The world has been very hot lately. Like, really hot. Much hotter than you might expect from climate change alone.
In 2023, the global average temperature was nearly 1.5 degrees Celsius, or 2.7 degrees Fahrenheit, above its pre-industrial level. It is nearly certain to exceed that milestone in 2024.
These are extreme leaps. For context, 2019 was the second-hottest year on record when it happened, and it was merely 0.95 degrees Celsius above the pre-industrial average. So Earth’s temperature has seemingly surged half a degree Celsius in five years.
These searingly hot temperatures aren’t completely outside the range climate models predict, but they are arriving sooner than most scientists thought, and climate researchers haven’t yet reached a consensus explanation for why they are happening.
This week, though, we got somewhat closer. A new study adds to a growing literature suggesting that a change to global shipping fuels has accidentally contributed to a surge in warming.
In 2020, the International Maritime Organization began enforcing rules that removed a toxic air pollutant, sulfur dioxide, from shipping fuels. Sulfur dioxide can inflame and irritate the heart and lungs, trigger asthma attacks, and can cause acid rain. But it can also reflect heat back into space, cooling the Earth.
These cooling effects of sulfur dioxide are very short-lived, and sulfur dioxide only sticks around in the atmosphere for about a week and a half. (Carbon dioxide, on the other hand, can persist in the atmosphere for a millennium.)
It now seems that all those sulfur aerosols were likely reflecting enough heat back into space to make a noticeable difference in the Earth’s temperature rise. The new study, written by the researchers Ilaria Quaglia and Daniele Visioni, finds that removing sulfur dioxide from shipping fuel likely increased the planet’s temperature by 0.08 degrees Celsius.
This change alone can’t explain the Earth’s recent surge in temperature rise. But the new rules likely made the record-breaking temperatures in 2023 roughly 12 times likelier than they would have been had the rules not changed, Visioni, an atmospheric chemistry professor at Cornell, told me.
“The likelihood of something like 2023 happening — was it made larger, was it made bigger, by this contribution? We found, yes,” he said.
The timing of the surge — and the fact that the most anomalously warm part of the planet has been the surface of the North Atlantic Ocean, a popular shipping route — also support the conclusion that the IMO rules are playing an effect.
Other factors — including natural fluctuations in Earth’s multi-year climate cycles, like El Niño — may have helped the surge along too, Visioni said. “If you take a probabilistic approach, you can say, even without the shipping rules, 2023 wouldn’t have been completely impossible,” he added. “But you cannot evaluate the truthfulness of probability from one outcome because you only have one world.” In other words, both climate change and our response to it are part of the same poorly designed experiment — and we can only run that experiment once.
Over the past 12 months, several other papers have reached a similar conclusion, although they disagree about the magnitude of the IMO’s accidental cooling effect. Quaglia and Visioni’s study finds one of the largest effects.
The literature suggests that sulfur dioxide’s effects are “in the range of three hundredths to eight hundredths of a degree Celsius, but I don’t know that we can say that we're on the high or low end of that,” Zeke Hausfather, a climate scientist who studies carbon removal technologies at the tech company Stripe, told me. Hausfather has his own estimate of how much shipping rules have affected the recent warming episode — about five hundredths of a degree Celsius — which he reached with Piers Forster, a climate physics professor at the University of Leeds.
The exact magnitude of the effect, though, might matter less than the fact that it happened at all. For Visioni, the results demonstrate that policymakers need to think more intentionally about the tradeoffs between cutting toxic air pollution emissions and losing the cooling effect those same toxic emissions produce.
Over the past few decades, humanity has gotten better at cutting toxic air pollution from power plants and industrial activities than previous climate models estimated. That means that, somewhat paradoxically, it might be more difficult to stay below the Paris Agreement’s 2 degrees C warming goal because the same levels of greenhouse gas emissions will now have a greater warming effect than they would have in 2015.
It’s time to discuss this trade-off frankly and head-on, he told me. That also means taking seriously — and beginning research — on the proposition that humanity may want to experiment with intentionally releasing some forms of aerosols to suppress the planet’s warming — something the international shipping community has historically been loath to do.
In 2013, a paper from Finnish researchers suggested that ships could retain the climate benefits of sulfur aerosol pollution — while mitigating most of their public health downsides — by burning clean fuels near the coasts, but dirtier fuel on the open ocean. Under that scenario, shipping emissions would actually have reflected even more heat than they did at the time. But the group downplayed that scenario in part because it was a potential form of geoengineering.
Is it? It’s not clear where the line of “intentionality” in geoengineering lies, Visioni said. If you stop doing something bad for the environment, but it has a warming effect on the climate, are you geoengineering? Or are you passing prudent environmental policy? The question of where geoengineering begins or ends gets harder and harder to adjudicate — especially while humanity conducts what is in essence the largest and most important geo-engineering experiment possible by burning fossil fuels and releasing billions of tons of greenhouse gas pollution into the atmosphere.
Visioni made a point to emphasize that he’s in favor of the IMO’s efforts to clean up shipping emissions. “Do we keep polluting? No. I think we should be forceful and say no,” he said.
“But on the other hand, my wish would be if we started discussions a bit more like, ‘Okay, so do we think that these [warming] thresholds are so important? And if so, are we willing to have a discussion about what we could do to prevent this warming from happening?”
Visioni’s paper is not the only new study that seeks to explain the warming blip. On Friday, a team of German researchers wrote in Science that a recent and mysterious decline in low-altitude clouds in the atmosphere has decreased the planet’s brightness. Clouds, like sulfur aerosol emissions, reflect heat back into space, and so their decline would also contribute to a warming surge.
They provide another piece of evidence that the surge in warming is caused by some fundamental change to the climate system and not by a multiu-seasonal hiccup like El Niño. “The big question that we have is: Is this a blip or not?” Hausfather said. “If we're in the world where El Nino is behaving weirdly, that’s kind of the comforting one, because it means we’ll go back to normal — normal here being a rapidly warming world. If the spike in warming over the past two years is due to natural variability, it means it will likely be shortlived.”
The more worrying possibility, he continued, is that something more fundamental has changed in the climate system. Climate scientists describe these shifts as a change in “radiative forcings,” meaning a change in the basic dynamics that force adjustments in the energy balance between the Earth, the Sun, and outer space.
“If this is a change in forcings — which clouds or aerosols would imply — then that change in forcing would likely persist. It would be a factor that continues affecting the climate in the future, rather than just a blip,” Hausfather said.
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