Dr Zeke Hausfather, climate science contributor for Carbon Brief.
Prof Piers Forster, professor of climate physics at the University of Leeds and director of the Priestley International Centre for Climate.
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.
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.