Climate change will make Colorado’s millennial rainstorm a lot more common

October 24, 2013
October 24, 2013

The rainfall that caused massive flooding in Colorado last month was a once-in-a-millennium event, according to a recent study (pdf). And climate change is making those kinds of extreme weather events more common.

The impressively named Hydrometeorological Design Studies Center, a division of the National Weather Service, has concluded with greater than 90% certainty that the rainfall was millennial in nature. Here’s the chart for one rain gauge in Boulder, Colorado, that was inundated over seven days:

Screen shot 2013-10-24 at 11.59.13 AM

The calculations, though, use a dataset “based on the assumption of stationary climate.” As I wrote while the rains were still falling, climate change tilts the odds in favor of extreme weather like the Colorado floods. That means, the actual odds of flooding of this magnitude are much higher. Problem is, no one knows exactly how much higher.

The recent assessment report by the Intergovernmental Panel on Climate Change (IPCC) concluded that heavier precipitation events in central North America were “very likely” to increase. Without factoring the effects of climate change into construction projects, there’s an increasing risk that roads, bridges, houses, and people will get in the way of bigger floods caused by increasingly heavier downpours. (The Boulder Daily Camera has a particularly fascinating feature-length piece on the Colorado floods and their aftermath.)

Though the storm was called a thousand-year event while rain was still falling, the new analysis is compelling. It’s now clear that in Colorado, the weather is becoming more extreme.

That trend is holding up across the United States, as well: In sharp contrast to two years of extreme drought, new data released Thursday by the National Climate Data Center showed that not a single one of the lower 48 states had a drier than normal warm season in 2013 (defined as April to September).

In fact, after two of the driest years on record, the US was only a quarter-inch away from breaking the record for the wettest six months ever measured, nationwide. The two years ending in September 2013 were also tied for the warmest in the nation’s recorded weather history.

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