Yes, reducing the sulfur emitted by ships has increased the temperature, but it is not enough to explain the record temperatures

On January 1, 2020, a new rule from the International Maritime Organization (IMO) came into force requiring the reduction of sulfur content in the fuel of ships circulating the planet’s oceans. This resulted in an 80% reduction in these aerosols, improving air quality, but it had a parallel effect: it affected cloud formation and decreased albedo (the amount of light reflected into space), which contributed to an increase in ocean temperatures.

To what extent is this related to recent temperature records? Climatologists are clear that this is a small and temporary effect, but the answer is very difficult due to the complexity of the processes and the many factors involved. The latest contribution is made by NASA researcher Tianle Yuan and his team in a study published this Thursday in the journal Planetary Physics. communication earth and environmentIn which they established that satellite observations showed that the emissions had decreased and affected the microphysics of the clouds, and this situation allowed for more accurate estimates of the amount of excess heat.

The underlying cause of this phenomenon is that sulfur dioxide reacts with water vapor in the atmosphere to produce sulfates, which cool the Earth’s surface in two ways: by directly reflecting sunlight back into space and by participating in the formation of clouds. By acting as condensation nuclei, these sulfur particles allow the formation of large cloud corridors visible from space, which were significantly reduced when the IMO limits were implemented. The authors say that the removal of this layer itself masked the warming, leading to a regional temperature increase that affects the average across the planet.

“We have combined observations and numerical models to estimate the impact of this event,” Yuan tells elDiario.es. “By effectively reducing aerosol emissions over the ocean, clouds darkened and this caused climate change.” According to the researchers, this has contributed to the warming we have experienced in recent years and will continue to do so throughout this decade. “The IMO’s action is expected to significantly increase the pace of global average temperature increase in the 2020s,” the authors write. And the record heat of 2023 is within the trajectory limits determined by their models, although it is only a point at the beginning of the series.

Our calculation is that in about 7 years we will have an additional warming of 0.17 ºC

Yuan and his colleagues have found substantial reductions in both atmospheric aerosol levels and the number density of cloud droplets, especially over the North Atlantic, the Caribbean Sea and the South China Sea, the regions with the busiest sea routes. “The IMO measure contributed to the warming in 2023 and even this year, although we cannot say for sure how much,” he explains. He clarifies that the record temperature observations are the result of many factors, from the El Niño episode to volcanic eruptions such as the Tonga volcano, so they themselves do not explain the temperature records of 2023 and previous years. “Our calculation is that in about 7 years we will have an additional warming of 0.17ºC for this reason, which is not much, but this is added to the 0.2ºC of average increase in each decade, generating a specific peak that doubles the rate of increase we have seen since the 1980s.”

Unintentional geoengineering

One of the most remarkable aspects of this new work is that it qualifies this situation as an “involuntary geoengineering experiment” that can help us better understand the role of aerosols in the radiation balance, which is a very difficult issue to measure because it depends on their size, the height at which these particles are located and even the angle at which the sun’s rays fall on them. “We know that the aerosols emitted by the ships have contributed to the cooling and the warming that has occurred, but we don’t know how much,” says Yuan. The sudden shutdown of these gases is a kind of laboratory that allows them to observe the before and after and improve the models.

In the opinion of Yuan and his team, suddenly stopping these emissions, hiding the background warming, is an example of “termination shock”.Termination shock), an expression that comes from science fiction, describing a scenario in which after implementing a geoengineering strategy to cool the Earth, it suddenly stopped, with rapid and very negative effects. “This step represents the final impact on an unintended experiment in geoengineering through marine cloud attenuation by reducing cloud droplet number concentration,” they write. Their proposal is to use this information to propose potential geoengineering strategies in the future, which will certainly not involve the emission of sulfurous pollutants into the atmosphere, but rather sea salt solutions such as those emitted by the ocean and reflecting large amounts of radiation.

“Small, but important”

“This article provides new independent evidence that new maritime transport regulations have caused a small, but significant acceleration in global warming,” Michael Diamond of the University of Florida’s Department of Atmospheric Sciences tells elDiario.es. The expert believes that the warming level they calculate is significant, but it is not an effect that can overcome the variability of natural phenomena such as the El Niño/La Niña cycle in the short term. “This makes it more difficult to meet the goals of the Paris Agreement to limit temperature rise to less than 1.5 or 2 degrees Celsius above pre-industrial levels,” he admits. “However, the scenarios analyzed in the IPCC report keep warming close to the goals of the Paris Agreement, already taking into account aerosol loss.”

The unintentional aerosol experiment provides a great opportunity to study the effect of aerosols on Earth’s climate

“He Use “The involuntary aerosol control introduced by the IMO with the regulation of sulfur content in ship fuels provides a great opportunity to study the effect of man-made aerosols on the Earth’s energy balance and climate,” says Jim Hansen, a climatologist at Columbia University and a pioneer in the study of climate. “But it will take many years for the scientific community to take advantage of it to obtain all the information that that experiment can potentially provide.” He finds Yuan’s study very interesting, but he considers its conclusion inconsistent, that the rate of global warming will double compared to previous decades, as was predicted.

It’s not the only factor

“This warming contributes to recent temperature anomalies, but it is not the only cause,” emphasized Edward Griespeardt, a researcher at Imperial College London, in statements to SMC. “Other factors, such as El Niño and increasing greenhouse gases, have also influenced global temperatures during this period, leading to the unprecedented heat we are currently experiencing.”

For Richard Engelen, director of the Atmospheric Motorization Service of the Copernicus programme, the time series of these comparisons is still too short to draw firm conclusions. “As we have described in Copernicus, the effect of aerosols on clouds and radiation is complex and more research is needed to fully understand the impact on climate of changes in the global distribution of aerosols,” he explains. “This study is clearly contributing to improving our understanding, but it is difficult to draw firm conclusions at this stage.”

This study is clearly contributing to our understanding, but it is difficult to draw firm conclusions at this stage.

A few months ago, the Copernicus Climate Change Service (C3S) concluded that it was too early to attribute the recent exceptional warming to the reduction in emissions from maritime transport carried out since 2020. In their report they reminded that maritime transport is only one of the sources of SO2 and represents only 3.5% of global emissions, according to some estimates. Additionally, the reductions in emissions from shipping caused much larger aerosol anomalies than the sulfate change recorded in June 2023, such as the transport of dust aerosols from the Sahara over the tropical Atlantic or particles from massive forest fires in Canada.

Santiago Gasso, a researcher at the University of Maryland and an expert in detecting aerosols with NASA-affiliated satellites, believes that this is a solid study on a very complex subject, and agrees with Hansen that Yuan’s estimates need to be developed further to be more accurate. “We will have to see how the series continues, but it must be emphasized that aerosols are short-term geophysical phenomena,” he commented from Washington. “You have to have something with a considerable impact on the climate, either it must be a volcanic eruption that reaches the stratosphere or something like the Sahara Desert, which throws dust for four months in a row.” And sulfur emissions from thousands of ships, even if there are many, may not have the same level of impact.

This increase in temperature was not predicted in the models, but we do not understand what contribution each of these events made.

“This issue has resurfaced because some people are saying that this unusual warming that happened in 2023, especially in the Atlantic, is due to the fact that the use of very high-sulfate diesel was phased out in 2020,” says scientist María José Sanz, director of BC3. “It’s true that this makes us need to better understand the role of aerosols, one of the most difficult issues in climate modeling.”

In Sanz’s opinion, the record temperature increases of the last two years have been a combination of events in the same time period. “This has resulted in a temperature increase that was not predicted in the models, but we also don’t understand what the contribution of each of these events is,” he concluded. “I think this work is a wake-up call for the need to better understand these aerosol-related processes and to continue improving the models.”