Have we accidentally made climate change worse by cutting pollution from big, ocean-going ships? A new paper from a team of NASA scientists has pushed one of the most controversial topics in climate science into high gear.
According to the research, published in the journal Nature, a major reduction in emissions of sulphur dioxide in 2020, following the introduction of new, international shipping fuel regulations, led to a “termination shock” that they say could add 0.16 degrees Celsius (0.29 Fahrenheit) of heat to the world’s oceans over seven years, greatly accelerating global warming.
The researchers say this is possible because sulphur dioxide, produced by the burning of high-sulphur fuels in ships, reacts with water vapour in the atmosphere to product aerosols that reflect sunlight back into space, and helps increase cloud coverage, shielding the ocean from the sun. Reducing those sulphur dioxide emissions, the researchers claim, has therefore led to more of the sun’s energy reaching the oceans. The authors further suggest that their findings support the viability of geoengineering technologies to brighten clouds, which some researchers have suggested could be used to help to cool the Earth.
Writing on social media, study author Tianle Yuan, a research scientist at the Joint Center for Earth Systems Technology, said the effect was akin to an “inadvertent geoengineering event.” Speaking to media of his team’s findings, he suggested: “If our calculation is right, that would suggest this decade will be really warm.”
The research is controversial on two counts: one is that some climate scientists dispute the extent to which global shipping emissions have impacted global heating. The second concerns geoengineering, which is controversial not least because it is not known what unintended consequences artificially cooling the Earth might cause.
Laura Wilcox, associate professor at the National Centre for Atmospheric Science at the University of Reading, had questions about the methodology used by the researchers, telling media that the paper made some “very bold statements about temperature changes and geoengineering which seem difficult to justify on the basis of the evidence.” Wilcox raised concerns about “some mathematical issues” with the researchers’ calculations, including the possibility that double-counting of the effect of sulphur emissions may have taken place. She went on to warn about the language used in the paper, saying: “Describing this as accidental geoengineering, and presenting figures which may overestimate the impacts, could lead to misguided assumptions about policies intended to curb future emissions.”
On social media site Bluesky, climate scientist Zeke Hausfather posted a breakdown of why, in his view, Yuan and colleagues were mistaken in their findings. Hausfather pointed out that, while oceans cover 71% of Earth’s surface, shipping regulations should have a minimal effect on land, so that total warming would be less than that suggested in the report. Furthermore, Hausfather wrote, the energy balance model used by the researchers “does not reflect real-world heat uptake by the ocean, and no actual climate model has equilibration times anywhere near that fast.”
Geoengineering efforts to cool the Earth, meanwhile, have been publicly objected to by hundreds of prominent climate scientists. Since 2021, the Solar Geoengineering Non-Use Agreement has been coordinating researchers to push back against the development of what it calls “a risky and poorly understood set of technologies that seek to intentionally manage incoming sunlight at planetary scale.”
Nevertheless, many researchers responded positively to the “termination shock” paper. Richard Allan, professor of climate science at the University of Reading said that, while the study was “not the last word and there are varying estimates of how potent this marine cleaning influence has been … the new findings do add still more urgency in massively and rapidly cutting greenhouse gas emissions that are the root cause of the ongoing warming of climate.”
Karsten Haustein, a climate scientist at Leipzig University, found much to appreciate about the research, saying: “The paper does contribute to understanding why the global radiative imbalance is increasing at the rate it has been increasing lately.” He went on: “The remainder of 2024 will tell us whether or not recent temperature records are indeed something to be even more worried about than the already worrisome pace of our human-made warming during the last few decades.”
