Cleaner air leads to more warming

Due to new regulations for cleaner marine fuels, the content of atmospheric sulfate aerosols and, as a result, the density of cloud droplets has decreased significantly. This in turn led to a darkening of the ocean clouds, resulting in less solar radiation being reflected back into space. The largest declines in aerosol concentrations were calculated for the North Atlantic Ocean, the Caribbean Sea and the South China Sea – regions with the busiest shipping lanes.

Skepticism about the results of the study
Independent researchers view the study with skepticism. A very short period was taken into account. Furthermore, the increase in human-induced greenhouse gases remains critical to climate change as a whole.

The fuel oil used for large ships has a significantly higher sulfur content than fuels used for other vehicles. Combustion produces sulfur dioxide, which reacts with water vapor in the atmosphere to produce sulfate aerosols. These cool the Earth’s surface in two ways: by reflecting sunlight directly into space and by influencing cloud cover.

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

Sulfur content in marine fuels reduced
In 2020, a new International Maritime Organization regulation (IMO2020) was introduced that reduced the maximum permitted sulfur content in marine fuels from 3.5 to 0.5 percent to reduce air pollution. As a result, an abrupt 80 percent decrease in sulfur dioxide emissions from shipping was observed, the study said.

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 radiated by the Earth as of 2020. Accordingly, IMO2020 represents a “strong energy balance”. temporary shock” for “The warming effect is consistent with the recently observed strong warming in 2023 and is expected to make the 2020s anomalously warm,” the report concludes.

“Significant boost” from global warming?
IMO2020 could therefore give global warming a “significant boost” in the coming years. The models suggest that 0.24 degrees of warming can be expected this decade – more than double the average since 1880.

However, experts not involved in the study are skeptical. “Caution is advised,” Anders Levermann of the Potsdam Institute for Climate Impact Research (PIK) told the German news agency (dpa). When you look at an effect for such a short period of time, it is generally more susceptible to errors than over longer periods of time. The share of thermal energy stored since 2020 could also be well below 80 percent. “The value could be overestimated in the modeling.” Other factors may also be decisive for the record values ​​observed last year.

“In fact, it is a mystery to science why the past twelve months on a global average have been so exceptionally warm, far beyond the usual,” Niklas Höhne of Wageningen University and the Newclimate Institute in Berlin told dpa. The main culprit is clearly the ever-increasing greenhouse gas emissions. “But an additional effect was previously unexplained.”

In addition to volcanic activity, the reduction of sulfur in ship exhaust fumes is also suspected to be the cause. In principle, it is not surprising that the current research shows a fairly large link between sulfur reduction and warming. Sulphate aerosols had a strong effect, but only for a short time.

“This kind of geoengineering is dangerous”
Tianle Yuan scientists also conclude from the models that the significant impact of IMO2020 demonstrates the potential effectiveness of lifting ocean clouds using aerosols as a strategy to temporarily cool the climate. But they also emphasize: “They (geoengineering programs) are not a solution to global warming caused by greenhouse gases and have uncertain and complex collateral consequences beyond the intended short-term cooling.”

Levermann also warns: “This kind of geoengineering is dangerous.” Introducing sulfate aerosols into the stratosphere from an altitude of about eight miles is cooling and also “relatively cheap,” but the stratosphere must be continually replenished. “If you use this to reduce man-made warming to zero, then you are sitting on a powder keg. Then you have to shoot the aerosols into the air for hundreds of years. As soon as you stop doing that, the temperature will skyrocket within a few years.” The consequences of this for life on earth cannot be estimated.

According to Levermann, the measure would also have direct geopolitical effects. “The country that starts this would be held responsible for any extreme weather that occurs anywhere in the world,” the climate scientist fears. The problem becomes even more complicated when the sulfates or salts enter the lower layers of the atmosphere, where the clouds are located. “This immediately changes the weather. An incalculable danger.”

Oceans with enormous heat buffers
The oceans form a huge heat buffer and, according to experts, absorb more than 90 percent of the heat caused by human-induced climate change. A liter of water can therefore absorb three thousand times more heat than a liter of air.

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

As man-made air pollutants affect health, many governments have implemented containment measures. An important open question is the trade-off between the benefits of better air quality and the potential costs of additional warming, Tianle Yuan’s team writes. Levermann thinks nothing of such statements. “It cannot be a decision: let’s protect the climate or reduce pollution,” he emphasizes. “The fact that climate change temporarily becomes more visible due to cleaner air should not lead to the conclusion that the air must become dirtier again.”