What’s happening? Regulations on shipping fuel introduced in 2020 may have increased ocean warming by decreasing cloud cover, according to research published in Communications Earth & Environment. The analysis found that International Maritime Organization (IMO) rules forcing shippers to reduce fuel sulphur content from 3.5% to 0.5% have led to an 80% reduction in sulphur dioxide (SO2) pollution. However, SO2 also creates aerosols that intensify clouds, reflecting the sun’s rays into space and combatting global warming. The study estimates that IMO fuel standards could have been responsible for 80% of total net heat uptake since 2020. (Reuters)

Why does this matter? Exposure to SO2 can be harmful to the human respiratory system and is linked to asthma and chronic bronchitis. The gas is also detrimental to the environment as it can combine with water vapour to form acid rain, which is damaging to ecosystems such as forests and freshwater habitats. SO2 can also contribute to ocean acidification.

Clean up operation – The IMO regulations, which came into effect on 1 January 2020, have brought about a significant reduction in total sulphur oxide emissions from shipping. To achieve the emissions reduction, companies have switched to compliant fuels such as very low sulphur fuel oil (VLSFO) and marine gas oil (MGO). Some ships have also had exhaust gas cleaning systems known as “scrubbers” installed. Certain ship engines are also able to use low or zero sulphur fuels, such as liquefied natural gas or biofuels.

Unintended consequences – However, the new research indicates that, as well as reducing SO2 pollution, the regulations could have affected aerosol formation, increasing ocean temperatures. This could result in at least a doubling of the warming rate this decade, compared to the rate since the 1980s. The scientists argue that such a “warming effect is consistent with the recent observed strong warming in 2023” and could make this decade “anomalously warm”.  

Unmasking – Last year also saw six leading scientists argue that the process of reducing air pollution – known as “unmasking” – could accelerate climate change more generally. The experts found that China’s decade-long “war on pollution” had reduced SO2 emissions by almost 90%, saving hundreds of thousands of lives. However, the loss of the polluting shield that reflects solar radiation also means that average temperatures in the country have risen by 0.7C since 2014, leading to stronger heatwaves. Similar effects could also occur in other highly polluted regions, such as India and the Middle East, if anti-pollution measures are introduced.  

No aerosols, no Paris Agreement? – Environmental physicist Paulo Artaxo claims that “Aerosols are masking one-third of the heating of the planet”. A 2021 report by the Intergovernmental Panel on Climate Change (IPCC) found that, without SO2 pollution, global average temperatures would already have increased by 1.6C above pre-industrial levels. This would exceed the 1.5C goal established under the Paris Agreement, which is designed to limit the worst impacts of climate change.

The fight against pollution must continue – Scientists argue that the findings do not imply that efforts to combat air pollution – which the World Health Organization claims is responsible for around 7 million premature deaths per year – should be reduced. Rather, stronger actions needs to be taken to lower greenhouse gas emissions, with methane reduction offering one of the most effective short-term ways of offsetting pollution unmasking.