Cleaning up air pollution could weaken vital AMOC ocean current

Cleaning up air pollution in Europe and North America could lead to further weakening of the Atlantic Meridional Overturning Circulation (AMOC), an ocean current critical to Europe’s climate.

The smog and soot that pollute the world’s air kill some 7 million people each year and cause illnesses that affect many more. Yet aerosols – small particles of substances like sulfur dioxide that make up the bulk of ground-level pollution – tend to reflect sunlight and brighten clouds, drawing some of the sun’s heat away.

In recent years, research has found that reductions in air pollution from sources such as shipping have caused global temperatures to rise even faster. “As we reduce aerosols, they will unmask the warming,” says Michael Diamond of Florida State University.

Until now, scientists’ understanding of the impact of aerosols on climate has been limited to running the same type of global simulations used to study the greenhouse effect. These global models showed that “if there is an increase in aerosols, it cools the surface of the North Atlantic, which strengthens the AMOC,” says Robert Allen of the University of California, Riverside, “but if you reduce global aerosol emissions, it warms the surface and weakens the AMOC.”

Yet these global simulations cannot account for the regional nature of air pollution. Unlike greenhouse gases, which persist in the atmosphere for decades or even centuries and eventually mix uniformly throughout the atmosphere, most aerosols last less than a week. So the impact of pollution on weather and climate is felt close to the source, as are the unintended consequences of its removal.

To get a better idea of ​​the effects of air protection policies, Allen and his colleagues used eight different climate models to understand how regional changes in aerosol emissions impact the climate locally and remotely. The models calculated the strength of the AMOC under a high emissions scenario defined by the Intergovernmental Panel on Climate Change, then assessed how the strength changed when the model was run under the same greenhouse gas conditions but with stricter air quality controls.

When these clean air conditions were taken into account, the researchers found that by mid-century – if greenhouse gas emissions continue to increase but aerosol pollutants decrease – the magnitude of AMOC weakening would be a third greater than if horizons remained grainy.

Although Allen and his colleagues did not assess what consequences this weakening would have on regional weather patterns, previous studies have shown that a collapse of the AMOC could worsen drought across Europe, worsen sea level rise in northeastern North America, disrupt monsoons around the world, and cause temperatures to plummet in northern Europe.

Looking at aerosol emissions region by region, Allen’s team found that the impact on AMOC was, unsurprisingly, greater when aerosols were removed from Europe and North America. Allen was surprised, however, to discover that abatement campaigns as far away as East Asia—where aggressive air-cleaning measures have already had an impact on planetary temperatures—can also weaken the AMOC because, however ephemeral the aerosols are, they still manage to drift long distances and mask warming wherever they reach.

“If we want to clean the air and improve air quality, there is a climate penalty,” says Allen. “So if we want to clean the air while minimizing climate consequences, we must simultaneously reduce other warming agents, such as CO2 and methane.”

Diamond echoes this view. “When we think about these air quality policies, it’s very important to think about decarbonization policies at the same time,” he says.