Since the turn of the millenium, we’ve been struck by some truly notable and very damaging heat waves. Probably foremost in the memory of those who follow such things are the 2003 European heat wave, 2010 in the Moscow region, and just this year in the U.S.
Those heat waves are very hot summertime events. But depending on what one calls a “heat wave,” it can strike at any time of year. The World Meteorological Organization defines a heat wave as when “the daily maximum temperature of more than five consecutive days exceeds the average maximum temperature by 5 °C (9 °F), the normal period being 1961–1990.” By this definition they can happen at any location during any season, even wintertime in Antarctica.
A different approach is to define a heat wave by an absolute rather than relative temperature threshhold. Wikipedia reports:
In 1900, A. T. Burrows more rigidly defined a “hot wave” as a spell of three or more days on each of which the maximum shade temperature reaches or exceeds 90 °F (32.2 °C). More realistically, the comfort criteria for any one region are dependent upon the normal conditions of that region.
Heat waves are one of the most dangerous aspects of global warming, as is emphasized in a new report from the World Bank. The danger is well illustrated by showing the impact of the 2003 European heat wave on daily mortality in France. Here’s the excess mortality, i.e., the number of deaths above what would otherwise be expected, along with a plot of daily high and low temperature (in Celsius):
Temperatures above 30°C (86°F) are associated with increased mortality, so the previously mentioned threshold of 90°F (32.2°C) is a reasonable level to indicate enhanced heat stress, at least in this region. Note also that during the peak of the heat wave when daily high temperature hovered around 35°C (95°F), mortality increased steadily in spite of temperatures remaining relatively constant. This illustrates that the longer a heat wave lasts, the deadlier it becomes. At its worst, this heat wave caused an extra 2200 deaths per day in France.
To determine the frequency and duration of heat waves, I took daily high temperature data from the European Climate Assessment and Dataset Network, then noted the occurence of heat waves according to the above definition (at least 3 days with temperature of at least 32.2°C). To gauge the extremity of heat waves, I computed “heat wave degree-days” as the accumulated total excess temperature (above 32.2°C) throughout each heat wave. Then I computed both the number of heat wave days, and the total heat wave degree-days, for each year, for every station which had at least 30 years of data with at least 300 daily observations per year.
I also restricted the analysis to stations which had actually had a minimum of heat waves (which leaves out Greenland for instance), requiring at least one heat wave during at least two years, and at least two heat waves during at least one year.
The results show just how unusual the 2003 European heat wave was. Here, for instance, is the number of heat wave degree-days from the data for Carcassonne, France:
Note how total heat wave degree-days during 2003 dwarfs that during any other year. The same is true at Nimes:
There’s something else to pay heed to: that although 2003 is the only astounding heat wave year observed at these locations, nonetheless heat wave activity has remained elevated. Even if we eliminate 2003 from these data records there’s still an increase in heat wave activity — and the extremity of 2003 cannot be denied.
The 2003 heat wave didn’t just affect France. It’s also evident in data from Spain:
and from Italy:
One of the most astounding data sets is that from Milan, Italy. It stretches back to the 1700s and shows, not just how remarkable was the 2003 heat wave, but how heat wave activity has “taken off like a bat out of hell” since 2000:
Like a bat out of hell, indeed.
As unprecedented as was the 2003 European heat wave, the Russian heat wave of 2010 was even more remarkable. It hit hard — very hard — in areas that don’t (or should I say “didn’t”) usually suffer from heat waves, like Moscow:
Some areas which had no previous heat waves on record have changed that since 2000, such as Voru in Estonia, which not only suffered from heat waves in 2010 but also in 2007 and 2001:
Even some areas which do regularly experience heat waves have seen them get worse since 2000, with 2010 a monster year:
I emphasize that the heat wave record isn’t just about the monsters of 2003 and 2010. It’s also about singificant increase over time, especially during the “modern global warming era” since about 1975. Here, for instance, is the data from Tortosa, Spain:
Bearing in mind that the risk from heat waves involves not just their extremity but their duration, note that the number of heat wave days has also steadily increased at this location:
They used to average fewer than 10 heat wave days per year. Now it’s over 50.
A total of 468 of the ECA data sets met the data selection criteria (minimum amount of data and of heat waves). I tested for statistically significant change in heat wave degree-days, albeit rather crudely, by linear regression on both the number of heat wave degree-days, and on the rank of heat wave degree-days. Of the 468 total, 15 showed a significant decrease in heat wave degree-days. That’s 3.2% — but in fact you would expect 2.5% to do so, simply by random fluctuation, for a test at 95% confidence — so the result is perfectly consistent with nothing more than random variation on a large number of samples. However, 197 out of 468 — fully 42% — showed significant increase in heat wave degree-days. That’s a helluva lot more than you could possibly get from random fluctuation.
As bad as it is already, the heat wave situation is going to get worse. Far worse. In the future, according to projections, most of the world will see a huge increase in the number of 3-sigma hot summers by 2080 (top panels):
Under “normal” conditions, a “3-sigma” hot event would only happen in 1 out of 740 years. Imagine a future in which most of the world suffers what used to be a once-in-ten-lifetimes hot summer, most years.
Perhaps most troubling is the large area that will experience 5-sigma hot summers. That’s something that under “normal” conditions would only happen once out of 3.5 million years. We’re headed for a future in which, for large areas of our planet, what used to be a once-in-a-million-years hot summer has become the “new norm.”
We’re headed into winter in the northern hemisphere, so temperatures are dropping and we’re preparing for the oncoming cold. That makes it all too easy to forget the heat waves we’ve suffered in the recent past, and to ignore the danger they bring. The dead remember.
Let’s not forget the risk of heat waves until the next one hits, who knows where, causing more thousands of unnecessary deaths.