January 31, 2007

European Heat Wave 2003: A Global Perspective

Filed under: Climate Extremes, Heat Waves

Although the event occurred over three years ago, the summer heat wave of 2003 is still prominently featured in every popular presentation of the global warming issue. A web search of “Europe Heat Wave 2003” produces nearly 950,000 sites to choose from, and if you take that plunge, you will see estimates of 35,000 deaths directly attributed to that heat wave, although that number varies considerably from one site to the next. Although the number of deaths may vary, virtually every one of the sites mentions global warming as an underlying contributor, and statements like “even more extreme weather events lie ahead” are commonplace in the thousands of essays on the topic. Not surprisingly, many of these thousands of heat wave articles end with something like “the world must cut the carbon dioxide emissions that contribute to global warming.”

We have covered heat waves many times in the past at World Climate Report and shown that the link between extreme heat waves and global warming (or, at least, increasing death) is not nearly as strong as we are led to believe. An article in the recent issue of Geophysical Research Letters dares to ask the question “Was the 2003 European summer heat wave unusual in a global context?” We saw that title and new this was going to be good.

A team of scientists from Colorado and France begin by noting “The European heat wave of summer 2003 has received considerable attention, both because of a potential link to larger scale warming patterns (e.g., “global warming”), and the large loss of life. Several studies find that this regional heat wave was quite unique and it has been suggested that such an extreme event could be accounted for only by a shift of statistical regime to one with higher variance.” Basically, others have argued that the climate has changed due to the buildup of greenhouse gases, variability has increased, and this increase in variability made the heat wave of 2003 more likely. The Chase et al. team decided to test this pillar of the greenhouse crusade, and as you might suspect, their findings are not going to make them popular with the greenhouse crusade.

The team collected data on the temperature of the atmosphere from throughout the world for the surface to 500 mb (half way up in the atmosphere) for the period 1979-2003. For each month, they computed the mean and standard deviation (SD) of the temperature thereby allowing them to map temperature anomalies in terms of standard deviations above or below the mean.

As seen in Figure 1 below for June, July, and August of 2003, Europe was definitely ground-zero for what is certainly an extreme event. However, it is interesting that far more than half the planet is portrayed in green tones indicating below normal temperature anomalies at that time. Europe was simply located in the wrong place at the wrong time, but it is immediately obvious that the heat wave was anything but global in nature.

Figure 1. 1000–500 mb thickness temperature anomaly for June, July, and August 2003. Areas exceeding 2.0, 2.5, and 3.0 standard deviations from the 1979–2003 mean are contoured in thick lines for anomalies of both sign (from Chase et al., 2003).

Chase et al. analyzed the anomalies for all parts of the globe for the 25-year study period and conclude “Extreme warm anomalies equally, or more, unusual than the 2003 heat wave occur regularly.” Of course, they rarely appear in summer, directly over Europe, where many residents do not have the defense of air conditioning. They also note “Extreme cold anomalies also occur regularly and can exceed the magnitude of the 2003 warm anomaly in terms of the value of SD.” Of course, cold anomalies are tougher to sell to the public in terms of linking to global warming, so they are not nearly as well publicized, but they are definitely in the record.

The global warming crowd will be thrilled to know that “There is a correlation between global and hemispheric average temperature and the presence of warm or cold regional anomalies of the same sign (i.e., warmer than average years have more regional heat waves and colder than average years have more cold waves).” This could make headlines – warm years tend to have heat waves and cold years tend to have cold waves!

The next two conclusions might give warming advocates reasons to sweep the Chase et al. piece under the rug. The team found “Natural variability in the form of El Niño and volcanism appears of much greater importance than any general warming trend in causing extreme regional temperature anomalies as regional extremes during 1998 in particular were larger than the anomalies seen in summer 2003 both in area affected and SD extremes exceeded.” They show that in 2003, only 2% of the planet had temperature anomalies above two standard deviations, but in the big El Niño year of 1998, nearly 30% of the planet had temperature anomalies above two standard deviations. If we look at three standard deviations above normal temperature for the year as a whole, 1998 had over 5% of the planet covered, 2003 had 0% of the planet at that level.

There is even more bad news for the global warmers. Chase et al. examined the trends in the data over the 25 years and found “Regression analyses do not provide strong support for the idea that regional heat or cold waves are significantly increasing or decreasing with time during the period considered here (1979–2003).” Sorry, but there is no evidence that things are getting worse when a global-scale analysis is conducted. They conclude that “our analysis does not support the contention that similar anomalies as seen in summer 2003 are unlikely to recur without invoking a non-stationary statistical regime with a higher average temperature and increased variability.” They basically show that heat waves like the one in Europe in 2003 can occur by chance even if temperature does not rise or the variability of temperature does not increase.

There is no question that the heat wave of 2003 was a natural disaster in Europe with a substantial loss of human life. Europe was not prepared for an event that, from a purely statistical view point, was inevitable, with or without global warming.


Chase, T. N., K. Wolter, R. A. Pielke Sr., and I. Rasool, 2006. Was the 2003 European summer heat wave unusual in a global context? Geophysical Research Letters, 33, L23709, doi:10.1029/2006GL027470.

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