Humans are influencing some extreme weather events, but not all

by Sara E. Pratt
Thursday, December 12, 2013

In 2012, the world experienced dozens of extreme weather events, including droughts, heat waves, cold spells, extreme rainfalls, big storms like Superstorm Sandy, and a record-low Arctic sea-ice extent. Teasing apart the factors that create extreme weather is a challenge for scientists, especially when it comes to determining whether human-induced climate change plays a role. Recently, 18 different research teams — comprising 80 scientists — took on that challenge.

In a new study compiling 19 separate studies on 12 extreme weather events of 2012, researchers reported that anthropogenic climate change was a contributing factor in about half of the events.

However, “the effects of natural fluctuations of weather and climate on the evolution of many of the extreme events played key roles as well,” wrote the editors of the report, “Explaining Extreme Events of 2012 from a Climate Perspective,” which was published as a special supplement to the Bulletin of the American Meteorological Society.

In addition to Superstorm Sandy and the Arctic sea-ice minimum, the studies analyzed the heat wave and drought that hit the U.S. during the spring and summer of 2012 — the worst drought in the U.S. since the 1950s — as well as droughts in Africa and Europe, extreme precipitation events in Asia, Europe and Australia, and both hot and cold spells in Europe.

Four of the events — the U.S. heat wave, the Arctic sea-ice minimum, and extreme rainfall events in both northern Europe and eastern Australia — were analyzed by more than one team, allowing a comparison of methods and results.

“In these cases, there was considerable agreement between the different assessments of the same event,” wrote editors Thomas Peterson and Stephanie Herring of NOAA’s National Climatic Data Center, Martin Hoerling of NOAA’s Earth System Research Lab, and Peter Stott of the United Kingdom’s Met Office Hadley Centre. “However, different events had very different causes.”

The research teams used various methods, including comparing climate model simulations with and without human factors, and calculating how the probability of a particular extreme event has changed over time, which allowed them to quantify the influence of climate change on the event.

Three teams that examined the U.S. drought and heat wave found that although climate change likely had little impact on the lack of precipitation in the Central U.S., it likely did account for about 35 percent of the extreme warmth experienced in the Eastern U.S. between March and May. In addition, researchers reported that in the future, high temperatures are likely to occur four times more frequently in an anthropogenically warmed world.

Two teams examined the record-low Arctic sea ice, using different methods but coming to the same conclusion: that the record low extent of Arctic sea ice was due to the melting of thinner, younger sea ice, an event that was extremely unlikely to have been caused by natural variability alone. Given the long-term warming of the Arctic atmosphere and ocean, the researchers predicted a continued decrease in the extent of sea ice in the future.

The findings would appear to contradict the oft-repeated notion that no single weather event can or cannot be attributed to climate change. But the science of attribution has been advancing in recent years and the distinction ultimately comes down to differences in how scientists and nonscientists use language, says Richard Alley, a Penn State climate researcher who was not involved in the research.

“Terms such as ‘extremely unlikely’ have technical meanings in probability,” Alley says. “Scientists develop such ‘language’ not to shut out other people, but to allow precise discussion of what is or is not known,” he says. “However, if someone asks a question using imprecise terms, or asks for absolute answers rather than probabilities, you get a different answer.”

The previous year’s report, “Explaining Extreme Events of 2011 from a Climate Perspective,” looked at six extreme weather events and found similar results: Some, but not all, extreme weather events are influenced by climate change. As the science of attribution continues to advance, it is expected that the methodologies used to assess the relative contribution of human and natural factors in extreme weather will become even more accurate.

Scientists who are allowed “to do science, develop measures of uncertainty, make estimates of how likely something is under natural conditions versus under the natural-plus-human world we now have, those scientists are making notable progress in estimating the human as well as natural contribution to extreme events,” Alley says.

If the causes of extreme weather events, both natural and human, can be attributed, then, in theory, they could possibly be predicted, the editors suggested, noting that in one instance at least, this has already occurred.

Over the last decade, researchers have discovered links between warming sea-surface temperatures (SST) in the Indo-Pacific Ocean and dry events in East Africa. In March 2012, when a strong western-to-eastern Pacific SST gradient was detected — which has previously been associated with below normal rainfall in East Africa — the Famine Early Warning Systems Network issued alerts for expected crop failures and food shortages in Kenya and Somalia. This allowed humanitarian aid workers to get disaster relief and food supplies in place ahead of time, mitigating the toll of the subsequent drought.

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