Scientists studied 12 extreme 2012 weather events to look for a global warming signal. Map courtesy NOAA.
Studies partly aimed at finding the best way to pursue climate attribution research
By Bob Berwyn
FRISCO — A new report asserts that global warming was a factor in several extreme weather events in 2012, including the spring and summer heatwave across the central U.S. and flooding in parts of New York and New Jersey from Hurricane Sandy.
But the researchers with the National Oceanic and Atmospheric Administration and the UK Meteorological Office also said they couldn’t find a global warming connection to the U.S. drought, summer extremes in Europe or floods in northern China.
The report covered 12 separate studies that looked for climate-weather links in different ways, including statistical analyses and comparison with analog years. It’s part of an effort to tease out how global warming may be affecting weather in the emerging field of climate attribution studies. View the full report online.
In other words, climate scientists want to know if they can identify a global warming fingerprint on heatwaves, droughts and flooding. Along with answering some of the more esoteric questions about the way best way to go about finding that fingerprint, the report, released by the Bulletin of the American Meteorological Society, is aimed at answering very specific questions raised by people who are affected by extreme weather.
Farmers, urban planners, ski area operators all want to know if a specific weather event was related to global warming caused by human production and emission of heat-trapping greenhouse gases, the scientists said during a Sept. 5 press call. They also want to know if the likelihood of weather events affecting their interests will increase as levels of greenhouse gases continue rising, said Thomas R. Karl, director of NOAA’s National Climatic Data Center.
“This report adds to a growing ability of climate science to untangle the complexities of understanding natural and human-induced factors contributing to specific extreme weather and climate events,” Karl said. “Nonetheless, determining the causes of extreme events remains challenging.”
Specifically, the studies showed that global warming was a factor in increasing the likelihood or intensity of the 2012 U.S. heat waves, flooding from Superstorm Sandy, a record minimum summer sea ice extent, drought in Spain and Portugal and extreme rainfall in Australia and New Zealand.
The studies reviewed in the report did not find signs of a link to the 2012 U.S. drought, summer extremes in Europe, an unusual cold snap in northwestern Europe drought in eastern Kenya and Somalia, floods in northern China and heavy rain in southwestern Japan.
Teasing out a global warming signal is a big challenge, given the complexity of annual weather patterns and the large range of natural variability. Nevertheless, as climate attribution research progresses, some of the evaluations showed compelling evidence that human-caused climate change contributed to some of the extreme events. Key findings include:
Heat Wave and Drought in United States:
- Human-induced climate change had little impact on the lack of precipitation in the central United States in 2012.
- The 2012 spring and summer heat waves in the U.S. can be mainly explained by natural atmospheric dynamics, however, human-induced climate change was found to be a factor in the magnitude of warmth and was found to have affected the likelihood of such heat waves. For example:
- High temperatures, such as those experienced in the U.S. in 2012 are now likely to occur four times as frequently due to human-induced climate change.
- Approximately 35 percent of the extreme warmth experienced in the eastern U.S. between March and May 2012 can be attributed to human-induced climate change.
Hurricane Sandy Inundation Probability:
- The record-setting impacts of Sandy were largely attributable to the massive storm surge and resulting inundation from the onshore-directed storm path coincident with high tide. However, climate-change related increases in sea level have nearly doubled today’s annual probability of a Sandy-level flood recurrence as compared to 1950. Ongoing natural and human-induced forcing of sea level ensures that Sandy-level inundation events will occur more frequently in the future from storms with less intensity and lower storm surge than Sandy.
Arctic Sea Ice:
- The extremely low Arctic sea ice extent in summer 2012 resulted primarily from the melting of younger, thin ice from a warmed atmosphere and ocean. This event cannot be explained by natural variability alone. Summer Arctic sea ice extent will continue to decrease in the future, and is expected to be largely absent by mid-century.
Global Rainfall Events:
- The unusually high amount of summer rainfall in the United Kingdom in 2012 was largely the result of natural variability. However, there is evidence that rainfall totals are influenced by increases in sea surface temperature and atmospheric moisture which may be linked to human influences on climate.
- The magnitude of the extreme rainfall experienced over southeastern Australia between October 2011 and March 2012 was mainly associated with La Niña conditions. However, the likelihood of above-average precipitation during March was found to have increased by 5 percent to 15 percent because of human influences on the climate.
- Extreme rainfall events such as the December 2011 two-day rainfall in Golden Bay, New Zealand, are more likely to occur due to a 1 percent to 5 percent increase in available moisture resulting from increased levels of greenhouse gases in the atmosphere.
- The July 2012 extreme rainfall events in North China and southwestern Japan were mainly due to natural variability.
