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Analysis: How low-sulphur shipping rules are affecting global warming

In 2020, international regulations to reduce air pollution from shipping imposed strict limits on the sulphur content of marine fuels.

The International Maritime Organization (IMO) rules have had some success in improving public health. Global emissions of sulphur dioxide (SO2) – a health-damaging air pollutant – have dropped by about 10% as a result.

But the shift to low-sulphur shipping fuel has had an additional consequence.

Sulphur particles contained in ships’ exhaust fumes have been counteracting some of the warming coming from greenhouse gases. But lowering the sulphur content of marine fuel has weakened the masking effect, effectively giving a boost to warming.

Some researchers have proposed that the drop in SO2 as a result of the IMO’s clean air regulations could be behind a recent spike in global sea surface temperature.

Carbon Brief analysis shows that the likely side-effect of the 2020 regulations to cut air pollution from shipping is to increase global temperatures by around 0.05C by 2050. This is equivalent to approximately two additional years of emissions.

While this will contribute to warming and make it even more difficult to avoid exceeding 1.5C in the coming decades, a number of other factors are likely contributing to the ocean heatwave.

These include a massive eruption of an underwater volcano in the south Pacific, an unusual absence of Saharan dust and a growing El Niño.

Phasing down

Nearly all SO2 emissions today are a by-product of fossil fuel combustion.

Globally, SO2 emissions from marine fuel increased from around 6 million tonnes (MtSO2) per year in the 1970s to more than 10MtSO2 per year in the 2000s and 2010s.

SO2 emissions from fossil fuels have long contributed to severe health impacts through the formation of particulate matter known as PM2.5.

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