Fatal effect

Cleaner air leads to increased warming

The significant decrease in sulphur dioxide emissions from shipping could be linked to a considerable warming of the atmosphere over certain ocean regions. (symbolic image)

(Bild: Spencer Platt/Getty Images via AFP)

The significant decrease in sulphur dioxide emissions from shipping could be linked to a considerable warming of the atmosphere over certain ocean regions. Around 80 percent of the increase in heat energy stored on Earth observed from 2020 could be due to this, a research team reports in the journal "Communications Earth & Environment" on the results of a modeling study.

According to the study, the content of atmospheric sulphate aerosols and, as a result, the density of cloud droplets decreased significantly in the wake of a new regulation for cleaner marine fuels. This in turn led to a darkening of the sea clouds, which reflected less solar radiation back into space. The greatest reduction in aerosol concentration was calculated for the North Atlantic, the Caribbean Sea and the South China Sea - regions with the busiest shipping routes.

Skepticism about the results of the study
Independent researchers are skeptical about the study. A very short period of time was considered. In addition, the increase in man-made greenhouse gases is still decisive for climate change as a whole.

The fuel oil used for large ships has a much higher sulphur content than fuels used for other vehicles. Combustion produces sulphur dioxide, which reacts with water vapor in the atmosphere and produces sulphate aerosols. These cool the earth's surface in two ways: by reflecting sunlight directly back into space and by influencing cloud cover.

With the new regulation of the International Maritime Organization, the maximum permissible sulphur content in marine fuels has been reduced from 3.5 to 0.5 percent in order to reduce air pollution.

(Bild: stock.adobe.com/Kalyakan)

As the amount of aerosols increases, the number of water droplets that form increases, while their size decreases. As a result, cloud cover increases and brighter clouds form, which reflect more sunlight back into space, as the researchers explain.

Sulphur content in marine fuels reduced
In 2020, a new regulation was introduced by the International Maritime Organization (IMO2020), which reduced the maximum permissible sulphur content in marine fuels from 3.5 to 0.5 percent in order to reduce air pollution. As a result, an abrupt drop of around 80 percent in sulphur dioxide emissions from shipping was observed, according to the study.

Based on this, the team led by Tianle Yuan from the University of Maryland in Baltimore estimated the Earth's energy budget, i.e. the difference between the energy received from the sun and the energy emitted by the Earth from 2020 onwards. According to the study, IMO2020 represents a "strong temporary shock" to the planet's net heat absorption. "The warming effect is consistent with the recently observed strong warming in 2023 and is likely to make the 2020s anomalously warm," it concludes.

"Significant boost" in global warming?
IMO2020 could therefore give global warming a "significant boost" in the coming years. According to the modeling, a warming rate of 0.24 degrees could be expected for the decade - more than twice as much as the average since 1880.

However, experts not involved in the study are skeptical. "Caution is called for," said Anders Levermann from the Potsdam Institute for Climate Impact Research (PIK) to the German Press Agency (dpa). If you look at an effect for such a short period of time, it is generally more prone to error than for longer periods of time. The proportion of thermal energy stored since 2020 could also be far below 80 percent. "The value could be overestimated in the modeling." Other factors could also be decisive for the record values observed last year.

"In fact, scientists are puzzling over why the last twelve months were so extraordinarily warm on a global average, far outside the usual range," Niklas Höhne from Wageningen University and the Newclimate Institute in Berlin told dpa. The main cause is clearly the continuing rise in greenhouse gas emissions. "But an additional effect was previously unexplained."

In addition to volcanic activity, the reduction of sulphur in ship exhaust gases had already been suspected as a cause. The fact that the current study now shows a fairly large correlation between sulphur reduction and warming is not surprising in principle. Sulphate aerosols have a strong effect - but only in the short term.

"This type of geoengineering is dangerous"
The scientists led by Tianle Yuan also conclude from the modeling that the significant impact of IMO2020 proves the potential effectiveness of brightening sea clouds using aerosols as a strategy for temporarily cooling the climate. However, they also emphasize: "They (geoengineering programs) are not a solution to global warming caused by greenhouse gases and have uncertain and complex additional consequences beyond the intended short-term cooling."

Levermann also warns: "This type of geoengineering is dangerous." Although introducing sulphate aerosols into the stratosphere from an altitude of around twelve kilometers is cooling and also "comparatively cheap", the stratosphere has to be constantly replenished. "If you use it to dampen man-made warming to zero, then you're sitting on a powder keg. You then have to shoot aeorosols into the air for hundreds of years, and as soon as you stop, the temperature will shoot up within a few years." The consequences of this for life on Earth cannot even be estimated.

According to Levermann, the measure would also have a direct geopolitical impact. "The country that starts doing this would be held responsible for every extreme weather event that happens anywhere in the world," fears the climate scientist. The problem is even more complicated if the sulphates or salts are introduced into the lower layers of the atmosphere, i.e. where the clouds are. "This then directly changes the weather. An incalculable danger."

Oceans a huge heat buffer
The oceans are a huge heat buffer and, according to experts, absorb more than 90 percent of the heat caused by man-made climate change. One liter of water can absorb three thousand times more heat than one liter of air.

The oceans are a huge heat buffer and, according to experts, absorb more than 90 percent of the heat caused by man-made climate change.

(Bild: erika8213 - stock.adobe.com)

Another research team recently linked the significant decline in man-made aerosols to the sharp rise in sea surface temperatures in the North Pacific. The dwindling cooling effect of particulate matter aerosols in China has changed the atmospheric circulation in the region, reported a group led by Xiao-Tong Zheng from the Ocean University of China in Qingdao in the scientific journal "PNAS".

Because man-made air pollutants affect health, many governments have introduced measures to curb them. The team led by Tianle Yuan writes that an important open question is the trade-off between the benefits of better air quality and the potential costs of additional warming. Levermann does not think much of such statements. "It can't be a decision: Do we do climate protection or less pollution," he emphasizes. "The fact that climate change becomes even more visible at times due to cleaner air should not lead to the conclusion that the air must become dirtier again."