The increase in simultaneous heatwaves

THIS WEEK record-breaking temperatures of more than 40°C were forecast in Britain. Schools closed and hospitals cancelled routine appointments. Trains ran less frequently for fear the tracks would buckle. The Royal Air Force had to rearrange flights from its biggest air base after the runway melted. In mainland Europe, things were bleaker still. After weeks of drought, a heatwave sparked wildfires in France, Greece, Portugal and Spain. Parts of America are slogging through one of their hottest summers ever. And swathes of eastern and southern China have seen unusually high temperatures in recent weeks. Why are all of these places suffering extreme heat at the same time?

Climate change has made heatwaves more frequent, and more brutal. Very hot temperatures that were once rare occur more frequently and heatwaves that were once impossible come to pass. The temperatures in Britain this week fit into the second category, says Richard Betts, a climate researcher with the British Met Office. Similarly, modelling carried out by the World Weather Attribution group, a research coalition, found that last year’s unprecedented heatwave in the Pacific Northwest would have been “virtually impossible” without climate change.

New records will be set as long as humans continue to burn fossil fuels and destroy carbon-storing ecosystems. This year’s sweltering summer comes in a world that is, on average, 1.1-1.3°C warmer than it was in pre-industrial times. Only the most optimistic assumptions about global action to reduce emissions result in projections of 1.5°C by 2100. In practice, much higher levels of warming are likely.

Heatwaves are also happening in more places simultaneously. A study published by American and Australian researchers this year found that between 1979 and 2019, the average number of days with simultaneous heatwaves in the mid to high latitudes of the northern hemisphere increased from around 20 to 143—about seven times more. The intensity and extent of these concurrent events also increased.

There are two explanations for this. First, greater odds of a heatwave in any given region at any given time makes simultaneous events more likely. Second, the atmospheric conditions that cause two separate events may be linked. For instance, the same study found that certain patterns of distribution were cropping up more frequently: concurrent heatwaves in North America, Europe and central and eastern Asia appear more often than is easily explained by warming alone. Other studies have suggested that climate change may be changing the undulations in the jet streams—the air currents in the upper atmosphere that move weather patterns from west to east—in a way that causes extreme heat to hit more places at once.

Whatever the cause, the consequences are disastrous. Heatwaves kill people and exacerbate existing health conditions. They also cause productivity to plummet. The impacts can cascade when multiple areas are affected. Consider food production. Heatwaves can cause crops to suffer, as happened this spring in India. In three northern states, yields probably fell by between 10-35%. If the failure is limited to one region, others can normally make up the shortfall. But if several “breadbasket” areas are affected there will be worldwide shortages, pushing up prices. Disruption caused by the war in Ukraine has shown that such problems can drive inflation and whip up unrest throughout the world.

Other systems are at risk, too. Heat can disrupt everything from manufacturing to transport. Last week factories in Zhejiang province in China were forced to ration power to ensure enough supply for air-conditioners in nearby homes. Drought in Europe has left the Rhine river, a major trade route, inches away from being too shallow for barges carrying commodities to pass through it. Heatwaves often stretch energy grids to breaking point. In places where electricity is moved across borders an influx of power from somewhere not affected can help stabilise the grid. But simultaneous heatwaves make that tricky. In an interconnected world, regions rely on each other to cope with disaster. That becomes much harder when the disasters are happening in many places at once. ■

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