As the Mississippi River crested at near-record levels near Memphis, Tennessee, a nagging question surfaced at a Capitol Hill briefing: are people to blame? According to one expert on water and hydrology, the answer is closer to yes than no.

“I’m not suggesting these (floods) are caused by climate change, but there’s very clear scientific evidence that the risk of flooding on the Mississippi River is increasing because of human influence,” said Peter Gleick, president of the California-based Pacific Institute.

Human influence comes in at least two ways, Gleick told a briefing that drew congressional staff and personnel from U.S. agencies including the National Oceanic and Atmospheric Administration. (You can see the briefing slides here.) First, the carbon dioxide emitted by the burning of fossil fuels — cars, factories, power plants — loads the atmosphere with climate-warming greenhouse gas, pushing global mean temperatures higher.

Climate scientists have projected that this will make wet areas wetter and dry ones dryer, and this appears to be happening in the continental United States. In some parts of the Mississippi River basin, there has been as much as 20 inches (50 cm) of rain in the last 30 days, which is up to 600 percent of the normal amount, Gleick said.

Projections indicate there will be less winter snowpack — which locks water away until the spring melt — and more rain, which drains quickly into rivers like the Mississippi.

People also build levees that channel the river, packing it into a narrower, deeper space when waters rise, and they put houses, farms and factories behind the levees, putting themselves in the path of any potential flood, Gleick said.

Projections can be wrong, of course. Take the projections of flood severity along the Mississippi. When the “Father of Waters” swelled disastrously in 1993, it was called a 500-year flood — a rise in water expected only twice in a millennium. Then came the flood of 2008, the second 500-year flood in two decades. This frequency of severe floods has prompted the U.S. Army Corps of Engineers to re-evaluate their statistics on Mississippi River flooding, according to Gleick.

“Flood frequency along Mississippi River is not a constant,” he said. “That flood risk is growing.”

Photo credits: REUTERS/ERIC THAYER (A man takes a picture of a flooded mobile home park as floodwaters slowly rise in Memphis, Tennessee May 8, 2011)

REUTERS/John Sommers II (The Belle of Louisville steamboat at the Louisville River Front after being inundated by flood waters from the Ohio River in Kentucky, April 29, 2011)