This year is on course to be the hottest on record, with an average global surface temperature more than 1.5掳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.
鈥淕iven the way that the numbers have evolved in the last year, it no longer looks like there鈥檚 anything dramatically wrong with the models,鈥� says at the CICERO Center for International Climate Research in Oslo, Norway. 鈥淪o it鈥檚 not the sort of potentially dramatic issue some people were saying it was a year ago.鈥�
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The latest satellite data showing the fall in heat gain was by at NASA. 鈥淚t doesn鈥檛 support a doomist narrative,鈥� 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. 鈥淭hat (IMO) is not justified.鈥�
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鈥檚 energy enters the planet鈥檚 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鈥檛 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鈥檚 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.
鈥淚t is on the high end compared to the models,鈥� says at NASA, who leads the CERES project.
But the energy imbalance also varies due to factors such as La Ni帽a and El Ni帽o. The big spike in 2023 was a result of a rare 鈥渢riple-dip鈥� La Ni帽a that continued for three winters, says Loeb.
During a La Ni帽a, 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 Ni帽a then gave way to an El Ni帽o, 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 Ni帽o developed are especially surprising.
鈥淗ad that [spike] continued, it would have very much looked like the real world was doing something which we weren鈥檛 seeing in any of the models,鈥� says Sanderson. 鈥淎s it actually turned out, it was a short spike, and we do see comparable spikes in the model data as well.鈥�
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鈥檚 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鈥檛 be able to tell, says Loeb, because the number of satellites carrying CERES instruments is falling, and they aren鈥檛 being replaced. 鈥淚t takes a long time from the time you start working on the satellite instrument until it actually launches,鈥� he says. 鈥淪o you have to really plan ahead, and I don鈥檛 think we鈥檙e doing a very good job on that.鈥�
