"Over the past 35 years in the Northern Hemisphere mid-latitudes, we've observed, on the whole, an increase in the occurrence of extremely hot days and a decrease in the occurrence of extremely cold days," Daniel Horton, of the University of Stanford, told environmentalresearchweb. "In some places, these trends are stronger than in others, and in some places they are of opposite sign. We wanted to understand the underlying causes of these trends."

Increasing trends in anticyclonic circulations over portions of Eurasia and North America have contributed to summer and autumn heat extremes, Horton and colleagues found, while a rise in flow towards central Asia from the north has enhanced winter cold extremes.

The researchers used self-organizing map cluster analysis to look at seasonal variations in 500 hPa geopotential heights, which represented atmospheric circulation patterns, for the satellite measurement era of 1979–2013 and the period of rapid Arctic sea-ice decline (1990–2013).

"We know that the occurrence of extreme events can be driven by changes in the motion/circulation of the atmosphere (atmospheric dynamics), and/or can be driven by changes in the heat content of the climate system (thermodynamics)," said Horton. "To understand which of these factors has driven the observed changes in extreme-temperature occurrence, we designed an approach that allows us to independently partition the dynamic and thermodynamic contributions."

This partitioning revealed that both an increased frequency of blocking circulations – persistent anticyclonic patterns – and changes in energy balance were responsible for increases in extreme summer heat over Europe.

"Our analysis indicates that a substantial portion of the observed change in extreme-temperature occurrence has resulted from changes in the heat content of the climate system, and that these changes are consistent with anthropogenic influences," said Horton. "However, we also find that the risk of extreme temperatures over some regions has been altered by changes in the motion of the atmosphere via changes in the frequency and duration of regional circulation patterns."

The researchers don't yet know whether these trends in atmospheric circulation are due to human or natural causes.

"We can say that the thermodynamic changes we observe are consistent with predictions of global warming, whereas we are uncertain if the observed changes in circulation patterns are driven by human influences or are simply the result of natural variability," said Horton. "We are hopeful that the methods we've designed will allow us to independently attribute both the dynamic and thermodynamic changes."

According to Horton, the greatest impacts of climate change to human, ecological, and environmental systems are often mediated through extreme climate events. "Heatwaves and cold snaps can have calamitous effects," he said. "Increasing our knowledge of the causes and trends in these types of events is essential for understanding what caused them in the past; forecasting their occurrence in the future; and informing policy makers, public-health officials, and individual citizens about the potential risks of extreme temperature occurrence."

The team reported the results in Nature.

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