Are heatwaves evidence that climate change is speeding up?

Normally highest in the southern summer (most of Earth’s water is in the south), temperatures are at record levels in the southern winter (Photo: Reuters)
Normally highest in the southern summer (most of Earth’s water is in the south), temperatures are at record levels in the southern winter (Photo: Reuters)


  • All sorts of records are being broken in all sorts of places

Estimates of Earth’s average temperature, having set a new record on July 3rd, have yet to fall back below the previous record, which was set just last year. That a run of very hot days should happen in July is, by itself, perhaps unsurprising. Two-thirds of the Earth’s land is in the northern hemisphere, and land warms up faster than water does, so northern summers are the hottest times of year for the planet as a whole. But the highest temperatures tend to come later in the season. That this year’s should start so early, rise so high and run so long is unprecedented.

So is what is happening in the oceans (see chart). Since March 13th the sea-surface temperature in low- and mid-latitudes has been higher than on the same day in any year since 1979. Normally highest in the southern summer (most of Earth’s water is in the south), temperatures are at record levels in the southern winter.

(Graphic: The Economist)
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(Graphic: The Economist)

Within the rising global averages lie savage peaks in particular places. On July 16th a site in the Turpan Depression in Xinjiang, sometimes called China’s Death Valley, reported a high of 52.2°C. In America, in Death Valley proper, the same day saw a peak of 53.9°C. Of more immediate concern than isolated spikes in deserts, temperatures have been dangerously high in places where hundreds of millions of people live, too. On July 6th, after the city measured its highest July temperature ever, authorities in Beijing announced their second red alert for heat in two weeks. July 19th marked the 19th day in a row that the temperature in Phoenix, Arizona, has exceeded 43°C. Things are similarly sweltering in Italy and many nearby countries (see map).

Life in the greenhouse

Asked how such a thing might be, one climate scientist replies drily “I suspect it might have something to do with accumulation of greenhouse gases in the atmosphere." More greenhouse gas in the atmosphere results in more of the warmth from the sun being trapped near the surface and absorbed by the oceans. The level of carbon dioxide, the most important long-lived greenhouse gas, as measured at Mauna Loa, a mountain peak in Hawaii, reached 424 parts per million in May, the highest it has been for over 3m years. Methane and nitrous oxide, two other long-lived greenhouse gases, have also reached levels never before experienced by humans. The world is now, on average, around 1.2°C warmer than it was before humans started thickening the glass in the greenhouse.

(Graphic: The Economist)
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(Graphic: The Economist)

The climate has natural variations, too, and the most famous of them, the El Niño Southern Oscillation (ENSO), is adding to the warmth. ENSO is a sloshing back and forth in the winds and currents of the tropical Pacific ocean which sometimes sees the waters suck up more heat, and sometimes sees them give more heat out. In June the world entered an “El Niño" phase, in which heat is released. The greatest effect of an El Niño on global temperatures tends to be seen after it has been in place for a year or so. But today’s ocean temperatures look like evidence of this one getting off to a flying start.

On top of these global effects, there is the fact that moving the top of a bell curve even a touch to the right can change the values in the tail a lot. According to James Hansen, a climate scientist at Columbia University, the sort of summer which would have been a once-in-a-century event between the 1950s and 1980s has become a once-every-five years event now. If sweltering summers are more likely everywhere, the chances of more than one region being affected at a time go up, too.

So are the thickening of the atmospheric blanket, an outpouring of heat from the Pacific and the random effects of year-on-year variation enough to explain this summer’s freakish temperatures? Or is there something more going on?

Dr Hansen thinks there is. He argues that the rate at which the world is warming seems to have gone through a step change in the 2010s, though he has not yet convinced his peers. This summer’s surprises, especially a run of record temperatures in the North Atlantic, might help change that. “I wouldn’t be surprised if we see papers appearing over the next few years saying [the Atlantic anomaly is] more than just another extreme," says Myles Allen, a climate modeller at Oxford University.

Several things could be speeding up warming. One is the change to the stratosphere brought about by the eruption of Hunga Tonga–Hunga Ha’apai, a submarine Pacific volcano, in January 2022. This was the largest eruption on Earth since Mount Pinatubo, in the Philippines. In 1991 Pinatubo injected tens of millions of tonnes of sulphur-dioxide gas into the stratosphere, where it reflected some of the sun’s light. The result was a worldwide cooling of about 0.5°C that lasted about a year.

The Hunga eruption did not throw anything like that much sulphur into the stratosphere. But it did pump in a great deal of water vapour; between 70m and 150m tonnes. Water vapour is a powerful greenhouse gas. In the lower atmosphere it condenses out into rain or snow fairly quickly. In the stratosphere, though, it lingers for longer. The Hunga eruption is thought to have increased the amount of water vapour in the stratosphere by 13%. That would have warmed the planet—though if Hunga is playing a role, it is one that is already waning.

Other possible influences are waxing. When ice ages end, methane levels in the atmosphere shoot up, ushering in the warmer climate of the “interglacial" to come. Some scientists cite recent increases in methane levels as evidence that something similar may be afoot today. Methane levels rose throughout the 20th century, mainly because of the rising use of fossil fuels and agriculture. They flattened off at the beginning of the 21st century, but are now growing faster than ever.

Some of this is doubtless still because of farming and fossil fuels. But a paper by Euan Nisbet, an Earth scientist at Royal Holloway, and his colleagues, and recently accepted for publication in Global Biogeochemical Cycles, argues that not all the extra methane can be explained that way.

The researchers think that the surplus may be coming from the growth of tropical wetlands, whose plants produce the gas when they rot. This is one candidate for the mechanism that drives the methane spikes seen at the end of ice ages. If true, it opens up the possibility of a feedback loop starting today similar to the ones that seem to have operated in the past. More methane means more warming, which means more wetlands, and therefore more methane.

That idea is speculative, for now. Perhaps a more plausible culprit is falling emissions of sulphur. The burning of coal and heavy fuel oil produces a lot of sulphur dioxide. Once in the atmosphere that gas forms sulphate particles. These particles cause air pollution leading to hundreds of thousands of deaths every year. Environmental regulators have been trying to reduce sulphur emissions for decades.

But sulphate particles in the lower atmosphere reflect sunlight, just like those created in the stratosphere after volcanic eruptions. And, unlike those in the normally bone-dry stratosphere, particles lower down can help create clouds which reflect away more sunshine still. Controls on pollution mean that this climate-cooling side effect has been weakening.

Of particular relevance are new regulations on the sulphur content in shipping fuel that came into force in 2020. The regulations were brought in by the International Maritime Organisation on the basis of estimates that they would save around 40,000 lives a year. They are thought to have reduced sulphur emissions from shipping by more than 80%. The evidence is visible as a worldwide decline in “ship tracks", long, thin clouds created when sulphate particles in a ship’s exhaust provide nuclei around which water droplets can form. Fewer, fainter ship tracks and other clouds mean less sunlight is bouncing back out to space, and is instead being absorbed by the oceans below.

(Graphic: The Economist)
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(Graphic: The Economist)

The indirect effects that aerosol particles have on cloud cover are notoriously hard to capture in climate models. Estimates of how much cooling shipping pollution might have caused vary by a factor of ten. But Dr Hansen thinks the changes could plausibly explain most of the quicker warming that he sees in the data. From 1970 to 2010 the warming trend was 0.18°C a decade. Since around 2015, Dr Hansen thinks it has been between 0.27°C and 0.36°C per decade—between half as high again and twice as high. A study by Dr Allen and his colleagues published last year sees a similar increase in the trend, but warns that it may be strongly influenced by natural variability, with aerosol effects playing a much smaller role than that which Dr Hansen would assign them. “Quantifying the role of human influence in these apparently unprecedented events is hard," Dr Allen cautions.

A sweltering world might try to find a way to keep the cooling properties of sulphates without the drawbacks for air quality and health. In 2006 Paul Crutzen, an atmospheric scientist, suggested this might be done by continuously injecting small amounts of sulphur directly into the stratosphere. Since there is no rain to flush them out, high-flying stratospheric particles last much longer than those in the lower atmosphere.

That means that a few million tonnes of sulphur dioxide added to the stratosphere—technically quite plausible—could provide as much cooling as the 100m tonnes or so that humans dump into the lower atmosphere each year. And as with warming itself, its effect on extremes would be greater than its effect on averages. Unwelcome things in the tail of the distribution could be made a lot less likely.

Sunscreen for the planet

This idea, a form of “solar geoengineering", is controversial, and with good reason. Its effects on stratospheric chemistry cannot yet be predicted accurately. Of particular concern are what it might do to the ozone layer, which screens out a good deal of the sun’s harmful ultraviolet radiation before it reaches the ground.

Because solar geoengineering’s effects on rainfall, as well as temperature, would differ from place to place, a cooling tailored to the needs of one country might not be to the taste of others. Settling such disputes is beyond any current system of global governance. Above all, a technology that could cool the planet without ending fossil-fuel use might well slow or even scupper that phase-out.

So far these worries have carried the day. Research on solar geoengineering has been side-lined, and its possible role in climate policy has gone largely undiscussed. All those who take part in such discussions as there are stress that solar geoengineering should at best be seen as a complement to decarbonisation, shaving off extreme risks while the world moves towards a fossil-free economy. But the fear that it would instead be treated as an alternative is sufficiently persuasive as to be pervasive.

If 2023 is not an aberration, though, and the world really is moving into an accelerated phase of warming, that reluctance might be reassessed. Emissions reduction should be able to slow the warming of the Earth within a few decades. Pursued with real zeal, it might bring it to an end this century. But it provides no cooling in the meantime. If that proves to be what the world wants, solar geoengineering is the only thing which looks able to provide it.

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© 2023, The Economist Newspaper Limited. All rights reserved. From The Economist, published under licence. The original content can be found on

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