diby Judith Curry
Western Europe is on track for a potential record breaking heat wave. Should any of this be blamed on human-caused global warming?
Details on the European heat wave can be found here: Significant, Prolonged Heat Wave to Blanket Europe Over the Next Week
The Telegraph warns that heat wave could buckle train tracks and melt roads [link]
So what is causing the heat waves? Are they increasing in severity/duration/frequency?
A new paper from Stanford University addresses some of these issues:
Contribution of changes in atmospheric circulation patterns to extreme temperature trends
Daniel E. Horton, Nathaniel C. Johnson, Deepti Singh, Daniel L. Swain, Bala Rajaratnam & Noah S. Diffenbaugh
Abstract. Surface weather conditions are closely governed by the large-scale circulation of the Earth’s atmosphere. Recent increases in the occurrence of some extreme weather phenomena have led to multiple mechanistic hypotheses linking changes in atmospheric circulation to increasing probability of extreme events. However, observed evidence of long-term change in atmospheric circulation remains inconclusive. Here we identify statistically significant trends in the occurrence of atmospheric circulation patterns, which partially explain observed trends in surface temperature extremes over seven mid-latitude regions of the Northern Hemisphere. Using self-organizing map cluster analysis, we detect robust circulation pattern trends in a subset of these regions during both the satellite observation era (1979–2013) and the recent period of rapid Arctic sea-ice decline. Particularly substantial influences include the contribution of increasing trends in anticyclonic circulations to summer and autumn hot extremes over portions of Eurasia and North America, and the contribution of increasing trends in northerly flow to winter cold extremes over central Asia. Our results indicate that although a substantial portion of the observed change in extreme temperature occurrence has resulted from regional- and global-scale thermodynamic changes, the risk of extreme temperatures over some regions has also been altered by recent changes in the frequency, persistence and maximum duration of regional circulation patterns.
Published in Nature [link]
The Daily Mail has an article on this study, excerpts:
For example, the type of summer weather pattern with a northeastern North American high pressure system that keeps it hotter than normal in the eastern U.S. used to happen about 18 days a summer in the early 1980s. It now occurs about 26 days a summer, the study found. ‘There are more of them each summer and on average they are lasting longer and the longest are lasting longer,’ Horton said. That pattern shift is even stronger in the summer in Europe and western Asia, Horton and co-author Noah Diffenbaugh found.
The patterns Horton and Diffenbaugh studied are different from the one responsible for the current southeastern U.S. heatwave, Horton said.
But the weather patterns were the type responsible for heatwaves that killed more than 50,000 people in western Russia in 2010 and more than 70,000 people Europe in 2003, the study said.
Diffenbaugh said the changes could be a result of random chance, or a side effect of climate change and melting sea ice as others have theorized.
See also Noah Diffenbaugh’s youtube discussion of the paper [link].
Because of the short time period of their analysis (since 1979), it is very difficult to attribute any trend in circulation patterns to AGW.
While in the UK attending the Hoskins@70 event, I heard a very interesting and relevant talk by Prashant Sardeshmukh of NOAA, entitled Extremes and climate change: some roadblocks to detection, attribution and projection. Prashant was kind enough to make his ppt presentation available to me for this blog post (their paper on this is under review).
The first part of his talk addresses the issue of circulation changes. A really important contribution is use of the 20th century reanalysis (going back to 1871), which is described in this previous post reanalyses.org. The findings of their analysis is summarized in this slide:
So only the PWC and AAO have long term trends over the period. Re the AAO, somewhere I spotted an article or presentation about a 300 yr oscillation in the Southern Ocean. At the recent Ringberg Workshop, Mojib Latif’s presentation also mentions Southern Ocean centennial variability. If anyone knows of more info on this, I would appreciate the links.
He then addresses issues related to attributing and predicting temperature extremes:
This issue was previously raised at CE in context of Greg Holland’s analysis How extreme can it get?
It is intuitively reasonable that as average temperatures warm, then the probability and intensity of heat waves should increase. Sardeshmukh writes:
Can we not be reasonably certain that 20th century global warming has resulted in increases of daily warm extremes and decreases of daily cold extremes in most regions of the globe ? Isn’t the mean shift (i.e. the mean warming) a useful guide in this context ?
However, this also depends on changes to the shape of the temperature distribution (skewness and dispersion). Sardeshmukh defines a new index:
In the 20th century reanalysis, the temperatures at the 850 hPa level (nominally 1.5 km above the surface) are more robust than surface temperatures, hence they are used to reflect changes in the temperature distributions:
The diagrams on the right shows that the temperature distribution are both negatively and positively skewed in different regions; consistent with Greg Holland’s analysis, the subtropical regions tend to have negatively skewed temperature distributions.
20th century changes in probabilities of extreme winter temperatures are shown in upper right quadrant. There is substantial regional variability in terms of more versus fewer extreme events. The regional variability looks all over the map; surprisingly the climate model simulations (second row) show similar regional variability.
I’ve stared at a blown up version of the upper right diagram for a long time, trying to make sense of it. Many of the most intense changes (dark blue or orange) are over the ocean. Dark blue over the Arctic Ocean makes sense; warming melts ice rather than increases extreme temps. Dark orange over Alaska makes sense since this is a cold region that has plenty of room to heat up for extremes. Dark blue over the tropical Pacific warm pool makes sense if more warming of this very warm region results in more evaporation rather than extreme warming.
Thermodynamics reasoning can only take you so far; the rest looks like dynamics or land use. The most intense orange blob is in South America; looks like the Amazon, no idea how to explain that one. Most of Europe is light blue, with a big blob of orange over France and Spain. South Asia looks pretty orange, with the exception of India.
The fact that changes in extreme anomaly risks cannot be deduced from the mean shiLs alone is not disturbing . . . . but entirely understandable in terms of basic weather dynamics and the Climate-‐Weather connection. Climate Models must adequately represent subseasonal atmospheric variability and its links with longer term changes to adequately capture the changes in the mean, width, and shape of the associated probability distributions. Currently they have difficulty in capturing even the mean changes in many regions. This is a challenge, but also an opportunity.
I find this to be a fascinating study; I hope that someone follows up and uses this methodology to analyze heat extremes using historical surface temperature data.
Bottom line is that the intuitively reasonable attribution of more heat waves to a higher average temperature doesn’t work in most land regions.
Looks like they need more air conditioning in Spain and France and also South Asia.
Does it make more sense to provide air conditioning or to limit CO2 emissions. I vote for more air conditioning in these susceptible regions. A good solution for South Asia is described in this previous post Tactical Adaptation to Indian Heat Waves.