Is cleaner air accelerating global warming more than we expected?

James Hansen, the climate scientist best known for alerting the US Congress to global warming in the 1980s, has redoubled his warnings that we are underestimating the climate impact of declining air pollution.

“Humanity made a bad deal, a Faustian bargain, when we used aerosols to offset almost half of greenhouse gas warming,” said Hansen at a briefing hosted by the United Nations Sustainable Development Solutions Network.

But other researchers say this conclusion is based on shaky foundations, and we still don’t know how much reductions in air pollution are contributing to global warming. Hansen’s conclusions are “hovering around the top end of what we’d consider to be plausible”, says Michael Diamond at Florida State University, who wasn’t involved with the research.

Record spikes in global average temperatures in 2023 and 2024 have spurred debate about whether the pace of global warming is accelerating faster than expected. Rising levels of greenhouse gases and a warming Pacific Ocean drove most of the temperature increase, but other unknown contributors pushed average temperatures even higher than can be explained by those factors alone.

Hansen and his colleagues previously linked the accelerating rate of warming with a reduction in air pollution. Now they offer a new analysis arguing that a decline in air pollution can explain the spike in temperatures over the past two years. Aerosols in air pollution can both reflect sunlight away from Earth directly and affect the reflective properties of clouds – changes in cloud cover have also been implicated as a factor in the heat.

The researchers focus in particular on the effect of a 2020 regulation that slashed the amount of harmful sulphur used in shipping fuels. That sudden drop in air pollution over the oceans has provided researchers with an unintended experiment that lets them determine the climate effects of aerosols with more precision.

Hansen and his colleagues looked at busy shipping corridors in the Pacific Ocean to estimate this effect, measuring the change in solar radiation absorbed by the planet in those areas as air pollution declined. From this, they estimate that the drop in shipping aerosols increased the heat reaching Earth by 0.5 watts per square metre. That is roughly equivalent to the warming effect of a decade of global carbon dioxide emissions at today’s levels.

That additional warming would be enough to account for the unexplained portion of the heat seen over the past two years, they found. But the implications are broader: it would also mean air pollution’s cooling effect has been masking the full extent of greenhouse gases’ warming effect – in other words, the warming experienced to date does not represent the full impact of our emissions.

Hansen and his colleagues warn that this means the climate is much more sensitive than expected to rising levels of greenhouse gases. As a result, they argue, the world is more rapidly approaching climate tipping points, such as the slowdown of key Atlantic Ocean currents and the collapse of the West Antarctic ice sheet. To combat this, they say we should more seriously consider how to cool the planet with interventions like solar geoengineering.

However, the 0.5 watts per square metre number at the core of the new analysis is far higher than other estimates of the warming effect of the change in shipping emissions, says Tianle Yuan at the University of Maryland Baltimore County. But he says it is not completely implausible.

Gavin Schmidt at NASA says the number is “very likely an overestimate” because it assumes all the change in absorbed sunlight is due to the change in shipping aerosols, rather than other changes like less air pollution from China or natural variability.

A change in aerosols may not even be necessary to explain the 2023 temperature spike, says Shiv Priyam Raghuraman at the University of Illinois Urbana-Champaign – he previously found it can be explained by changes in Pacific Ocean temperatures alone. He says more work is needed to reconcile different estimates of the warming effects of aerosols.