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<\/div>A 2020 rule put limits on harmful sulphur dioxide pollution in shipping emissions<\/p>\n
Robert McGouey \/ Industry \/ Alamy<\/p>\n<\/div>\n<\/figcaption><\/figure>\n<\/p>\n
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.<\/p>\n
\u201cIf our calculation is right, that would suggest this decade will be really warm,\u201d says Tianle Yuan<\/a> at the University of Maryland, Baltimore County. Combined with background warming due to rising greenhouse gas concentrations, the added heat could mean 2023\u2019s record-breaking temperatures<\/a> will be the \u201cnorm\u201d in coming years, he says. Yuan described the change as an unintended \u201cgeoengineering termination shock\u201d.<\/p>\n However, other climate researchers say there are issues with the new numbers. \u201cThis 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,\u201d says Laura Wilcox<\/a> at the University of Reading in the UK.<\/p>\n 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<\/a> to tens of thousands of deaths each year.<\/p>\n <\/p>\n 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<\/a> that slashing those emissions would result in some warming due to the loss of sulphur dioxide\u2019s cooling effects. But the magnitude of anticipated warming ranged widely.<\/p>\n 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<\/a> in response to the expected reduction in sulphur aerosols.<\/p>\n 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<\/a>, experienced a warming influence more than triple the average, according to the study.<\/p>\n The researchers then calculated how this warming influence, known as \u201cradiative forcing\u201d, 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\u00b0C in global average temperatures in the seven years after emissions dropped, effectively doubling the rate of warming during that period compared with previous decades.<\/p>\n \u201cThis forcing is not a greenhouse gas forcing. It\u2019s a shock,\u201d says Yuan. \u201cSo it\u2019s going to be a blip in the temperature record for this decade.\u201d<\/p>\n The new numbers are on the high end, but are in line with estimates <\/a>using other methods, says Michael Diamond<\/a> at Florida State University. The modelled results match those from a study<\/a> that directly measured the change in clouds after 2020 in one region of the Atlantic Ocean, for instance.<\/p>\n However, other researchers dispute how the team calculated the resulting change in global temperatures. Zeke Hausfather<\/a> 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. \u201cIt\u2019s really hard to justify more than 0.1\u00b0C warming in the near term using modern climate models,\u201d says Hausfather.<\/p>\n 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 Ni\u00f1o conditions were responsible for most of the heat, but a still unexplained gap has fuelled discussion about whether climate change<\/a> may be accelerating.<\/p>\n \u201c[The change in shipping emissions] goes some way towards closing the gap that we perceive,\u201d says Gavin Schmidt <\/a>at NASA. But \u201cit\u2019s not the whole story\u201d.<\/p>\n Topics:<\/p>\n<\/section><\/div>\n