Posted on 4 July 2015 by John Abraham

Just this week, a new article appeared in the journal Nature that provides more evidence of a connection between extreme weather and global warming. This falls on the heels of last week’s article which made a similar connection. So, what is new with the second paper? A lot.

Extreme weather can be exacerbated by global warming either because the currents of atmosphere and oceans change, or it can be exacerbated through thermodynamics (the interaction of heat, energy, moisture, etc.). Last week’s study dealt with just the thermodynamics. This week’s study presents a method to deal with both.

The authors, Daniel Horton, Noah Diffenbaugh and colleagues used a new technique to tease apart the complex influences of warming on changes to atmospheric circulation. Dr. Horton told me,

Our study focuses on the need to understand the underlying physical causes of extreme weather events, and to systematically test whether the probability of those underlying conditions has changed in recent decades. Events that are so extreme that they fall outside of our historical experience often result from a suite of complex interacting factors. To better understand these factors we’ve developed a method that allows us to partition the climate influences.

In particular, the authors focused on pressure levels up into the atmosphere (heights of approximately 5 km) from 1979 onwards. Those patterns gave information about atmospheric circulation. The authors grouped the patterns, using seven geographical regions (Europe, Western Asia, Central Asia, Eastern Asia, Western North America, Central North America, and Eastern North America) and four different periods of the year (winter, spring, summer and fall).

They separated changes in circulation from changes in thermodynamic effects. What they found is that most regions have seen increases in summertime warm temperatures in the past three decades. Furthermore, they found that in some regions, a large part of this trend is due to the increases in anticyclonic circulation and atmospheric blocking. The blocking that has been associated with extreme swings of weather (bringing very warm weather to the Western USA and simultaneous cold weather to the east for instance). 

The authors show that as the Earth warms, we expect fewer cold temperature events generally. But, in some cases the circulation changes has led to extreme cold outbreaks in some regions. What has happened is that the arctic front, which typically confines cold weather to the Arctic region, has undulated sufficiently to allow cold-air breakouts to the south. Think of the polar vortex from last year. 

These findings support the commonly-heard term that has emerged in the past few years of “weather whiplash - wild swings from one extreme to another. Importantly, the authors show that the trends are “statistically significant” and are unlikely just random occurrences.

That said, the authors clarify,

The majority of the observed changes in extreme temperature occurrence have resulted from changes in the heat content of the climate system. 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. 

It’s important to note that the authors do not explicitly attribute the trends to human causes or natural causes. The authors state clearly that we need a deeper understanding of the causes of the trends they’ve found. In particular, a future step will be to separate human-causes from natural variability in the climate on the decadal scale. At the same time, they write,

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