by Conrad Fox
- Aerosols are fine particulates that float in the atmosphere. Many are natural, but those haven’t increased or decreased much over the centuries. But human-caused aerosols — emitted from smokestacks, car exhausts, wildfires, and even clothes dryers — have increased rapidly, largely in step with greenhouse gases responsible for climate change.
- Aerosol pollution kills 4.2 million people annually, 200,000 in the U.S. alone. So curbing them rapidly makes sense. However, there’s a problem with that: The aerosols humanity sends into the atmosphere presently help cool the climate. So they protect us from some of the warming that is being produced by continually emitted greenhouse gases.
- But scientists still don’t know how big this cooling effect is, or whether rapidly reducing aerosols would lead to a disastrous increase in warming. That uncertainty is caused by aerosol complexity. Atmospheric particulates vary in size, shape and color, in their interactions with other particles, and most importantly, in their impacts.
- Scientists say that accurately modeling the intensity of aerosol effects on climate change is vital to humanity’s future. But aerosols are very difficult to model, and so are likely the least understood of the nine planetary boundaries whose destabilization could threaten Earth’s operating systems.
China has seen dynasties rise and fall over the last two millennia. Such are the vagaries of human history. But researchers at Trinity College, Dublin, and Zhejiang University, Hangzhou, recently suggested a surprising natural explanation: volcanoes. Of 68 dynastic collapses since 0 AD, they found that 62 were preceded by major volcanic eruptions around the world.
Volcanoes throw tons of tiny particles known as aerosols skyward. These float in the atmosphere with sometimes huge effects: scattering sunlight, absorbing solar radiation, cooling the earth, and changing rainfall patterns. When Mount Pinatubo erupted in the Philippines in 1991, for example, the resulting ash cloud lowered the planet’s temperature by 0.6° Celsius (1.1° Fahrenheit) for at least two years.
The team behind the Chinese dynasty research surmises that volcanic aerosols triggered drought and ruined crops, leading to catastrophic social unrest across China’s agricultural economy. This causality is hard to prove conclusively, but the results suggests just how powerful an effect aerosols may have had on climate and civilization in the past — and today.
When we talk about the causes of climate change, we typically think of greenhouse gases like carbon dioxide or methane that form homogeneous mixtures spread evenly across the globe, which lead to relatively uniform and well-understood effects, namely, the warming of the planet. But scientists are increasingly realizing that greenhouse gases only tell part of the story.
Another major climate story is told by aerosols. These are a mixed bag of substances, liquid and solid, that differ from their gassy brethren in almost every way. Aerosols tend to hang in the atmosphere near their source, or move as localized or regional masses via air currents, and they can affect the climate in a host of contradictory ways, both cooling or warming, triggering drought or intense rainfall. Aerosols’ effects are tough to quantify and characterize, but have the potential to fill many gaps in climate science.
“That’s why the community wants to understand [them],” says Stephen Schwartz, a senior scientist in the Environmental and Climate Sciences Department of Brookhaven National Laboratory. “That’s the big unknown.”
A look through a microscope reveals the complexity of aerosols. They range in size from a few atoms across to the width of a human hair. They include crystals of sulfate, balls of almost pure black carbon (commonly, though not entirely accurately, called soot), droplets of nitric or sulfuric acid, spores of pollen. They may be salt freed from the crests of breaking waves, or desert sand whipped up by the wind. One of the largest natural sources of aerosols are plankton, which breathe out dimethyl sulfide (DMS), a strong-smelling chemical that gives the sea it’s familiar pungent odor. DMS reacts with oxygen to produce clouds of sulfuric acid. Sulfur dioxide released by volcanoes does the same.
Ninety percent of aerosols in the atmosphere are naturally occurring, but their levels have remained relatively constant over time, says physicist, Yi Ming a Princeton University lecturer and researcher at the Geophysical Fluid Dynamics Laboratory of the U.S. National Oceanic and Atmospheric Administration (NOAA). “If they’re not changing that much, you don’t have to worry about them.”
On the other hand, he says, we do have to worry about anthropogenic, or human-made aerosols. These are emitted from vehicle exhausts; the smokestacks of factories, ships and coal-burning powerplants; by farmers burning field stubble and land grabbers clearing Amazon forest with fire; by gas flares on oil rigs and discarded plastic shopping bags. Even tumble driers release microplastic fibers that float skyward. These sources have increased dramatically over the industrial period, roughly in step with greenhouse gases.
Aerosols: Climate change wild card
Like greenhouse gases, there are good reasons to curb aerosol pollution. One motivation: human health. “[Aerosols] impact almost every part of the human body, depending upon the composition, exposure amount and size,” says Bhupesh Adhikary, an air pollution specialist at the Kathmandu-based International Centre for Integrated Mountain Development and a lead author for the most recent assessment report by the Intergovernmental Panel on Climate Change (IPCC).
The worst aerosols, he says, are very fine particulates, “that can penetrate deep into the lungs and may even enter the blood stream,” exacerbating respiratory and cardiovascular conditions.
According to current research, atmospheric particulate matter kills 4.2 million people a year, 200,000 in the U.S. alone, making it “a leading source of premature mortality globally.” Other sources have put the global premature deaths from air pollution as high as 6 million annually.
In that regard, there is some good news. Unlike greenhouse gases, aerosols don’t last long in the atmosphere. “If you turn off all the greenhouse gas emitters, [those gases] would just keep on persisting in the atmosphere for hundreds of years,” says Ming. “If you turn off all the [human-contributed] aerosol emissions today, they will be gone in [about] a week.”
But now that we are producing them, eliminating aerosols isn’t so simple. Most aerosols help cool the planet by reflecting sunlight back out into space, reducing the amount of radiant energy that reaches Earth’s surface. They also help create clouds or brighten existing clouds, by acting as condensation nuclei around which water vapor condenses. This phenomenon is clearly visible in satellite images of so-called “ship tracks,” where sulfur-spewing ships have passed under clouds. Clouds reflect light back into space even better than the particles themselves.
Here’s the important bit: The cooling effect of anthropogenic aerosols is thought to have counteracted global warming significantly, potentially halving the warming effect of our greenhouse gas emissions. Without this masking effect, the world might already have surpassed 1.5°C (2.7°F) of warming above pre-industrial levels — the threshold for avoiding planetary disaster specified in the 2015 Paris climate agreement.
Climate scientist James Hansen, who famously predicted the cooling effect of Mount Pinatubo in the 1990s, made headlines last year by warning that eliminating sulfate aerosols could double the rate of global warning over the next 25 years.
But the actual amount of increased warming that could result from a drastic curbing of aerosols is still very much up in the air, contend many other scientists. The wispy nature of aerosols eludes observation and makes them tricky to predict, despite recent advances in modeling. They can’t be easily plotted on a grid map, it’s hard to distinguish anthropogenic from natural aerosols, and their effects depend on whether they are above or below clouds, over land or over sea, and on the size and shape of individual particles. Even the color of the fabric from which airborne microplastic fibers are derived affects how much light they reflect.
The IPCC estimates greenhouse gases trap an extra 3.3 watts per square meter of solar energy beyond what the Earth received in the pre-industrial era, a process called radiative forcing. Anthropogenic aerosols, meanwhile, reflect anywhere between 0.4 and 1.7 watts per square meter of solar energy.
“That’s huge,” says Schwartz of the spread. “If it’s at the high end, you subtract 1.7 from 3.3 and get 1.6, and if it’s 0.4 you get 2.9. So that’s almost a factor of two.” He adds: “that’s why some people call aerosol forcing the wild card in understanding human influence on temperature.”
Overshooting a planetary boundary?
Is there an optimal level of aerosol pollution we can tolerate, an uneasy trade-off between human health and the cooling? The Stockholm Resilience Centre, an international group of scientists, counts aerosol pollution among the nine planetary boundaries — critical limits to key natural processes currently being adversely influenced and overshot by our species.
Destabilizing any one of these boundaries could threaten the stability of Earth’s operating system, bring down civilization and even put life as we know it at risk. Among the nine: climate change, ocean acidification, the release of novel chemical entities (including heavy metals and plastics), the loss of biodiversity integrity, and, of course, atmospheric aerosol pollution.
At the global level, the Stockholm Centre admits there is just not enough science to say exactly how much aerosol pollution is too much. At the regional level, however, these tiny particles may already be causing complex and interacting impacts.
Aerosols first came to public attention in the 1970s, not so much because of their cooling impact, but due to acid rain. Cars and coal-burning power plants in North America were then churning out seven Mount Pinatubos’ worth of sulfur dioxide annually. That sulfur dioxide reacted with water in the atmosphere to produce sulfuric acid, which rained down on forests and fish. Spruce and fir trees died in huge numbers at high altitudes in the U.S. Appalachians, while lakes turned lifeless in New York’s Adirondack Mountains.
That disaster spurred feverish scientific research into sulfate aerosols, along with intense media scrutiny and public dialogue, even in schools. As a young child some of the adult talk had me fearing my skin would melt off.
But by the 1990s, the U.S. Clean Air Act, and similar legislation in Canada and Europe, forced scrubbers onto smokestacks, catalytic converters into cars, and imposed fines on polluters. The results were dramatic. Since 1980, sulfur dioxide emissions in North America and Europe have declined by about 80%, and by 2018 New England’s evergreens were recovering. Sometime around my adolescence, people stopped talking about acid rain and I forgot my fear.
Bullet dodged, right? Wrong!
While my family was polluting the Canadian air with our enormous V6 station wagon, Africa’s Sahel reigon was also suffering, as the nations of Mauritania, Mali, Chad, Niger and Burkina Faso endured one of the worst droughts of the 20th century. Crops failed and an estimated 100,000 people starved to death. From the 1960s to the ’90s, Lake Chad shrank to 5% of its original size.
At the time, the drought was blamed entirely on deforestation and desertification. But in its latest report published last year, the IPCC now says a major contributing factor was the cooling in the Northern Hemisphere induced by human-produced aerosols from North America and Europe. That cooling caused changes in air circulation near the equator, shifting the tropical rain belt southward and cutting off the Sahel from its annual source of precipitation.
As sulfate emissions in North America diminished, precipitation returned to much of the Sahel, though not all, and in places rain patterns are now more erratic than before. Homegrown African aerosols could be to blame for these persisting precipitation anomalies, says Charles Ichoku, professor of Earth and atmospheric sciences at Howard University and a former researcher at NASA’s Goddard Space Center.
Today, the West African pollution is “tremendous,” says Ichoku, who often visits family in his native Nigeria at year’s end, when acrid clouds of wood smoke from cooking fires mix with dust blown off the drying bed of Lake Chad. “There’s a lot of smoke in the atmosphere. I smell it. I feel this is not normal. But people are like, ‘this is life.’”
According to one study, air pollution killed a million people on the continent in 2019. During the burning season, satellite imaging reveals a continent-sized smoke plume obscuring the coastline, and visibly pushing other air masses aside.
But the mix of soot, dust and organic carbon acts very differently from other aerosols. Although they cool Earth’s surface beneath them by blocking solar energy, their dark color causes clouds to absorb that energy instead of reflecting it back to space, warming the lower atmosphere. “When it heats up the atmosphere, there is no longer a [temperature] gradient,” explains Ichoku. “It stabilizes the atmosphere, and prevents convection.” That effect retards the water cycle and supresses rain.
While at NASA, Ichoku led a study that found climate models were underestimating the atmospheric warming, caused by black carbon from bush fires, partly because not all wood smoke is equal. Burning African savanna, it turns out, has different climate effects than burning Amazon rainforest, because the particulates have different properties. Ichoku also found some indication that burning in the dry season can reduce precipitation in the rainy season.
“We proved correlation, not causation,” cautions Ichoku. The difficulty here is one that plagues aerosol science in general: lack of fine-grained data. Reliable weather stations in Africa are few and far between. Instruments his team installed near Lake Chad, for example, fell into disrepair due to insurgent activity in the area.
Brown cloud over Asia
About the same time as North America and Europe were reining in smog, China, India and other parts of Asia were industrializing at breakneck speed. By the early 2000s, sulfur dioxide emissions in this region had quadrupled, and aerosol-induced cooling had reached the same level as North America 30 years earlier. “It was horrible,” says Yi Ming. “You rarely saw blue sky.”
Since 2010, China has reduced sulfur and other emissions through stricter pollution controls, though the country continues building an average of one new coal-fired powerplant almost every week. Emissions have remained so pronounced that when COVID-19 restrictions slowed China’s industrial output in 2020, local temperatures warmed by 0.3°C (0.5°F).
India’s aerosol emissions continue growing, as do those in other Asian nations. It isn’t only heavy industry to blame. “Before, in South Asia everyone was on a bicycle or walking,” says Bhupesh Adhikary. “Earlier [people] were traveling by bus, but now a family will have three cars, or two motorbikes.” As populations grow, he notes, aerosol emissions from even traditional energy sources, such as wood and dung burning for cooking and heat, also increase.
Today, an immense cloud swirls over India, China and Southeast Asia, and wafts out over the Indian Ocean — fed by dark, radiation-absorbing particles of ash, soot and organic carbon compounds from cooking stoves, coal plants, cars and the ubiquitous motorcycle taxis known as tuk-tuks.
Dubbed the Asian Brown Cloud, the 3-kilometer-thick (1.9-mile) mass blocks as much as 10% of sunlight from reaching Earth’s surface regionally; the dense cloud has been linked to a dramatic weakening of the winter monsoon. It stalls convection — much the way Charles Ichoku suspects burning biomass and blown dust have done over Africa — inhibiting rainfall as cloud droplets cling to particles instead of falling.
When these aerosols finally fall out of the air, they can land on alpine snow and ice, and hasten melting by absorbing solar radiation. Himalayan glaciers are retreating by up to 1 meter (3 feet) a year, and are predicted to halve in size by 2050. Some experts attribute almost of all this loss to aerosols rather than greenhouse gas warming. Those Himalayan glaciers feed rivers that are the water source for some 750 million people. Loss of that water would be a human and ecological calamity and could destabilize China, much as volcanic eruptions may have done through history.
Aerosols clouding the future
Meanwhile, back in North America, the continent has tamed its sulfur emissions but wildfires are on the rise, posing the same kinds of threats as the Asian Brown Cloud and as soot and dust pollution over Africa. There are already signs that brown carbon — remnant unburnt biomass from grass and trees — is accumulating in the upper atmosphere, while black carbon deposition is warming up Arctic ice and snow, though there are few local sources of the aerosol.
If melting sea ice opens the Arctic to industry and seagoing vessels, black carbon plumes in the polar region (arising from transpolar shipping, gas flaring at oil wells, and other industrial sources) could potentially cause a fivefold increase in Arctic surface temperature response as compared to the same amount of black carbon emitted at midlatitudes — resulting in a major impact on snow and ice melt.
Despite the uncertainty around aerosol impacts, cooling still seems to be their dominant effect. Improved modeling in recent years has revealed many ways in which aerosol cooling is holding the worst effects of greenhouse gas warming in check. Without aerosol cooling, for example, the atmospheric rivers carrying vast amounts of moisture and causing calamitous flooding around the world could be even stronger; the Atlantic Meridional Overturning Circulation (ocean currents that include the Gulf Stream) should be weaker; and the waters of the eastern equatorial Pacific Ocean should be warmer.
“We were scratching our heads, like this ‘doesn’t make sense,’” says Ulla Heede, a graduate student at Yale University’s Department of Earth & Planetary Sciences who is studying the Pacific Ocean anomaly. Earlier models solely based on greenhouse gas effects suggested that global warming should have weakened easterly trade winds causing them to draw up less of the deep cool ocean water that regulates regional temperatures. That is, “Unless aerosols play a role,” she says. Heede plugged aerosol cooling estimates into the models, and discovered a perfect fit. “It was the only thing left.”
But what happens if aerosols, with their cooling effect, are reduced? Heede predicts flooding for Pacific islanders, plus other unknowable outcomes. “If the circulation slows down, a lot of other things are going to adjust, and I don’t think we know, as humans, how those are going to be experienced.”
All this thorny uncertainty could diminish as modeling and observational data continue being refined. But if we don’t soon figure out how much power the mysterious aerosol wild card holds over our planet’s atmosphere, humanity could end up in the same predicament as a Chinese ruler 1,000 years ago — watching our dynasty crumble in the face of climate chaos, with no clear understanding as to causes and effects occurring at opposite ends of the world.
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