A look at weather extremes and the big-picture climate connections.

Did Global Warming Set Stage for Duluth Flooding?

As the people of Duluth, Minn. — a community of about 86,000 tucked away at the southwest corner of Lake Superior — try to recover from the record flooding of the past week, it’s reasonable for them to ask whether global warming may have played a role in the floodwaters that so heavily damaged their city.

Given the unusual nature of the rainfall, and the prevalence of extreme weather in Minnesota and other states so far this year and during recent decades, the answer, according to the scientific evidence, is “maybe.” (That the jury is still out is reason enough for concern.).

A car that was partially swallowed by floodwaters in Duluth, Minn. this week. Credit: National Weather Service.

Here are some of the facts regarding the unprecedented and devastating flooding event that took place this week in Duluth. A cold front sparked slow-moving thunderstorms that repeatedly moved over the Duluth area between June 17-19, dumping between 8 and 10 inches of rain in a 24-to 36-hour period on Duluth and neighboring communities in Minnesota and Wisconsin.

An all-time record 24-hour rainfall was set in Duluth, with 7.24 inches of rain falling during that period. The rainfall came during an already wet month in Minnesota, as the state rapidly lurched from drought conditions during the spring to suddenly having a precipitation surplus.

The rainfall washed out numerous roads in the Duluth metro area and nearby counties, and a state of emergency was declared in the city. The heavy rains caused rapid increases in the levels of local rivers and creeks. The St. Louis River at Scanlon, Minn., crested at an all-time record high of 16.62 feet on June 21, up from 5.5 feet just two days prior.

In other words, this was not your ordinary heavy downpour, and the flooding the rains caused were not your typical floods, either. It’s likely that the flooding will go down as among the most destructive in Duluth’s history.

The U.S. Climate Extremes Index, showing an increase in 1-day precipitation extremes in recent years across the Lower-48 states. Click on the image for a larger version. Credit: NCDC.

It’s been well documented that global warming is already contributing to an increase in the amount of water vapor in the atmosphere, and an increase in the number of heavy precipitation events across large parts of the globe. A 2008 report from the U.S. Climate Change Science Program found that there has been a 31 percent increase in very heavy precipitation events from 1958 through 2007 in the Upper Midwest.

As Paul Huttner of Minnesota Public Radio wrote: “What we can credibly say and support with facts is that events like the Great Duluth Flood of 2012 ‘fit’ within the overall pattern of climate changes we're observing in Minnesota.”

Some recent studies that have assessed global warming’s relative contribution to specific extreme precipitation events have shown that by putting more moisture into the air, global warming made them more likely to occur.

Given the studies showing changes already occurring in the planet’s water cycle as a result of global warming, it’s quite possible that global warming aided and abetted the extreme rainfall event such as the one that occurred in Duluth by making more moisture available for the thunderstorms to wring out of the air as heavy rainfall.

Chart showing the rapid rise and record crest of the St. Louis River at Scanlon, Minn. Credit: USGS.

Global warming did not cause the thunderstorms, of course, and they would likely have occurred regardless, but it may have made the rainfall heavier than it would otherwise have been.

As I’ve previously written, one can think of global warming’s role in extreme weather events as a suspected accomplice to a crime, not necessarily pulling the trigger, but still playing a role for which it could be held accountable.

As in a courtroom, in meteorology and climate science, it’s important to examine all of the possible factors that led to a certain outcome, be it a crime or an extreme weather event, and the scientific evidence to date suggests that global warming may have left some fingerprints at the scene of this particular crime.

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