Before 2020, global shipping used high-sulfur fuels that caused air pollution. These pollution particles blocked sunlight and helped form more clouds, thereby reducing global heating. However, new regulations at the start of 2020 reduced the sulfur content of fuels by over 80%.
The new analysis indicates that the subsequent decrease in pollution particles has significantly increased the amount of heat being trapped at the Earth's surface, exacerbating the climate crisis. Researchers noted that the abrupt end to decades of shipping pollution acted as an unintended geoengineering experiment, providing new insights into its effectiveness and risks.
In 2023, ocean surface temperatures reached record highs, alarming experts who have struggled to explain these significant increases. Scientists have mixed opinions on the role played by the reduction in shipping pollution.
The study's authors suggest that this factor could be "pretty substantial," while others believe it is minor, with the reasons for the extraordinary rises in sea and global temperatures remaining an alarming mystery.
Dr. Tianle Yuan, from the University of Maryland, who led the study, stated that the estimated 0.2 watts per square meter of additional heat trapped over the oceans after the pollution reduction was "a big number, and it happened in one year, so it’s a big shock to the system."
"We will experience about double the warming rate compared to the long-term average" since 1880 as a result, he said. The heating effect of the pollution cut is expected to last about seven years.
The research, published in the journal Communications Earth & Environment, combined satellite observations of sulfur pollution and computer modeling to calculate the impact of the reduction. It found the short-term shock was equivalent to 80% of the total extra heating the planet has experienced since 2020 due to long-term factors like rising fossil-fuel emissions.
The scientists used relatively simple climate models to estimate how much this would increase average global temperatures at the Earth’s surface, finding a rise of about 0.16°C over seven years. This is a significant rise and matches the margin by which 2023 broke the temperature record compared to the previous hottest year.
However, other scientists believe the temperature impact of the pollution reduction will be significantly lower due to feedback mechanisms in the climate system, which are accounted for in the most advanced climate models. Results from this type of analysis are expected later in 2024.
"Pollution particles are one of the largest uncertainties in the climate system, and pretty hard to measure," said Dr. Zeke Hausfather of Carbon Brief. He said the new analysis did a good job of using satellite data to estimate the change in trapped heat after the pollution cut, but he disagreed on how that translated into a temperature rise. Hausfather’s estimate of the temperature rise due to the pollution cut was 0.05°C over 30 years.
"The pollution cut is certainly a contributing factor to the recent warmth, but it only goes a small way toward explaining the 0.3°C, 0.4°C, and 0.5°C margins of monthly records set in the second half of 2023," he said.
Dr. Gavin Schmidt, from NASA’s Goddard Institute for Space Studies, said the new research was "definitely a positive contribution, but it’s not using a fully coupled climate model, so there is still more work to be done. We’ll see how this all gets reconciled over the coming months."
In March, Schmidt warned: "We need answers for why 2023 turned out to be the warmest year in possibly the past 100,000 years. And we need them quickly." He stated that the recent El Niño event and a rise in solar activity were not sufficient explanations.
Deliberately pumping aerosols into the air over the oceans to stimulate more cloud cover has been proposed as a way of cooling the Earth. Yuan said years of shipping pollution followed by a sharp cut was an accidental large-scale experiment: "We did inadvertent geoengineering for 50 or 100 years over the ocean."
The new analysis suggests that this type of geoengineering would reduce temperatures but would also bring serious risks. These include a sharp temperature rise when the pumping of aerosols stopped – the termination shock – and potential changes to global precipitation patterns, which could disrupt the monsoon rains that billions of people depend on.
"We should definitely do research on this, because it’s a tool for situations where we really want to cool down the Earth temporarily," like an emergency brake, Yuan said. "But this is not going to be a long-term solution, because it doesn’t address the root cause of global warming," which is emissions from fossil fuel burning.
