mississippi.

"This is all quite expected and due in part to a warmer atmosphere and climate change," according to meteorologist Jeff Masters. There are near-record temperatures at the surface of the ocean on the Gulf of Mexico. The warm air from there is colliding with cold air from Canada to create extreme weather.

"If you look at the history of precipitation over the Mississippi Valley region over the past century, it has increased ten to twenty percent. That is something we expect to see in a warmer climate -- warmer air holds more water vapor, and climate models predict that by the end of this century, we'll see another 20 percent increase in the rainfall over the Mississippi Valley."

That 20 percent increase in rainfall might sound like it would lead to 20 percent more flooding. In fact, it could lead to about 50 percent more flooding, according to Masters, "because what happens when you start getting heavier rains is now you've got a saturated soil that can't absorb rain anymore - so you tend to get more runoff."

Climate change could also be connected to the recent rash of tornadoes, according to Masters. "We had two outbreaks in April that each generated more than 150 tornadoes," Masters told Living on Earth. "Two of the top four events going back to 1950 occurred last month. So we had a very unusual atmospheric situation in April that's unprecedented."

The role of climate change in tornadoes, however, is mixed. "I think it's a flip of a coin. And that's because with climate change, we expect it to increase instability, which is favorable for generating intense thunderstorms," Masters says. "The flipside of that is that in order to get a tornado, you need more than just unstable air - you need something to get it spinning. And the way you get it spinning is if you have a very strong jet stream that changes direction with speed and height. But in the future, we expect the jet stream to weaken with climate change - that's one of the things that these climate models tell us."

Climate change could push tornadoes into places where they don't normally occur, including up into Canada. Overall, Masters says, "my motto for the coming decades is going to be: expect the unprecedented."


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