DENVER — Climate change activists were quick to blame last year’s unprecedented, devastating Colorado flooding on increased human-caused emissions in the atmosphere, but a study released this week says that’s not the case.

A report from National Oceanic and Atmospheric Administration scientists concludes that a fluky weather pattern, not climate change, produced the system that dropped 17 inches of rain in five days and flooded a 200-mile stretch along Colorado’s Front Range.

“There’s clear evidence that overall, our greenhouse gas emissions are making the planet warmer and moister, but we found such climate factors had little appreciable effect on the frequency of heavy five-day rainfall events in this area during September,” said a statement by Martin Hoerling, a research meteorologist at NOAA’s Earth System Research Laboratory in Boulder.

The report, released Monday, was the second in as many weeks to cast doubt on the role of man-made climate change in major environmental shifts. Researchers last week said the warming of waters off the Pacific Coast over the past century could be explained entirely by shifting wind patterns without any reference to increased emissions in the atmosphere.

The Colorado analysis was included in “Explaining Extreme Events of 2013 From a Climate Perspective,” a supplement to the Bulletin of the American Meteorological Society that included 20 scientific papers examining 16 worldwide “extreme weather events.”

A number of environmentalists put the blame squarely on climate change as the culprit in the aftermath of the Colorado flooding, which resulted in 10 deaths and $2 billion in damage.

“In the Colorado floods of 2013, we got a sobering glimpse of a future that will only worsen if we don’t act now,” Susan Secord of Citizens’ Climate Lobby in Boulder said in a Dec. 27 op-ed in The Denver Post. “A planned, orderly economic adjustment now will save us from having to make very expensive and chaotic adjustments later.”

The floods along the Front Range also have become a campaign issue in this year’s tight U.S. Senate race. The liberal environmental group NextGen Climate, founded by billionaire Tom Steyer, has cited the floods as an example of climate change devastation in campaign materials attacking Rep. Cory Gardner, a Republican who is running against Democratic Sen. Mark Udall.

Warmer surface temperatures trigger moisture in the atmosphere, which tends to cause heavy rains, but the study said that was not what prompted the floods last year. A slow-moving, wet air mass, possibly the remnant of a tropical system, pushed up against Boulder’s steep mountains and was trapped, producing five straight days of rain, researchers concluded.

In fact, the scientists behind the study added that the probability of such an event “has likely decreased due to climate change.” They noted that similar systems hit the Front Range long before the advent of elevated carbon dioxide levels in the atmosphere.

“From an observational perspective, analogous events have occurred before in the Front Range, perhaps most strikingly similar in September 1938, long before appreciable climate change,” the study said.

In analyzing the 16 extreme weather events, the studies found that last year’s heat waves — in Australia, Asia and Europe — were more likely to be linked to changes in the climate than other disasters, such as droughts, storms and hurricanes, “indicating that natural variability likely played a much larger role in these extremes.”

In fact, the report included three studies on the California drought, but none of those could find a definitive link between climate change and the state’s long-standing below-average rainfall.

“The comparison of the three studies for the same extreme event, each using different methods and metrics, revealed sources of uncertainty,” the report says.

Stanford University researchers issued a study Monday — the same day as the release of the American Meteorological Society report — saying that the high-pressure system blocking California rains is “very likely” connected to human-caused climate change.

Paul Knappenberger, assistant director of the Cato Institute’s Center for the Study of Science, said the report pokes holes in efforts to chalk up a variety of disparate weather events to climate change.

“It rained here, it didn’t rain there. There was snow here, there was a hurricane there. Those types of things are much more complicated,” Mr. Knappenberger said. “The science isn’t clear one way or the other how global warming is going to affect those things.”

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