Extreme Weather Heat Waves

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Otto, F.E.L., Massey, N., van Oldenborgh, G.J., Jones, R.G. and Allan, M.R. 2012. Reconciling two approaches to attribution of the 2010 Russian heat wave. Geophysical Research Letters 39: L04702 doi:10.1029/2011GL05042.

The 2010 Russian heat wave was an extra-ordinary event which killed an estimated 55,000 people during July and early August of that year. The heat wave (and associated drought) also led to crop reductions in Russia by 25%, with an estimated loss in revenue of about 25 billion US dollars. Such an exceptional climate event has prompted many scientists and climate modelers in the US and UK to analyze the large-scale atmospheric sequence of events that led to this exceptionally hot spell in western Russia and, in particular, in and around the city of Moscow, all in attempt to determine if its cause was of natural or anthropogenic origin. In an earlier paper (see Dole et al., 2011) concluded that the 2010 summer Russian heat wave was mainly due to “natural internal atmospheric variability that produced and maintained an intense long-lived blocking event,” further adding that “it is very unlikely that the ‘warming’ attributable to increasing greenhouse gas concentrations contributed substantially to the heat wave.” Other scientist however, have come to a different conclusion.

Rahmstorff and Coumou (2011), for example, developed a mathematical model to quantify the effect of long-term trends on the expected number of extremes in generic time series, using analytical solutions and Monte Carlo simulations. The mathematical model then used mean temperatures for July over Moscow to show that the local and regional warming trends experienced there have substantially increased the probability of extreme records like the Russian heat wave. And contrary to the Dole et al. study, Rahmstorf and Coumo linked the 2010 exceptional heat wave over Russia to recent global warming.

A more recent model-based analysis on the subject comes from Otto et al. (2012), who examined the results from a large ensemble simulation to compare the return period of a 2010-like heat wave. Specifically, they compared the return period for a 1200-member ensemble of model runs for the 2000s with the return period of such an event in a 1600-member ensemble for the 1960s using temperature/geopotential anomalies. The comparison suggested that “in the 1960s a 2010-magnitude heat wave was to be expected every 99 years, while in the 2000s this has changed to every 33 years.” Using these simulation results, Otto et al. conclude that there is no contradiction between the studies by Dole et al. using natural variability and the mathematical modeling study by Rahmstorf and Coumou, since both the studies point to “a higher probability” of a 2010-like Russian heat wave occurring in the recent decade when compared to the decade of the 1960s.

However, the two model-based studies described above are of limited utility in predicting future heat waves and their spatial and temporal distribution, since these studies do not make a comprehensive historical analysis of heat waves over various regions of the world. In the conterminous U.S. (and possibly for the entire North America), for example, the 1930s were the hottest years of the twentieth century yet a number of heat waves occurred during that time. The 1950s and the 1980s also saw a many heat waves throughout the U.S. and Canada, while the most recent decade (of the new millennium) witnessed relatively fewer heat waves.

Elsewhere, Southern Europe (Italy and Spain in particular) experienced severe heat waves in the 1940s and in the early 2000s. In the monsoonal climate of India, heat waves of a week to longer duration were observed many times throughout the twentieth century. Such heat waves often develop during the pre-monsoon months because of a delay in the arrival of monsoonal rains, and as such, are not likely linked to (anthropogenic) warming of the climate. In the Southern Hemisphere, parts of Australia have also witnessed heat waves during the 1930s, 1980s, and in the recent decade. Several U.S. researchers have also analyzed heat waves and their mechanism over the conterminous U.S. In an important paper, Namias (1982) shows how the protracted heat wave on the U.S. Great Plains in the summer of 1980 was caused due to ‘an abnormal form of (atmospheric) general circulation’ which developed in response to significantly colder waters of the North Pacific.

In summary, although recent model-based studies suggest recent anthropogenic climate warming is the most probable reason for the Russian heat wave of 2010, they fail to adequately explain the proper mechanism for many heat waves of the historic past, which could not have been forced by recent anthropogenic warming of the climate. As a result, any linkage between future heat waves and present climate warming remains questionable at present.

Additional References
Dole, R., Hoerling, M., Perlwitz, J., Eischeid, J., Pegion, P., Zhang, T., Quan, X.-W., Xu, T. and Murray, D. 2011. Was there a basis for anticipating the 2010 Russian heat wave? Geophysical Research Letters 38: 10.1029/2010GL046582.

Namias, J. 1982. Anatomy of Great Plains protracted heat waves (especially the 1980 US summer drought). Monthly Weather Review 110 824-838.

Rahmstorf, S. and Coumou, D. 2011. Increase of extreme events in a warming world. Proceedings of the National Academy of Sciences (USA) 108: 17,905-17,909.