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 on September 5 by the Bulletin of the American Meteorological Society. Overall, 18 different research teams from around the world contributed to the peer-reviewed report that examined the causes of 12 extreme weather events that occurred on five continents and in the Arctic during 2012. Scientists from 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 weather 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, L.H.D, Director of NOAA’s National Climatic Data Center (NCDC). “Nonetheless, determining the causes of extreme events remains challenging.”
In addition to investigating the causes of these extreme weather 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 and contrast 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, Ph.D., Principal Scientist at NOAA’s NCDC 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.”
Location and type of events analyzed in the Paper. © NOAA
Key findings include:
Heatwave 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 heatwaves 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 heatwaves. 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 Hurricane 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.