Climate Change and Extreme Weather Events

Colorado Floods in 2013

Scientists Worldwide Examine If and How Humans Have Influenced Recent Extreme Climate Events
September 29, 2014

Scientists can point to strong links between human-caused climate change and recent heat waves, but links to droughts, blizzards and extreme rainfall are more difficult to demonstrate.

These are, in general, the findings of “Explaining Extreme Events of 2013 from a Climate Perspective,” published today in a special issue of the Bulletin of the American Meteorological Society.

The report is a collection of 22 separate peer-reviewed studies on 16 extreme weather events from 2013. Ninety-two scientists from 14 countries, including editors from the National Climatic Data Center (NCDC) in Asheville, contributed research on events from California’s drought, to South Pacific heat waves, to blizzards in Europe.

“What this work shows is just the strengths and limitations we have,” said Thomas Karl, director of the NCDC. “It’s a lot easier to associate heat extremes with climate change than the other events.”

The analyses of heat extremes focused mainly on Australia’s record heat in September of 2013. Five studies examining computer models, air circulation patterns and Australia’s dry landscape found that human-induced climate change most likely contributed to the record heat.

“The chances of observing such high temperatures [in Australia] is almost impossible without climate change,” said Peter Stott, a climate researcher from the U.K. and coeditor of the report.

Other studies of heat waves in Japan, Korea, China and Western Europe echo that sentiment, uniformly finding that anthropogenic warming most likely influenced the extreme heat. The Korean study found that climate change makes similar heat waves up to 10 times more likely. In total, nine studies found links between man-made climate change and extreme heat events.

“The evidence in these papers is very strong,” said Martin Hoerling, a climate scientist with NOAA’s Earth System Research Laboratory and a coeditor of the report.

That strong link to man-made climate change begins to wane, however, in studies of other extreme weather events. After the Australian heat wave, the next most studied event was California’s extreme drought. The three papers on California arrive at different conclusions about how climate change affects drought.

One found that climate change creates conditions favorable for a dry California, but not necessarily for a lack of rainfall. Another found no link between rising sea-surface temperatures — caused by climate change — and the recent drought. The final paper found that climate change drove large rainstorms away from California but also increased humidity, decreasing drought risk.

Stephanie Herring, a researcher at NCDC and the lead editor of the report, says this uncertainty stems from the large number of factors that contribute to droughts. Isolating which factors are significant trends and which are random anomalies, what science climatologists call attribution research, becomes more difficult when there are more contributing factors, says Herring.

“The attribution research remains incredibly challenging,” Herring said. “A clear picture [of climate change and drought] is yet to emerge.”

A clear picture of climate change’s influence on extreme precipitation also does not emerge in this report. Two studies found that increased heavy rainfall over the continental United States and India in 2013 can be at least partially attributed to climate change. Two other studies, however, looking at Europe found no link between climate change and increased precipitation. Another study of Colorado’s extreme rains that led to widespread flooding last year found the event was a freak occurrence and that climate change should actually decrease the likelihood of similar events.

Even though seven of the 22 studies found either an uncertain relationship between man-made climate change and extreme weather or no relationship at all, climate change may still play a role in similar events. NCDC Director Thomas Karl says it is possible that climate change may effect those events in a way not investigated in the study or that the limits of current technology may prevent scientists from seeing climate change’s influence.

As this report is a small, non-random collection of studies, it is by no means a global image of how anthropogenic climate change influences all extreme weather events, everywhere. Karl says, however, that this report is still an important tool for expanding our knowledge of how climate change influences extreme weather and providing in-depth analysis for policy-makers.

“The science remains challenging, but the environmental intelligence it yields for decision makers is invaluable and the demand is ever growing,” he said.

- Daniel Lane

Daniel Lane covers science, engineering, medicine and the environment in North Carolina.

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