'We're changing the clouds': Unintended geoengineering test is warming the Atlantic

The Atlantic Ocean is experiencing unprecedented warming, with waters off Florida resembling a hot tub, causing bleaching of the world’s third-largest barrier reef. Similarly, extreme heat off Ireland’s coast has been linked to the mass death of seabirds. 

Warming trend in the Atlantic Ocean

Historically, the North Atlantic warmed more slowly than other global regions, but it has now not only caught up but surpassed previous records. 

Last month, the sea surface temperature there surged to a record 25°C – nearly 1°C higher than the previous record set in 2020 – and the temperatures are expected to rise even further. “This year it’s been crazy,” said Tianle Yuan, an atmospheric physicist at NASA’s Goddard Space Flight Center.

The primary cause of this alarming trend is the increased greenhouse gas emissions from human activities, which trap heat that the oceans gradually absorb. 

Additionally, recent weather patterns, especially stationary high-pressure systems that prevent cloud formation, have allowed the oceans to absorb more sunlight, intensifying the warming.

Low-lying clouds called ship tracks

Researchers have also identified another contributing factor, which can be seen as an unintended consequence: the reduction of clouds known as ship tracks. 

Regulations implemented in 2020 by the United Nations International Maritime Organization (IMO) significantly reduced sulfur pollution from ships by over 80%, improving air quality globally.

However, this reduction has also diminished the formation of low-lying, reflective clouds that follow in ships’ wakes and play a crucial role in cooling the planet. 

“The 2020 IMO rule ‘is a big natural experiment,’” said Duncan Watson-Parris, an atmospheric physicist at the Scripps Institution of Oceanography. “We’re changing the clouds.”

Warming effects of human activities

Studies have shown that by drastically reducing the number of ship tracks, the planet has warmed at a faster rate, particularly in the Atlantic, where maritime traffic is dense. 

In these shipping corridors, the reduced cloud cover has enhanced the warming effect of human carbon emissions by 50 percent. Michael Diamond, an atmospheric scientist at Florida State University, compared the impact to the loss of the cooling effect equivalent to that of a significant volcanic eruption each year.

Geoengineering experiment gone wrong 

This natural experiment created by the IMO rules offers climate scientists a rare opportunity to study a geoengineering scenario – though it’s one that’s unfortunately working in the wrong direction. 

One such proposed strategy to mitigate global warming, called marine cloud brightening, involves ships spraying salt particles into the air to make clouds more reflective. 

According to Diamond, the sharp decline in ship tracks demonstrates that humanity could, in theory, significantly cool the planet by brightening clouds. “It suggests pretty strongly that if you wanted to do it on purpose, you could,” he said.

Warming in the Atlantic and beyond

The influence of pollution on clouds remains one of the largest uncertainties in predicting the rate of global warming, according to Franziska Glassmeier, an atmospheric scientist at the Delft University of Technology. Progress in understanding these complex interactions has been slow because “clouds are so variable,” she explained.

Some basic principles are well understood: particles like sulfate or salt can seed clouds by providing nuclei around which water vapor condenses into droplets. These seeds also brighten clouds by increasing the number of smaller droplets. However, the effects don’t stop there, according to Robert Wood, an atmospheric scientist at the University of Washington. 

Smaller droplets are less likely to merge, potentially reducing rainfall, which could enlarge clouds and further increase their reflectivity. However, larger clouds may mix with dry air, which could decrease their brightness.

Ship tracks decreased by more than 50 percent 

Even before the IMO regulations, ship tracks were a focus for testing these ideas. With their striking visual appearance, these linear clouds were prime candidates for artificial intelligence-based image recognition. 

By employing such techniques and analyzing 20 years of calibrated imagery from NASA’s Terra and Aqua satellites, Yuan and his colleagues identified ten times more ship tracks than had previously been documented manually. 

The study, published last year in Science Advances, revealed that ship tracks in major shipping corridors decreased by more than 50% after the IMO regulations.

Extent of warming in the North Atlantic 

In subsequent research, Yuan’s team took this analysis further, calculating the cooling effect associated with the brightening of these clouds and the pollution’s impact on cloud longevity. 

The researchers concluded that the IMO rules have caused a warming of 0.1 watts per square meter – double the warming impact caused by changes to clouds from airplanes, as detailed in a paper currently under review. 

This impact is especially pronounced in heavily trafficked areas like the North Atlantic, where the disappearance of clouds is a “shock to the system,” according to Yuan. The increase in sunlight, exacerbated by a lack of reflective Saharan dust over the ocean this year, “can account for most of the warming observed” in the Atlantic this summer.

Air pollution cools the climate

Watson-Parris and his colleagues approached the problem differently, starting with ship location data and combining it with weather records to project where ship pollution traveled. 

The experts compared clouds at these locations with nearby clouds unaffected by ship pollution. In Nature last year, they reported that these “invisible” ship tracks not only enhanced low-lying marine clouds but also significantly increased the volume of higher-altitude cumulus clouds, which were previously thought to be unaffected by ship pollution. 

The findings suggested that air pollution could be causing clouds to cool the climate at nearly twice the previously estimated strength.

However, their follow-up study on the effect of the IMO rules on these invisible tracks delivered a surprise: the reduction in pollution did not reduce the puffiness of the cumulus clouds, according to a new preprint in the journal Atmospheric Chemistry and Physics (ACP). This suggests that these clouds have a saturation point, beyond which additional pollution has little effect on their development. 

“We removed 80% of the aerosols, but that’s still not taking us close to the preindustrial state,” Watson-Parris said.

Impact of ship pollution on clouds 

Another approach to understanding the impact of ship pollution on clouds involves zooming in on specific ocean regions where winds align with shipping lanes, concentrating the pollution. 

One such region is off the coast of Angola in the southeast Atlantic. Observing this area with the Terra satellite, Diamond found that with reduced pollution, cloud droplet sizes had increased to their largest in two decades. 

Based on these findings, Diamond estimates in a recent paper in ACP that the IMO rules have caused warming globally at levels comparable to those observed by Yuan.

Later this year, Diamond, Yuan, and other researchers will begin comparing their methods for studying the interaction of pollution and clouds under the National Oceanic and Atmospheric Administration’s small geoengineering research program. “After a few more years, we’re really going to have something to say about these cloud adjustments,” Wood concluded.

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