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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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Guest post: Why low-end ‘climate sensitivity’ can now be ruled out
Guest post: Why low-end ‘climate sensitivity’ can now be ruled out
After four years of labour and detailed discussions by an international team of scientists, we are able to quantify better than ever before how the world’s surface temperature responds to increasing CO2 levels.
Our findings, published in Reviews of Geophysics, narrow the likely range in “equilibrium climate sensitivity” (ECS) – a measure of how much the world can be expected to warm for a doubling of CO2 above pre-industrial levels.
Constraining ECS has remained a holy grail in climate science ever since US meteorologist Jules Charney suggested a possible range of 1.5C to 4.5C in his 1979 report. His estimate was largely based on the world’s first two global climate models, which gave different estimates of 2C and 4C when they performed a simple experiment where atmospheric CO2 levels were doubled.
Since then, despite more than 40 years of research, much improved understanding of atmospheric processes, as well as many more detailed observations, this range has stubbornly persisted.
Now, bringing together evidence from observed warming, Earth’s distant past and climate models, as well as advances in our scientific understanding of the climate, our findings suggest that the range of ECS is likely to be between 2.6C and 4.1C.
This narrowed range indicates that human society will not be able to rely on a low sensitivity to give us more time to tackle climate change. But the silver lining to this cloud is that our findings also suggest that very high ECS estimates are unlikely.
Stubbornly wide range
How sensitive the Earth’s climate is to increasing atmospheric CO2 is a fundamental question for climate science. It essentially dictates how much the Earth’s surface will warm in response to human-caused CO2 emissions.
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