Air pollution can have a cooling effect on the climateCheunghyo/Getty ImagesJames 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.January 2025 sets surprise record as hottest ever start to a year“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.AdvertisementBut 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 both reflect sunlight away from Earth directly and interact with clouds – which 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 total change in shipping aerosols increased Earth’s energy imbalance 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 expecte
Mer. Feb 5th, 2025
