Several US scientists have claimed in a paper published this week that a rule imposed by the International Maritime Organization in 2020 could have increased global temperatures, explaining last year’s high temperatures. The IMO 2020 fuel regulations, also known as the global sulphur cap, abruptly reduced the emission of sulfur dioxide from international shipping from 3.5% to 0.5%. This 80% drop, according to scientists from Maryland and Washington, created an inadvertent so-called geoengineering termination shock with a global impact. The warming effect of anthropogenic greenhouse gases has been partially balanced by the cooling effect of anthropogenic aerosols. The IMO2020 rule was intended to benefit public health by decreasing aerosol loading, but this aerosol decrease can temporarily accelerate global warming by dimming clouds across the global oceans. The regulations took effect quickly and the scientists argue in the paper that it had a termination shock through marine cloud dimming and reducing cloud droplet concentration. More recent studies show that aerosols can also change liquid water paths and total cloud fraction, which also greatly affect the amount of solar radiation reflected by clouds The scientists claim that ship tracking data suggests that IMO2020 reduced the occurrence and modified the properties of ship tracks across global oceans, demonstrating that a regulation intended to reduce pollution had collateral effects on cloud microphysics. 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 such as the North Atlantic, which has been anomalously hot since last year and, according to the study, experienced a warming influence more than triple the average. The researchers added that 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. This paper follows research done by Michael Diamond, an atmospheric scientist at Florida State University, in 2023 who claimed that the lack of ship tracks warmed the planet up faster and the trend is magnified in the Atlantic, where maritime traffic is particularly dense. In shipping corridors, the increased light represents a 50% boost to the warming effect of human carbon emissions. The 2020 regulations 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. Nature.com The scientists calculated the ratio between the aerosol optical depth change due to IMO2020 and that between pre-industrial and present-day. Over most of the ocean, the ratio is smaller than 10% because of sparse shipping outside the major shipping routes. Over the North Pacific and North Atlantic, on the other hand, it can exceed 10% and reach 25% in the Norwegian Sea and off the western European and northwestern African coasts. In these regions, the total anthropogenic aerosol concentration is relatively low because of declining emissions of aerosols and their precursors since the 1980s, making ship-emitted aerosols an important component of the anthropogenic maritime aerosols. These calculations are somewhat confirmed by data from