After months of frenetic research, scientists are edging closer to understanding why the planet has been so hot since 2023.
That year, the average sea surface temperature across the world hit a record high, smashing the previous figure by around 0.25掳C. It remained at record-breaking levels well into 2024, driving overall global temperatures to new recorded highs.
The scale and persistence of the heat has shocked many experts, who struggled to explain the abrupt spike.
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Long-term warming caused by greenhouse gas emissions was clearly a major factor, as was the start of an El Ni帽o phase in 2023 鈥� a change in conditions in the Pacific Ocean that generally pushes up global temperatures. But it is only in recent months that ocean cloud cover has emerged as a crucial piece of the puzzle.
and , both at the University of Reading, UK, have investigated why increasing amounts of heat are being absorbed by Earth鈥檚 atmosphere each year. This so-called Earth energy imbalance, the difference between incoming solar energy and energy escaping back into space from our planet, has nearly doubled since 2014.
This is because clouds over parts of the oceans are becoming less reflective, allowing the planet to absorb more sunlight, explains Allan. 鈥淲e were trying to look in a bit more detail at what鈥� regions and processes were involved in explaining this big increase in Earth鈥檚 energy balance that has been going on in the past few decades,鈥� says Allan. 鈥淲e could see it was a cloud effect.鈥�
Using a combination of modelling and observational data, the team pinpointed areas off the coasts of California, Namibia and the fringes of Antarctica where changes to cloud cover caused more heat to be trapped in the atmosphere in 2023.
The findings build on work published in December 2024聽that also highlighted a sharp drop in low-lying cloud cover as a driver of the recent heat.
鈥淭here鈥檚 now a consensus that it is the clouds that are changing,鈥� says at Imperial College London, 鈥渁t least in the context of these last few years and the exceptional energy imbalance.鈥�
The key question is: what is causing the cloud cover changes? There are three main contenders. It could be natural variability in the climate system. Or it could be down to recent, dramatic reductions in sulphate aerosol pollution, mainly as a result of cleaner shipping fuel, that are causing clouds to be less reflective. Alternatively, it could be a nasty surprise, a feedback effect of global warming.
鈥淢y overall feeling from the research that we have done, is that it is going to be a combination of factors that are coming together,鈥� says Allan. 鈥淎erosol is probably part of the story 鈥� reduced aerosols make clouds less bright. Clouds are [also] reducing in reflectivity as the planet warms. When you combine these two effects with natural variability, I think you can explain events like the recent jump in temperatures.鈥�
Teasing out the exact role of aerosols, compared with global warming feedbacks and natural variability, will be a key area of focus for researchers in the coming months.
The surge in ocean heat has prompted fears that the rate of climate change is increasing beyond that predicted by climate models. But while the jump in sea surface temperatures has been extreme, it isn鈥檛 totally unexpected, according to at the University of Bern in Switzerland.
Together with colleagues, Terhaar has drawn on 100 years of real observations to create a simulated record of sea surface temperatures stretching back 100 million years, in order to see how likely it would be to have such a large jump in ocean temperatures from one year to the next. The team also looked at whether climate models would replicate a jump in temperatures of the same scale between 2000 and 2040.
When considering the observation-based data, they found a temperature jump like that recorded in 2023-2024 would be expected once every 512 years given current levels of warming. According to the climate model simulations, it was the equivalent of a 1-in-1000-year event. The researchers therefore describe the jump as 鈥渦nlikely but not unexpected鈥�. 鈥淭he jump [in temperatures] that happened was very extreme,鈥� says Terhaar. 鈥淲hat we see is these kinds of jumps happen in climate models.鈥�
suggest global warming hasn鈥檛 spiralled beyond the parameters of climate models, says Terhaar, who describes this as 鈥渟cientifically reassuring鈥�. Rather, the recent spike in ocean temperatures is a rare but possible event under current warming, he concludes, suggesting current climate models are fit for purpose. 鈥淲e don鈥檛 believe it is something that has thrown us into uncharted territory,鈥� he says.
, a climate scientist at US software firm Stripe, thinks it is too early to conclude that the sudden jump in ocean temperatures was merely the product of natural variability in a warming climate. 鈥淎 1-in-512-year event for the planet is a pretty infrequent thing,鈥� he says. 鈥淚t鈥檚 like saying: 鈥業 could be struck by lightning.鈥� It鈥檚 possible. But it鈥檚 not very likely.鈥�
Climate models don鈥檛 currently account for the effect of aerosol pollution reductions, says Terhaar. Including that would probably make such a sharp jump in sea surface temperatures more likely in models, he notes.
Environmental Research Letters
Nature
