Only time will tell if the great flood of 2008 turns out to be a once-in-an-epoch aberration or the harbinger of a trend.

It took an unthinkable chain of weather events to swell the Cedar River to the 31.12-foot-tall, 1.6 mile-wide tsunami that swamped Cedar Rapids on June 13.

Now those events — heavy autumn rains followed by near record snowfall and three spring deluges in three weeks — are eminently thinkable.

The last of those deluges, the June 12 dumping of 5 inches on an already record-shattering flood, spelled the difference between the National Weather Service’s predicted 24.9-foot crest and the epic surge that engulfed 10 square miles of the city.

While no one can know if and when another epic flood will occur, Iowa climate and weather experts believe massive floods will be more likely in the future.

“If I had to hazard a guess, I’d say the likelihood of this type of event is increasing,” said Eugene Takle, a professor of atmospheric science and agricultural meteorology at Iowa State University,

Takle’s Iowa State colleague, meteorology professor Elwynn Taylor, said he believes documented increases in Iowa’s annual precipitation, coupled with anticipated precipitation increases linked to global warming, greatly increase the likelihood of more frequent and severe flooding.

Precipitation increase

Since 1950, Iowa’s annual precipitation has increased by 10 percent, which in turn has doubled the amount of water carried by rivers in the state, according to Taylor.

State Climatologist Harry Hillaker, who pegs the increase in annual statewide precipitation at 8 percent since 1959, said rainfall increases translate directly into increased storm runoff, which fuels flooding.

A 10 percent precipitation increase, roughly 3 more inches per year, makes rivers six times more prone to flooding because it all runs off, rather than soaking in for use by vegetation, according to Taylor.

What had been considered a 100-year flood — a flood with a 1 in 100 chance of happening any given year — is now a 17-year flood, Taylor said.

It gets worse, when you factor in global warming, he said.

Taylor said the consensus among scientists working on global warming is that a doubling of carbon dioxide in the atmosphere — it has increased by one-third since 1960 and at its present rate of increase will double in 50 years — will yield an increase of 5 degrees Fahrenheit in the global temperature.

“That means air coming to Iowa from the Gulf of Mexico (the source of most of Iowa’s precipitation) will hold 17 percent more moisture, which translates into 17 percent more precipitation,” Taylor said.

If that occurs, what had been considered a 100-year flood will become a 5-year flood by 2058, Taylor said.

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