This year is on course to be the hottest on record, with an average global surface temperature more than 1.5°C above pre-industrial levels. But there is some better news: the overall amount of heat energy being gained by the planet has fallen sharply from a record spike early in 2023.
At the time, there were suggestions that the spike in heat gain showed there are serious flaws in climate models that mean they are underestimating how fast the planet will warm. But the fall since then makes this much less likely.
“Given the way that the numbers have evolved in the last year, it no longer looks like there’s anything dramatically wrong with the models,” says at the CICERO Center for International Climate Research in Oslo, Norway. “So it’s not the sort of potentially dramatic issue some people were saying it was a year ago.”
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The latest satellite data showing the fall in heat gain was by at NASA. “It doesn’t support a doomist narrative,” Schmidt .
There is a big difference between tracking the increasing impacts that climate change and habitat loss are having, and buying into a notion that everything is spiralling out of control and we no longer have agency, he said. “That (IMO) is not justified.”
Studies of global warming naturally focus on the surface temperature, which accounts for the air a couple of metres above the land or sea. But this thin layer of air in which we live is just a small part of the climate system, which includes the entire atmosphere and oceans.
To get a measure of how much the entire climate system is warming, climate scientists can look at how much of the sun’s energy enters the planet’s atmosphere versus how much leaves it. Some sunlight is reflected immediately by, say, clouds or ice. The rest is absorbed and may later be emitted as heat energy.
If as much energy is reflected or emitted back into space as hits the atmosphere, the planet doesn’t gain any heat. But because rising greenhouse gas levels are blocking heat emissions, the planet has been gaining more heat than it loses to space.
Since around 2001, Earth’s energy imbalance, as this difference is known, has been measured directly by instruments on satellites as part of a NASA project called CERES. Over this time, the average energy imbalance has more than doubled.
“It is on the high end compared to the models,” says at NASA, who leads the CERES project.
But the energy imbalance also varies due to factors such as La Niña and El Niño. The big spike in 2023 was a result of a rare “triple-dip” La Niña that continued for three winters, says Loeb.
During a La Niña, cold ocean waters spread across the Pacific, soaking up a lot more heat from the sun and atmosphere than they emit, which increases the energy imbalance. This La Niña then gave way to an El Niño, in which warm waters spread across the Pacific, emitting more heat and reducing the energy imbalance.
As far as Loeb is concerned, neither the spike in 2023 nor the decline since the El Niño developed are especially surprising.
“Had that [spike] continued, it would have very much looked like the real world was doing something which we weren’t seeing in any of the models,” says Sanderson. “As it actually turned out, it was a short spike, and we do see comparable spikes in the model data as well.”
That said, many questions remain to be resolved, he says. For instance, reductions in air pollution in many parts of the world are thought to have contributed to the rise in the energy imbalance. Aerosol pollutants reflect sunlight back into space, so less pollution from, say, shipping allows more sunlight to reach the planet’s surface. But there is a lot of uncertainty about the size of these effects.
Then there is the question of how the energy imbalance will change in the future. If greenhouse gas emissions remain at roughly the same level, rather than continuing to rise, the energy imbalance should stop rising too, says Sanderson.
There is a danger that we won’t be able to tell, says Loeb, because the number of satellites carrying CERES instruments is falling, and they aren’t being replaced. “It takes a long time from the time you start working on the satellite instrument until it actually launches,” he says. “So you have to really plan ahead, and I don’t think we’re doing a very good job on that.”
