In 2015, in Paris, the nations of the world committed themselves to trying their best to prevent the planet warming by more than 1.5°C from its pre-industrial state. Even at the time, the goal looked ambitious. In recent years, it has come to seem almost impossible.
On May 17th the World Meteorological Organisation (WMO), an arm of the United Nations, added to the gloom. It said there was a 66% chance that the world would exceed the 1.5°C threshold in at least one of the next five years. That is a big jump from its estimates of even a year ago, when the WMO assessed the likelihood at 48%. Even if the 1.5°C target is not breached, the WMO thinks it is virtually certain that one of the coming five years will be the hottest in human history. (That record is now held by 2016, which was 1.28°C warmer than the pre-industrial average.) “It’s the first time in history that this level of global temperature is within reach,” said Adam Scaife of Britain’s Met Office, whose data and calculations are central to the WMO’s report. “It shows we’re getting very, very close to the Paris threshold.”
Optimists point out even if the 1.5°C threshold is breached in the coming half-decade, temperatures will likely fall back again, at least for a while. The Paris agreement would not, technically be breached. (That would require exceeding 1.5°C for several years.) Over the next few years the growing level of human-driven warming will be amplified further by natural, but transient, changes.
The biggest of those variations is the El Niño Southern Oscillation (ENSO), a natural cycle of warming and cooling in the waters of the eastern Pacific ocean that has widespread effects on the climate. The world has just seen three consecutive “La Niña” years, the name given to the cooler phase of the cycle, helping hold global temperatures down. It now seems almost certain that a warmer “El Niño” phase will begin sometime later this year, setting up 2024 to be a scorcher. (One reason for the 2016 record is that the year coincided with an especially strong El Niño.)
But the ENSO is not the only factor. An additional temporary nudge could come from the eruption last year of the Hunga Tonga-Hunga Ha’apai volcano, near Tonga. It was one of the biggest eruptions since that of Mount Pinatubo in the Philippines in the 1990s, and injected an estimated 146m tonnes of water vapour into the stratosphere. Water vapour, like carbon dioxide, is a greenhouse gas. Unlike carbon dioxide, it will gradually fall out of the stratosphere over the next few years. But Stuart Jenkins, a climatologist at Oxford University, reckons that while it persists it could increase the odds of passing 1.5°C by a few percentage points.
Some tentatively encouraging signs gleam amid the gloom. Global emissions of greenhouse gases from fossil fuels seem to have gone sideways for several years, leading some researchers to speak cautiously of a possible peak. Rystad Energy, a Norwegian think-tank, predicted earlier this year that global emissions of carbon dioxide from industry could peak in 2025 and then begin a slow decline.
Even so, the world’s actions still fall far short of its promises. Even the upper end of the goals agreed in Paris—of limiting warming to “well below 2°C”—will be achievable only with drastic action. For Europe and America to meet their commitments, for instance, would require them to switch off all their fossil-fired power plants within the next three decades.
And simply stopping emissions will not be enough. Somewhere between 3.5bn and 5.4bn tonnes of carbon dioxide will need to be sucked out of the atmosphere every year, rising to 4.7bn to 9.8bn tonnes within 30 years. All that is a big ask, to put it mildly. But the optimist’s take might be that the psychological impact of breaching the 1.5°C goal, even if only temporarily, could help focus minds. ■
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This article appeared in the Science & technology section of the print edition under the headline "Mercury rising"
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