The Planet Has a Fever

July 31, 2020

By Marco Tedesco

An increase in heat waves is among the most predicted consequences of a world that continues to overheat at a dizzying rate. The consequences of heat waves range from reduced worker productivity (especially those who work outside regularly), higher electricity use due to, for example, increased use of air conditioners, and, unfortunately, an increase in heat-related deaths.

Despite the importance of this topic, until a little while ago there was no global map of heat waves showing heatwave changes at a regional level. Such a map was recently published in an article in Nature Communications showing how, since the 1950s, heat waves have increased in almost every part of the world, both in frequency and in duration.

maps showing heat wave trends

Trends in seasonal heatwave days (a, b); length of longest heatwave (c, d); average heatwave intensity (e, f); and cumulative heat (g, h) over the period 1950−2014. Source: Nature Communications

The researchers also produced a new metric, called cumulative heat, which reveals precisely the amount of heat contained in the individual waves of heat during the different seasons. “Not only have we seen more and longer heatwaves worldwide in the past 70 years, but this trend has markedly accelerated,” said study co-author Sarah Perkins Kirkpatrick from the ARC Center of Excellence for Climate Extremes. “Cumulative heat shows a similar acceleration, increasing globally on average by 1°C – 4.5°C each decade but in some places, like the Middle East and parts of Africa and South America, the trend is up to 10°C a decade,” she continued.

In the worst heat wave season in Australia, an additional 80°C of cumulative heat occurred across the country. In Russia and in the Mediterranean, their most extreme seasons have simmered in an extra 200°C or more of cumulative heat.

The only metric that hasn’t seen an acceleration (but it has still been growing) is the intensity of heat waves, which is strongly associated with the average temperature. This is not because things have not gotten worse, but because globally there have been several days of heat waves and, at the same time, the heat waves lasted longer. In synthesis: Taking the average temperature across more days can hide extreme shifts in temperature. Only southern Australia and small areas of Africa and South America showed an increase in the average intensity of heat waves.

The study also focused on the potential impact of natural variability, showing that it is necessary to use more than a decade’s worth of data for results to be reliable. For this reason, they focused on a period that extends for about 70 years, from 1950 until 2017.

The results showed that in the Mediterranean, the number of days of heat waves has grown by about two days per decade between 1950 and 2017. This value grows up to about 7 days per decade when considering the period from 1980 to 2017. Regions such as the Amazon, northeastern Brazil, western Asia and the Mediterranean are experiencing rapid changes in heat waves, while areas such as Southern Australia and North Asia continue to see changes but at a slower pace.

Regardless of the pace of the change, nations with reduced infrastructure will be hit hardest by the extreme heat. In Gordon’s words: “The dramatic region-by-region change in heat waves we have witnessed over the past 70 years, and the rapid increase in the number of these events, are unequivocal indicators that global warming is now with us and accelerating. This research is just the latest piece of evidence that should act as a clarion call to policymakers that urgent action is needed now if we are to prevent the worst outcomes of global warming. The time for inaction is over.”

Marco Tedesco is a research professor at Columbia University’s Lamont-Doherty Earth Observatory.

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