Worldwide news reports of extreme weather events – oppressive heat, parching droughts, destructive storms – are increasingly common. A new study co-authored by Stanford and Princeton University researchers finds that trends in atmospheric circulation patterns can partially explain Earth’s increasingly severe weather. While scientists had previously surmised that the link existed, robust empirical evidence was lacking.

The study finds that overall increases in hot extremes and decreases in cold extremes in the Northern Hemisphere mid-latitudes are driven by a combination of changes in the amount of heat and moisture in the atmosphere as well as changes in atmospheric circulation patterns. Changes in the heat and moisture content of the climate system – called “thermodynamics” – can account for the majority of the observed changes in extreme temperature. However, shifts in the circulation of the atmosphere – called “dynamics” – have also altered the risk of extreme temperatures in some regions, according to the study published in Nature. The researchers suggest that the thermodynamic changes are consistent with rising greenhouse gas concentrations, but indicate that the cause of the changes in circulation patterns remains uncertain.

“Understanding the underlying causes of extreme weather events is essential for risk-based adaptation planning,” said study lead author Daniel Horton, a postdoctoral scholar in Stanford’s School of Earth, Energy & Environmental Sciences. “Identifying the dynamic and thermodynamic contributions will help us separate the anthropogenic or human influences from the changes that result from natural variability.”

Among the study’s findings:

  • Rates of extreme heat are increasing and rates of extreme cold are decreasing over most Northern Hemisphere mid-latitude regions.
  • Increasing trends in blocking anticyclonic circulation patterns – patterns that redirect the jet stream north or south and allow heat to build up over a region – have contributed to more frequent summer and autumn heat extremes in Eurasia and North America.
  • Increasing trends in the flow of frigid Arctic air masses have contributed to more frequent winter cold extremes in Central Asia.

The researchers isolated dynamic and thermodynamic contributions to extreme temperature trends during the past 35 years using a combination of atmospheric observations and pattern clustering analyses analogous to those used in some facial recognition algorithms. This approach offers the potential to “fingerprint” the causes of regional climatic change. However, the researchers caution that at this point in time the identified changes have not been rigorously attributed to human or natural causes.

“It’s important to note that our results don’t invalidate the basic observation that global warming is causing an overall increase in hot events and decrease in cold events,” said co-author Noah Diffenbaugh, an associate professor of Earth system science and a senior fellow at the Stanford Woods Institute for the Environment. “That said, we have found that some of the observed trend in temperature extremes has happened because of changes in the kinds of weather patterns that you see on the evening news. The extent to which those changes in atmospheric circulation are themselves due to global warming or are instead part of the long-term fluctuations of the climate system is something we’re looking into now.”

Still, the researchers expect the general trend of increasing hot extremes and decreasing cold extremes to continue if greenhouse gases continue to accumulate in the atmosphere. “This reality suggests increasing risk of the kinds of extreme hot events that we know can cause a lot of damage to people and ecosystems,” said Horton. “There are options for managing that risk, both through greenhouse gas mitigation policies like the U.N. is currently considering, and through approaches to reducing society's vulnerability to extreme temperature events.”

Co-authors of “Contribution of Changes in Atmospheric Circulation Patterns to Extreme Temperature Trends” include Nathaniel Johnson of Princeton University; Deepti Singh and Daniel Swain, Stanford graduate students in Earth system science in Stanford’s School of Earth, Energy & Environmental Sciences; and Bala Rajaratnam, an assistant professor of statistics and Earth system science.