A drop in harmful sulphur emissions from global shipping is argued by some researchers to be a factor behind the record-breaking heat seen in 2023, particularly in the North Atlantic Ocean. While the drop has probably had only a very small warming effect on global average temperatures so far, it may have a more significant warming effect over coming decades.
To explain the unprecedented heat observed across much of the world in 2023, climate researchers point to two primary factors: the background warming trend due to rising concentrations of greenhouse gases, and the shift from years of a cool La Ni帽a climate pattern in the Pacific Ocean to a hotter El Ni帽o pattern.
鈥淏ut those two aren鈥檛 enough to explain how weird 2023 is,鈥� says at Florida State University. 鈥淪o we start reaching to these other explanations.鈥� Those have included wavy patterns in the jet stream, a deficit of and an injection of water vapour 鈥� a greenhouse gas 鈥� into the atmosphere during the 2022 eruption of Hunga Tonga-Hunga Ha鈥檃pai.
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Another possible factor is a drop in sulphur emissions from ship exhaust due to a 2020 rule established by the International Maritime Organization (IMO), the UN body that regulates global shipping. The rule may have cut sulphur from shipping by around 70 per cent, resulting in a , according to an estimate published by the UK-based website Carbon Brief. Sulphur emissions are also generated by burning coal and other industrial activities.
The reduction is good news for human health, because sulphur emissions contribute to harmful air pollution that is responsible for each year. However, the sulphur particles also reflect sunlight, both directly and by changing the size and concentration of droplets within clouds, among other changes, says Diamond. It follows that reducing the amount of sulphur emitted into the atmosphere could change the reflectivity of clouds and increase the amount of solar radiation absorbed by the planet, thus accelerating global warming.
鈥淸The IMO rule] strongly hits the air pollution part without hitting the greenhouse gas part,鈥� says Diamond.
Exactly how much warming the world will experience because of the reduction in sulphur emissions is unclear from climate models. This is largely due to the challenge of measuring the precise effects of shipping emissions on clouds.
To better understand the effect, Diamond analysed satellite measurements of clouds in an isolated shipping corridor in the south-eastern Atlantic Ocean in the years before and after the IMO sulphur rule came into effect, using a statistical method to estimate cloud properties that affect reflectivity without shipping emissions.
He found the rule change in 2020 led to an 鈥渦nambiguous鈥� reduction in the brightness 鈥� and thus the reflectivity 鈥� of the clouds. The change corresponded to an average warming effect of 1 watt per square metre within the shipping corridor, amounting to a 0.1 watt per square metre warming effect when applied across global shipping. A report in Science suggests another team has in yet-to-be published work.
at the University of Leeds in the UK and a co-author of the Carbon Brief report says the new research 鈥� along with 鈥� substantiates the amount of warming projected in climate modelling he performed with his colleagues. Their modelling suggests the warming influence of the change in 2023 is likely to be extremely small, raising global average temperatures by just one or two hundredths of a degree Celsius.
Other climate researchers agree the warming effect has probably been negligible so far. at the National Oceanography Centre in the UK says any effect this year would be 鈥渄warfed鈥� by the influence of climate change and El Ni帽o.
However, Forster says the warming effect resulting from the 2020 rule change could be more significant in local areas with lots of shipping, especially the northern Pacific Ocean and in the North Atlantic, which has seen extremely hot surface temperatures this year. He says the warming effect is also likely to increase with time as heat accumulates, and the lower-sulphur emissions circulate through the atmosphere. By 2050, Forster鈥檚 models project that the reduction in sulphur emissions will probably have led to a global warming effect of 0.05掳C, although high-end estimates show warming of as much as 0.25掳C, a substantial amount for a world just a few tenths of a degree below 1.5掳C.
Current climate models already take a gradual reduction in sulphur from both shipping and other sources into account, but Forster says they don鈥檛 model the sharp reduction seen after 2020. 鈥淚t鈥檚 an interesting bit of detective work to try to work out what鈥檚 going on.鈥�
ACP Letters
