‘Shock of the end’: decrease in ship pollution caused global warming explosion

The reduction in transport pollution in 2020 led to a “big bottom hit” that is estimated to have pushed the rate of global warming to double the long-term average, according to the research.

Until 2020, global transportation used dirty, high-sulfur fuels that produced air pollution. The pollution particles blocked the sun’s rays and helped form more clouds, thus curbing global warming. But new regulations in early 2020 cut the sulfur content of fuels by more than 80%.

The new analysis calculates that the subsequent fall in particulate pollution has significantly increased the amount of heat trapped at Earth’s surface that is fueling the climate crisis. Researchers said the sharp end to decades of shipping pollution was an unintended geoengineering experiment, revealing new information about its effectiveness and dangers.

High ocean surface temperatures break records in 2023, alarming experts who have struggled to explain the huge increases. But scientists have mixed views on the role that landings play in ship pollution.

Those behind the new study say it could be a “very substantial” factor. Others say it is only a small factor and that the reasons for the extraordinary rise in sea and global temperatures remain an alarming mystery.

Dr Tianle Yuan, at the University of Maryland, US, who led the study, said the 0.2 watts per square meter of extra heat trapped over the oceans after pollution cuts was “a huge number and it happened in one year, so it’s a shock great for the system”.

“We will experience about twice the rate of warming compared to the long-term average” since 1880 as a result, he said. The warming effect of reducing pollution 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 logging. It found that the short-term hit was equal to 80% of the total additional warming the planet has seen since 2020 from long-term factors such as increased fossil fuel emissions.

Scientists used relatively simple climate models to estimate how much this would increase global average temperatures at Earth’s surface, finding an increase of about 0.16C over seven years. That’s a huge increase and the same margin by which 2023 beat the temperature record compared to the previous hottest year.

However, other scientists think that the temperature impact of reducing pollution will be significantly lower because of feedbacks in the climate system, which are included in more sophisticated climate models. The results of this type of analysis are expected later in 2024.

“[Pollution particles] are one of the biggest uncertainties in the climate system and quite difficult to measure,” said Dr Zeke Hausfather, at Carbon Brief analysts. He said the new analysis did a good job of using satellite data to estimated the change in trapped heat after pollution reduction, but he disagreed on how this translated into a temperature increase.Hausfather’s estimate of the temperature increase due to pollution reduction was 0.05C over 30 years.

“The [pollution cut] is certainly a contributing factor to the recent warmth, but it helps only slightly towards explaining the 0.3C, 0.4C and 0.5C limits of monthly records set in the second half of 2023,” he said.

Dr Gavin Schmidt, at 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’s still more work to do. We’ll see how it all comes together over the next few months.”

In March, Schmidt warned: “We need answers as to why 2023 turned out to be the warmest year in perhaps the last 100,000 years. And we need them fast.” He said the recent El Niño event and an increase 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 to cool the Earth. Yuan said the years of ship pollution followed by a sharp landing was an accidental experiment on a large scale: “We did unintentional geoengineering for 50 or 100 years over the ocean.”

The new analysis shows that this type of geoengineering would lower temperatures, but would also bring serious risks. These include the sharp increase in temperature when aerosol pumping stopped – the shock of the ending – and also possible changes in global rainfall patterns, which could disrupt the monsoon rains on which billions of people depend.

“We definitely need to do research on this because it’s a tool for situations where we really want to cool the Earth temporarily,” like an emergency stop, he said. “But that won’t be a long-term solution because it doesn’t address the root cause of global warming,” which is emissions from burning fossil fuels.