2017's Extreme Heat, Flooding Carried Clear Fingerprints of Climate Change

Some weather extremes are now 2-3 times more likely because of global warming, research shows. Several 2017 events would have been virtually impossible without it.

The remnants of Hurricane Harvey dropped more than 50 inches of rain on parts of Houston in August 2017 as it stalled over the region, flooding several neighborhoods. Credit: Scott Olson/Getty Images

The remnants of Hurricane Harvey dropped more than 50 inches of rain on parts of Houston in August 2017 as it stalled over the region, flooding several neighborhoods. Credit: Scott Olson/Getty Images

Many of the world's most extreme weather events witnessed in 2017, from Europe's "Lucifer" heat wave to Hurricane Harvey's record-breaking rainfall, were made much more likely by the influence of the global warming caused by human activities, meteorologists reported on Monday.

In a series of studies published in the American Meteorological Society's annual review of climate attribution science, the scientists found that some of the year's heat waves, flooding and other extremes that occurred only rarely in the past are now two or three times more likely than in a world without warming.

Without the underlying trends of global climate change, some notable recent disasters would have been virtually impossible, they said. Now, some of these extremes can be expected to hit every few years.

For example, heat waves like the one known as "Lucifer" that wracked Europe with dangerous record temperatures, are now three times more likely than they were in 1950, and in any given year there's now a one-in-10 chance of an event like that.

In China, where record-breaking heat also struck in 2017, that kind of episode can be expected once every five years thanks to climate change.

Civilization Out of Sync with Changing Climate

This was the seventh annual compilation of this kind of research by the American Meteorological Society, published in the group's peer-reviewed Bulletin. Its editors said this year's collection displays their increased confidence in the attribution of individual weather extremes to human causesnamely the buildup of greenhouse gases in the atmosphere.

"A warming Earth is continuing to send us new and more extreme weather events every year," said Jeff Rosenfeld, the Bulletin's editor-in -chief. "Our civilization is increasingly out of sync with our changing climate."

Martin Hoerling, a NOAA researcher who edited this year's collection, said the arrival of these damages has been forecast for nearly 30 years, since the first IPCC report predicted that "radical departures from 20th century weather and climate would be happening now."

Not every weather extreme carries the same global warming fingerprint. For example, the drought in the U.S. High Plains in 2017, which did extensive damage to farming and affected regional water supplies, chiefly reflected low rainfall that was within the norms of natural variabilitynot clearly a result of warming.

Even so, the dry weather in those months was magnified by evaporation and transpiration due to warmer temperatures, so the drought's overall intensity was amplified by the warming climate.

Warnings Can Help Guide Government Planners

Even when there's little doubt that climate change is contributing to weather extremes, the nuances are worth heeding, because what's most important about studies like these may be the lessons they hold for government planners as they prepare for worse to come.

That was the point of an essay that examined the near-failure of the Oroville Dam in Northern California and the calamitous flooding around Houston when Harvey stalled and dumped more than 4 feet of rain.

Those storms "exposed dangerous weaknesses" in water management and land-use practices, said the authors, most from government agencies.

What hit Oroville was not a single big rain storm but an unusual pattern of several storms, adding up to "record-breaking cumulative precipitation totals that were hard to manage and threatened infrastructure throughout northern California," the authors said.

Thus the near-disaster, as is often the case, wasn't purely the result of extreme weather, but also of engineering compromises and such risk factors as people building homes below the dam.

In Houston, where homes had been built inside a normally dry reservoir, "although the extraordinary precipitation amounts surely drove the disaster, impacts were magnified by land-use decisions decades in the making, decisions that placed people, homes and infrastructures in harm's way," the authors said.

Attribution studies should not just place the blame on pollution-driven climate change for increasingly likely weather extremes, the authors said. They should help society "better navigate such unprecedented extremes."

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