Cleaner ship emissions may warm the planet far faster than expected

A sharp drop in sulphur dioxide emissions from ships since 2020 may warm the planet more than expected this decade, although researchers disagree on the magnitude of this change in temperature.

“If our calculation is right, that would suggest this decade will be really warm,” says Tianle Yuan at the University of Maryland, Baltimore County. Combined with background warming due to rising greenhouse gas concentrations, the added heat could mean 2023’s record-breaking temperatures will be the “norm” in coming years, he says. Yuan described the change as an unintended “geoengineering termination shock”.

However, other climate researchers say there are issues with the new numbers. “This is a timely study, but it makes very bold statements about temperature changes and geoengineering which seem difficult to justify on the basis of the evidence,” says Laura Wilcox at the University of Reading in the UK.

The study adds to an ongoing debate among climate scientists about the consequences of an International Maritime Organization (IMO) rule that slashed the amount of sulphur dioxide pollution in shipping emissions after 2020. That added air pollution from burning heavy marine fuel was linked to tens of thousands of deaths each year.

However, those aerosols also had a cooling effect on the climate by reflecting solar radiation directly as well as through their brightening influence on clouds over the ocean. Researchers expected that slashing those emissions would result in some warming due to the loss of sulphur dioxide’s cooling effects. But the magnitude of anticipated warming ranged widely.

Yuan and his colleagues have now estimated the warming effect of the 2020 rule using satellite observations of cloud conditions, along with mathematical models of how clouds might change in response to the expected reduction in sulphur aerosols.

The researchers calculate the drop increased the amount of solar energy heating the oceans by between 0.1 and 0.3 watts per square metre, around double that of some earlier estimates. This effect was more acute in areas of the ocean with lots of shipping activity: the North Atlantic, which has been anomalously hot since last year, experienced a warming influence more than triple the average, according to the study.

The researchers then calculated how this warming influence, known as “radiative forcing”, would change global temperatures, using a simplified climate model that leaves out the influence of the deep ocean. They found the 2020 change translated to an additional rise of about 0.16°C in global average temperatures in the seven years after emissions dropped, effectively doubling the rate of warming during that period compared with previous decades.

“This forcing is not a greenhouse gas forcing. It’s a shock,” says Yuan. “So it’s going to be a blip in the temperature record for this decade.”

The new numbers are on the high end, but are in line with estimates using other methods, says Michael Diamond at Florida State University. The modelled results match those from a study that directly measured the change in clouds after 2020 in one region of the Atlantic Ocean, for instance.

However, other researchers dispute how the team calculated the resulting change in global temperatures. Zeke Hausfather at Berkeley Earth, a climate think tank, says the researchers conflated warming influence over the oceans with warming over the entire planet, and that their simplified climate model found a more rapid temperature rise than would occur in reality. “It’s really hard to justify more than 0.1°C warming in the near term using modern climate models,” says Hausfather.

If the new estimates prove accurate, however, it could help explain some of the huge jump in temperatures seen over the past year. Rising concentrations of greenhouse gases from burning fossil fuels and a shift to El Niño conditions were responsible for most of the heat, but a still unexplained gap has fuelled discussion about whether climate change may be accelerating.

“[The change in shipping emissions] goes some way towards closing the gap that we perceive,” says Gavin Schmidt at NASA. But “it’s not the whole story”.