Estimates about the average temperature of the planet, after the new record reached on July 3, have not yet fallen below the previous record, established just a year ago. It may come as no surprise that a series of very hot days in a row occurs in July. Two-thirds of the earth’s surface is in the northern hemisphere, and land warms faster than water; so northern summers are the hottest for the planet as a whole. However, the highest temperatures usually arrive later in the season. What is unprecedented is that this year’s series of consecutive hot days started so early, reached such high temperatures, and lasted so long.
This is what is happening in the oceans (see graph). Since March 13, sea surface temperatures in low and mid-latitudes have been hotter than on the same day of any year since 1981. Normally hotter in the austral summer (most of Earth’s water is in the south), temperatures are now reaching record levels in the austral winter.
Within the trend of increasing world averages, wild peaks occur in specific sites. On July 16, a spot in Xinjiang’s Turfan Depression, sometimes called the Chinese Death Valley, recorded a high of 52.2°C. In the United States, in Death Valley itself, 53.9°C was reached that same day. More worrisome than isolated spikes in deserts are dangerously high temperatures in areas where hundreds of millions of people live. On July 6, after Beijing registered the highest temperature in that month in its history, the city authorities announced the second red alert for heat in a period of two weeks. July 19 marked the 19th day in a row that the temperature in Phoenix, Arizona has exceeded 43°C. And the temperatures are equally sweltering in Italy and many neighboring countries (see map).
When asked how something like this can happen, a climatologist sardonically replies: “I suspect it may have something to do with the buildup of greenhouse gases in the atmosphere.” More greenhouse gases in the atmosphere cause more of the sun’s heat to be trapped near the surface and absorbed by the oceans. The level of carbon dioxide, the most important of the long-lived greenhouse gases, measured at Mauna Loa, a Hawaiian volcano, reached 424 parts per million in May, the highest figure for more than 3 million years. . Methane and nitrous oxide, two other long-lived greenhouse gases, have also reached levels never experienced by humans. Today, the planet is, on average, 1.2ºC warmer than it was before humans began to increase the thickness of greenhouse glass.
The climate also suffers natural variations; and the most famous of these, the El Niño/Southern Oscillation (ENSO), contributes to warming. ENSO is a fluctuation in the winds and currents of the tropical Pacific Ocean that causes the waters to sometimes absorb more heat and sometimes to expel it. In June the world entered an El Niño phase in which heat is released. The greatest effect of El Niño on global temperatures is usually observed after it has been in effect for a year or so. However, current ocean temperatures seem to indicate that this time it hasn’t waited as long.
Added to these global effects is the fact that shifting the top of a Gaussian bell to the right, even very small, can greatly change the values ??of the tail. According to James Hansen, a climatologist at Columbia University, the kind of summer that would have been a once-in-a-century phenomenon between the 1950s and 1980s has now become a once-in-five-year event. If sweltering summers are more likely everywhere, the chances of more than one region being affected at the same time also increase.
So, are a thickening atmospheric blanket, the influx of heat from the Pacific, and the random effects of year-on-year variations enough to explain this summer’s freaky temperatures? Or is there something else?
Hansen thinks so, that there is something else. Although he has not yet convinced his colleagues, he argues that the rate at which the planet is warming appears to have undergone a radical change in the 2010s. The surprises of this summer and, in particular, the wave of record temperatures in the North Atlantic could help change the situation. “I wouldn’t be surprised if scientific papers appear in the next few years claiming that [the Atlantic anomaly is] more than just another extreme event,” says Myles Allen, a climate modeller at the University of Oxford.
Several factors could be accelerating warming. One is the change in the stratosphere caused by the January 2022 eruption of the Hunga Tonga-Hunga Ha’apai, an underwater volcano in the Pacific. It was the planet’s largest eruption since Mount Pinatubo in the Philippines in 1991. Pinatubo injected tens of millions of tons of sulfur dioxide into the stratosphere, where it reflected some of the sunlight. The result was a global cooling of about 0.5°C that lasted for about a year.
The Hunga eruption did not spew nearly as much sulfur into the stratosphere, but it did pump out a large amount of water vapor, between 70 and 150 million tons. Water vapor is a potent greenhouse gas. In the lower atmosphere it quickly condenses as rain or snow. However, in the stratosphere it remains longer. The Hunga eruption is believed to have increased the amount of water vapor in the stratosphere by 13%. That would have caused global warming; Now, if so, that role would already be dwindling.
And other possible influences grow. At the end of the ice ages, atmospheric methane levels skyrocket, giving rise to the warmer climate of the next “interglacial” period. Some scientists cite recent increases in methane levels as proof that something similar may be afoot today. Methane levels increased throughout the 20th century; mainly due to the increasing use of fossil fuels and agriculture. They stabilized at the beginning of the 21st century, but now they are increasing faster than ever.
There is no doubt that a part of the increase is due to agriculture and fossil fuels. However, a paper recently accepted for publication in Global Biogeochemical Cycles by Euan Nisbet, an earth scientist at Royal Holloway, University of London, and colleagues, argues that not all of the additional methane would be explained in this way.
The researchers believe that the surplus may come from the growth of tropical wetlands, whose plants produce the gas when they rot. This is a possible candidate for the mechanism that would explain the methane peaks that are observed at the end of the ice ages. If so, the possibility opens up that we are now facing a feedback loop similar to those that seem to have operated in the past. More methane means more warming, which means more wetlands, and therefore more methane.
The idea is, for now, a mere speculation. Perhaps a more likely culprit is declining sulfur emissions. Burning coal and heavy fuel oil produces a lot of sulfur dioxide. Once in the atmosphere, that gas forms sulfate particles. These particles cause air pollution that causes hundreds of thousands of deaths a year. Environmental regulators have been trying to reduce sulfur emissions for decades.
Now, the sulfate particles in the lower layers of the atmosphere reflect sunlight, just like those created in the stratosphere after volcanic eruptions. And unlike those in the typically very dry stratosphere, particles lower down can help create clouds that reflect more of the sun’s light. Pollution controls have weakened that cooling side effect of the climate.
Of particular relevance are the new regulations on sulfur content in marine fuel that came into force in 2020. The International Maritime Organization introduced these regulations based on the estimate that it would save some 40,000 lives a year. The move is believed to have reduced sulfur emissions from shipping by more than 80%. The proof is the worldwide decline in “condensation contrails” from ships, long, thin clouds created when sulfate particles in ship exhaust provide nuclei around which water droplets form. . The fact that there are fewer contrails and other clouds means that less sunlight is returned to space and more is absorbed by the oceans.
The indirect effects of aerosol particles on cloudiness are very difficult to capture in climate models. Estimates of the cooling caused by maritime pollution vary by a factor of ten. However, Hansen believes that the changes could plausibly explain most of the faster warming that he sees in the data. Between 1970 and 2010, the warming trend was 0.18°C per decade. Since 2015 or so, Hansen thinks it has been between 0.27°C and 0.36°C per decade; that is, between half and double more. A study by Allen and colleagues published last year sees a similar increase in the trend, but cautions that it may be heavily influenced by natural variability, with aerosol effects playing a much smaller role than Hansen assigns to them. “It’s difficult to quantify the role of human influence in these seemingly unprecedented phenomena,” Allen cautions.
A sweltering world might search for a way to maintain the cooling properties of sulfates without their air quality and health drawbacks. In 2006, Paul Crutzen, an atmospheric scientist, suggested that this could be achieved by continuously injecting small amounts of sulfur directly into the stratosphere. In the absence of rain to wash them away, stratospheric particles last much longer than those located in the lower layers of the atmosphere.
That means that a few million tons of sulfur dioxide added to the stratosphere (in technical terms, a pretty doable thing) could provide as much cooling as the 100 million tons or so that we humans dump each year into the lower atmosphere. And, as with the warming itself, its effect on the extremes would be greater than on the average. Unpleasant events at the tail of the distribution would be much less likely.
That idea, a form of “solar geoengineering,” is controversial, and for good reason. We cannot yet accurately predict its effects on stratospheric chemistry. Of particular concern is what could happen to the ozone layer, which filters out much of the sun’s damaging ultraviolet radiation before it reaches the ground. Furthermore, since the effects of solar geoengineering on precipitation (as well as on temperature) would differ from place to place, cooling tailored to the needs of one country might not be to the taste of another. The resolution of such disputes exceeds any current system of global governance. Above all, a technology capable of cooling the planet without ending the use of fossil fuels could slow or even undo its phasing out.
So far, those fears have prevailed. Research on solar geoengineering has been sidelined and its potential role in climate policy has hardly been discussed. Everyone in those kinds of discussions insists that solar geoengineering should be considered, at best, as a complement to decarbonization to reduce extreme risks as the world moves toward a fossil fuel-free economy. However, the fear that it could be considered as an alternative is compelling enough to be widespread.
Still, if 2023 isn’t an aberration and the world does indeed enter an accelerated warming phase, that resistance might end up being reconsidered. Reducing emissions should be able to stop global warming in a few decades. If pursued with true zeal, it would be possible to put an end to it this century. Meanwhile, it does not provide any cooling. If that’s what the world wants, solar geoengineering is the only thing that seems capable of achieving it.
© 2023 The Economist Newspaper Limited. All rights reserved
Translation: Juan Gabriel López Guix
