Pollution Paradox: How Cleaning Up Smog Drives Ocean Warming

The Blob first formed in 2013 and spread across an area of the northeast Pacific the size of Canada. It lasted for three years and keeps coming back — most recently last summer. Until now, scientists have been unable to explain this abrupt ocean heating. Climate change, even combined with natural cycles such as El Niño, is not enough.

But new analysis suggests an unexpected cause. Xiaotong Zheng, a meteorologist at the Ocean University of China, and international colleagues argue that this extraordinary heating is the result of a dramatic cleanup of Chinese air pollution. The decline in smog particles, which shield the planet from the sun’s rays, has accelerated warming and set off a chain of atmospheric events across the Pacific that have, in effect, cooked the ocean.

Other researchers spoken to by Yale Environment 360 see the finding, made with the help of in-depth climate modeling, as having potentially critical implications for future climate in the Pacific and elsewhere. Emissions of the tiny particles that cause smogs, collectively known as aerosols, are in decline across most of the world — apart from South Asia and Africa. Scientists are concerned that the cleanups will both heat the global atmosphere and lead to more intense regional ocean heat waves.

Indeed, that may already be happening in the Atlantic. Some researchers we spoke to argue that the exceptional heat wave that spread across the North Atlantic from spring last year until April this year, sending fish fleeing for cooler Arctic waters, may have owed its intensity to international efforts to reduce aerosol emissions from ships crossing the ocean.

The idea that cleaning up air pollution can worsen atmospheric warming sounds counterintuitive. But small particles suspended in the atmosphere, collectively known as aerosols, are very different from greenhouse gases. Instead of warming the planet by trapping solar radiation, they shade it by scattering incoming sunlight and sometimes creating clouds.

In recent years, however, this cooling influence has begun to decline in much of the world. Thanks to clean-air legislation intended to protect public health, aerosol emissions have been reduced in Europe and North America since the 1980s. And over the past decade, the same has happened in China, where tough government controls on dirty industries, introduced by President Xi Jinping in 2013, have cut overall aerosol emissions by 70 percent, according to Zheng.

Globally, there are now fewer anthropogenic aerosols in the air at any one time than for decades. Susanne Bauer, a climate modeler at the NASA Goddard Institute for Space Studies, says this “turning point of the aerosol era” occurred in the first decade of this century, and seems set to continue, as more countries seek to banish smogs.

As a result, scientists say, the aerosol mask is slipping, causing a boost to global warming in many regions. “We are currently experiencing greenhouse-gas driven global warming enhanced by aerosol removal,” says Ben Booth, a climate modeler at the U.K. Met Office.

The climatic repercussions of this are not unexpected. Predicted declines in aerosol cooling are already factored into projections of future global warming by the Intergovernmental Panel on Climate Change (IPCC). But Zheng’s new findings on the cause of the warm Pacific blob suggest that we can also expect more and bigger regional climatic surprises.

Why so? The answer lies in the fact that aerosols do not remain aloft for long enough in the air to mix thoroughly in the atmosphere. So national pollution cleanups will create radically new maps of aerosol distribution.

Some areas will heat much more than others, and this differential warming has the potential to destabilise atmospheric circulation patterns, which are largely heat-driven. This is what appears to have been happening in the northeast Pacific, says Zheng.

When he and Hai Wang, also of the Ocean University of China, along with colleagues in the United States and Germany, modeled the likely impacts on circulation systems of the recent cleaning of the air over eastern China, they found that clearing the country’s smogs caused exceptional atmospheric heating downwind over the Pacific.

This altered air pressures and intensified the Aleutian Low, a semi-permanent area of low pressure in the Bering Sea. This in turn reduced wind speeds further east, limiting the ability of the winds to cool the ocean below, providing “a favorable condition for extreme ocean warming.”

Zheng and colleagues warn that the findings are a harbinger of future “disproportionately large” warm-blob events.

Aerosols come in many shapes and sizes, from dust and soot to tiny particles invisible to the eye. They have many natural sources, such as forest fires and dust storms. But since the Industrial Revolution the aerosol load in the atmosphere has been dramatically increased by anthropogenic sources, primarily the burning of fossil fuels such as coal and oil.

These emissions include large volumes of sulfur dioxide (SO2), a gas that reacts readily with other compounds in the air to create tiny particles that both shade the planet and can act as condensation nuclei that cause atmospheric moisture to coalesce into water droplets that form clouds.

Burning fossil fuels produces both planet-warming carbon dioxide and aerosols that mask much of the warming. Atmospheric temperatures depend on the balance between the two. The last IPCC assessment of climate science, published in 2021, calculated that greenhouse gases were producing a warming effect of around 1.5 degrees C, with 0.4 degrees of this masked by aerosols.

“Without the cooling effect of the aerosols, the world would already have reached the 1.5- degree temperature threshold of ‘dangerous’ climate change as set out by the Paris agreement,” says Johannes Quaas, a meteorologist at the University of Leipzig and former IPCC lead author.

But the balance is shifting as ever more countries act to reduce aerosol emissions.

Indeed, that may already be happening in the Atlantic. Some researchers we spoke to argue that the exceptional heat wave that spread across the North Atlantic from spring last year until April this year, sending fish fleeing for cooler Arctic waters, may have owed its intensity to international efforts to reduce aerosol emissions from ships crossing the ocean.

The idea that cleaning up air pollution can worsen atmospheric warming sounds counterintuitive. But small particles suspended in the atmosphere, collectively known as aerosols, are very different from greenhouse gases. Instead of warming the planet by trapping solar radiation, they shade it by scattering incoming sunlight and sometimes creating clouds.

In 2020, the U.N.’s International Maritime Organization (IMO) responded to rising pressure to clear the air around ports by reducing the sulfur content allowed in shipping fuel from 3.5 percent to 0.5 percent. Reduced ships’ SO2 emissions have already resulted in fewer clouds over shipping lanes and and higher ocean temperatures.

Diamond says he has a paper currently under peer review whose “takeaway is that something like a third of the North Atlantic marine heat wave [of the past year] might be attributable to the IMO regulations.” Booth, meanwhile, is coauthor of a paper preprinted online this month which argues that shipping emissions reductions “may help explain part of the rapid jump in global temperatures over the last 12 months.”

Where are we headed?

If the world works successfully toward lowering greenhouse gas emissions in the coming decades, while also continuing to curb aerosols, then we can still expect continued warming for which aerosol reductions are a growing cause.

Most scientists spoken to for this article agreed that the best route remains doubling down on reducing greenhouse gas emissions. But Diamond suggests the aerosol dilemma shines a spotlight on the need to give priority to cutting methane emissions.