Is there evidence of acceleration?

Some say that the massive temperature spike might end up being a blip in the climate data, owing in large part to new regulations covering air pollution from ocean-going ships. In 2020, the United Nations International Maritime Organisation implemented a rule requiring an 80% reduction in sulfur emissions from ships sailing in international waters. One analysis of satellite imagery, published in August, suggests that there has been a clear reduction in ship tracks, which are formed when sulfur-containing pollution particles seed low-lying mists. The changes seem to correlate with the broader reduction in cloud cover pinpointed by Goessling’s team.

Not everyone is convinced, however. If the pollution reduction was the primary explanation, warmer temperatures should coincide with the areas most frequented by ships, says Yoko Tsushima, a climate scientist at the Met Office in Exeter, UK. “We do not see that pattern,” she says. “The warming is almost everywhere.”

Nor do the numbers necessarily add up. It is clear that shipping regulations have played a part, Goessling says. But his calculations suggest that, to explain the 2023 temperature spike, all forms of air pollution on Earth, rather than just shipping pollution, would need to be almost completely eliminated.

This leaves the possibility that warmer temperatures are somehow driving reductions in low-lying cloud cover, which in turn promotes further warming — a positive feedback loop.