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Global warming makes 2008-level flooding more likely

Agriculture.com Staff 06/20/2008 @ 10:31am

Is a changing climate responsible for the devastating floods and excess rain that hit Iowa and the upper Midwest in the spring of this year?

Gene Takle, a professor of atmospheric science and agricultural meteorology at Iowa State University wouldn't make that claim. The study of climate isn't so precise that it can predict individual storms years in advance.

But a scientific paper that Takle and others published in the Journal of Geophysical Science in 2004 sounds eerily on target.

It was based on computer modeling of what will happen as the climate of the upper Mississippi region warms from the 1990s through 2040. The model showed increased snowfall in warmer winters that could have more freezing and thawing, as well as increased rainfall.

As Takle said at the time, "That's pretty major. The most significant outcome of this model is that it projects a 21% increase in rainfall. That translates into a 50% increase in stream flow."

So that makes a repeat of the floods of 1993, called a 500-year event, a bit more likely just 15 years later.

"We can't attribute any particular event to climate change. But the dice is loaded. The probability that climate change is affecting these events is increasing," Takle told Agriculture Online this week.

Many climate models, not just the one Takle published in 2004, are showing increases in precipitation in the region as temperatures warm. Researchers test the models by feeding in historic data and seeing if they match up with past weather trends.

The trends alone offer evidence that the researchers are right. Annual rainfall in central Iowa has already increased by about an inch over the past 40 years, Takle says. And average annual temperatures have been rising at the rate of about a degree per decade over the past 30 to 40 years.

On average, another inch of rain sounds like a good thing for crop production, especially if it came during July and August. Unfortunately, most of the increase seems to be hitting in the planting season, and coming in storms that dump more rainfall quickly. Takle said that the increased precipitation seems to be more likely in the first half of the year, before July.

"We get a lot of the moisture-laden tropical air coming into the Midwest earlier in the spring," he tells Agriculture Online. And, if that warm air happens to bump up against a cold air mass, as it did this year, the state is also more vulnerable to tornadoes.

The implications of climate change for farmers are obvious this year in the planting delays as well as flooding. But there are many others -- the potential for increased aflatoxin and other fungal contamination in hot years, an increase in insect pests, faster weed growth, and the need for grassed waterways and conservation structures to deal with more runoff.

Takle will soon be heading up a new climate science initiative at Iowa State University to help farmers prepare for the changes ahead.

Is a changing climate responsible for the devastating floods and excess rain that hit Iowa and the upper Midwest in the spring of this year?

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