Human-Caused Climate Change Seen in 2012 Extreme Weather

Human-Caused Climate Change Seen in 2012 Extreme Weather

Washington, 9 September 2013 - Human influences are having an impact on some extreme weather and climate events, according to the report Explaining Extreme Events of 2012 from a Climate Perspective released September 5 by the American Meteorological Society.

Eighteen different research teams from around the world contributed to the peer-reviewed report that examined the causes of 12 extreme events which occurred on five continents and in the Arctic during 2012. Scientists from the U.S. National Oceanic and Atmospheric Administration (NOAA) served as three of the four lead editors on the report.

The report shows that the effects of natural weather and climate fluctuations played a key role in the intensity and evolution of the 2012 extreme events. However, in some events, the analyses revealed compelling evidence that human-caused climate change, through the emission of heat-trapping gases, also contributed to the extreme event.

“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,” said Thomas R. Karl, director of NOAA’s National Climatic Data Center. “Nonetheless, determining the causes of extreme events remains challenging.”

In addition to investigating the causes of these extreme events, the multiple analyses of four of the events — the warm temperatures in the United States, the record-low levels of Arctic sea ice and the heavy rain in both northern Europe and eastern Australia — allowed the scientists to compare the strengths and weaknesses of their various methods of analysis. Despite their different strategies, there was considerable agreement between the assessments of the same events.

Thomas Peterson, principal scientist at the National Climatic Data Center, and one of the lead editors on the report, said, “Scientists around the world assessed a wide variety of potential contributing factors to these major extreme events that, in many cases, had large impacts on society. Understanding the range of influences on extreme events helps us to better understand how and why extremes are changing."

Key findings include:

• Human-induced climate change had little impact on the lack of precipitation in the central United States in 2012.

• 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

• The record-setting impacts of Hurricane Sandy were largely attributable to the massive storm surge that coincided with high tide. However, climate-change-related increases in sea level have nearly doubled the 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.

• 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.

• 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 that 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.

The report (PDF, 26MB) is available on the American Meteorological Society website.

ENDS

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