Monday, March 14, 2016

Extreme weather events show signal of climate change

The rapid warming of Earth may not have directly caused all of the extreme weather events that have taken place in the past two decades – from the European heat wave of 2003 to Hurricane Katrina – but climate change has in some way had an impact on them, a new report concludes.

A 10-person committee of the U.S. National Research Council has issued a report that examined the influence of humans on recent extreme weather events. Though the committee stopped short of saying that climate change is causing more frequent and severe events -- a link difficult to prove in a short time frame -- the connection, it acknowledges, is unmistakable.

"Scientists used to say that we can't attribute any one event to climate change," said Philip Mote, an Oregon State University (OSU) climatologist and co-author on the report.

"But that is a copout. Every extreme weather event has the fingerprint of climate change. The question is not whether global warming caused Hurricane Sandy; but rather how much stronger it was because of global warming."

"There is little doubt that Hurricane Sandy would have had less impact without climate change," Mote said.

The committee issued its report on Friday in the National Academies Press, published by the National Academies of Science, Engineering and Medicine.

David W. Titley, who chaired the Committee of Extreme Weather Events and Climate Change Attribution, noted in the report's preface that "the consequences of this change to the climate are seemingly everywhere: average temperatures are rising, precipitation patterns are changing, ice sheets are melting and sea levels are rising."

Despite progress on understanding these changes, scientists are trying many different approaches to understanding the causes of extreme events.

Since 2012, the number of research groups issuing studies on the attribution of extreme weather events has exploded, shedding new light on the external "forcing" mechanisms of events and how they are similar or different from other events.

"The clearest tie between climate change and weather is in heat-related events," said Mote, who wrote the sections on heat and drought in the report.

"Droughts are getting worse and some aspect of every major heat-related event is stronger today because of climate change. In fact, most types of extreme events are getting stronger or more frequent, except those related to cold events, which are weaker or less frequent."

A warming planet does not affect every region uniformly, he added, nor does it make every season warmer than average.

The three U.S. west coast states -- California, Oregon and Washington -- experienced major drought in 2014-15.

"I'm frequently asked if we can expect more of the same in the future for the West Coast," said Mote. "The answer is yes. The weather we had this past year, which was the warmest on record in Oregon, is the type of year we can expect to call 'norm' in the decade of the 2040s."

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