The World Meteorological Organization’s fifth annual Air Quality and Climate Bulletin, issued in September, called attention to the effects of air pollution on climate change and human and ecosystem health.
Aerosols were a primary topic of concern. Also called particulate matter, aerosols are tiny particles that come from both human and natural sources, including fossil fuels and industrial emissions, as well as volcanic eruptions, pollen, and dust. They can be harmful to human health, causing respiratory and cardiovascular diseases, and pollute soil and water.
However, their effects on climate change are more complicated. Some aerosols intensify warming, while others reflect sunlight and mask warming.
Jessica Seddon, a senior lecturer at the Yale Jackson School of Global Affairs, chaired the Bulletin’s editorial board and coauthored an article in the bulletin. She is an expert on environmental governance and institutional design for integrating science and policymaking.
Seddon, who also directs the Jackson School’s Deitz Family Initiative on Environment and Global Affairs, recently spoke to Yale News about the new report and the science and policy of aerosols more generally. The interview has been edited and condensed.
Jessica Seddon
What are the complexities in the relationship between aerosols and climate change?
Jessica Seddon: Basically, the effects of aerosols on the climate differ depending on their color. Dark-colored aerosols, such as the black carbon in soot, contribute to global warming by absorbing sunlight and heating up the atmosphere wherever they are. This localized warming in the atmosphere can then affect winds, rainfall, and even larger seasonal patterns like monsoons. Black carbon in the air does not always lead to higher ground level temperatures, but it can. Light-colored aerosols such as sulfates, on the other hand, have a cooling effect. Aerosols of all colors affect the formation of clouds, which then reflect sunlight back into space.
Some people talk about the reflection as “cooling,” but I think “masking” is a better way to put it: Reflecting sunlight back into space essentially prevents the extra energy in the sunlight from becoming trapped in the Earth system by greenhouse gases. Its impact on ground level temperatures is stronger in the regions that the aerosols cover. CO2 is “globally well-mixed,” meaning it is pretty uniformly distributed throughout the atmosphere. Aerosols are more like a really lumpy blanket with holes.
How are aerosols regulated?
Seddon: Aerosols are regulated in two ways. The first and most common is through air-quality policies that seek to protect human health and the environment by reducing particulate matter levels in the lower atmosphere where people can breathe it. Some of the longest-standing air quality policies specifically focus on reducing the lighter, reflecting, sulphate aerosols because they contribute to acid rain as well as damaging human health. Often these regulations aimed at air quality also contribute to climate mitigation because the same sources of air pollution (coal plants, for example, or cars) also emit CO2 and greenhouse gases at the same time.
From a climate perspective, regulation is a trickier question because the relationship between emissions — the activities that policies can shape — and climate impacts has a lot of “it depends” based on all the different types of impacts I just mentioned. So, there’s not a universal “climate return on investment” in the same way that there is for CO2.
Still, there are climate policies that aim to affect aerosols, especially black carbon. I think now 17 countries include black carbon reduction in their Nationally Determined Contributions, the commitments that countries make under the 2015 Paris Agreement.
You coauthored an article in the bulletin looking at the effects of recent international regulations to reduce sulphate aerosol emissions in shipping fuels. What did you find?
Seddon: The 2020 regulations [implemented by the International Maritime Organization] mandated the use of low-sulfur fuels for shipping. This reduced sulfate emissions — protecting human health in Asia and Africa in particular and reducing acid rain. The new fuels are also better for engine efficiency, so there may be some related reductions in greenhouse gas emissions.
But the sulfate emissions are the light-colored aerosols, so this reduced some of the masking and contributed to warming surface temperatures. We summarized some recent analysis, including by my co-authors, that highlights these tradeoffs. Most of the recent warming is still caused by the accumulated (and rising) greenhouse gases, but the effect of unmasking was detectable. On top of that, some of the emissions abatement systems used to comply with the regulations are polluting the water, creating a threat to marine life.
In the end, taking a comprehensive look at air, water, and climate impacts of the policy might have led to a different course of action to reduce shipping emissions. The lesson here is that we need a more holistic approach.
How do we achieve that holistic approach?
Seddon: The biggest challenge is the tradeoff between two tightly held environmental goals: cleaning up the air and addressing climate change. It’s not something that we can wish away; it’s just part of how the Earth system works.
The first step is to acknowledge that this tradeoff exists. It’s well-known in the scientific literature but more general communication has aimed for simplicity. The message has been: “Particulate matter is a harmful pollutant that damages people’s (and ecosystems’) health. Reducing emissions will also be good for climate.” The tricky part is that the last part has exceptions like the lighter-colored aerosols. The same policies aimed at protecting health by reducing particulate matter often also contribute to climate mitigation because they reduce dark particles or co-emitted greenhouse gases, but not always.
Acknowledging the tradeoff allows us to factor it into policy discussions about air quality. That doesn’t mean we slow down on cleaning up the air. It would be kind of pathetic to hide from climate in a toxic fog when there are so many other options to limit warming and its impacts. But we do need to recognize that increased warming is a potential outcome of such policies and make plans for that.
Maybe we take a more integrated approach that seeks to compensate for the tradeoff. We could focus on reducing methane from oil and gas production, unmanaged municipal solid waste, and some parts of agriculture, for example, which would act quickly to reduce warming because it doesn’t last long in the atmosphere. Or we could prioritize sectors that produce dark particles and greenhouse gases along with the lighter masking particles — diesel engines, biomass burning for cooking and heating, for example. These can and should be done now.
Some have suggested adding aerosols to the upper atmosphere to compensate for the unmasking that happens when aerosols in the lower atmosphere are removed. Is that a viable approach?
Seddon: This approach, known as stratospheric aerosol injection, would be a major infrastructure project. The particles would need to be delivered to the stratosphere and repeatedly replaced as they fall out of the upper atmosphere. It would also have other effects on regional temperatures, precipitation, agriculture yields, and more. It also raises a lot of new ethics and governance challenges.
Could climate policies that promote reductions in greenhouse-gas emissions be repurposed to manage aerosols?
Seddon: Unfortunately, the policy infrastructure that was built to recognize and reward reductions in greenhouse-gas emissions doesn’t easily carry over to aerosols. Carbon dioxide and other greenhouse gases spread uniformly in the atmosphere and stay there for decades to centuries, while aerosols tend to concentrate more near the point of emission and have varying time (days to weeks) in the atmosphere. Their impact on climate change, health, and ecosystems varies depending on where and when they are emitted and what the background conditions are.
One way to resolve this is to measure aerosols more consistently and in greater detail.
Right now, the main metric for aerosols lumps them all together into a single indicator: aerosol optical depth. That’s a measure of how much the particles in a given column of air scatter or absorb light. It provides some insight into the collective impact of all the aerosols, but not so much into the impacts of specific activities or emission sources that policies can affect. More investments in satellites and ground-, balloon-, or aircraft-based sensors to better distinguish aerosol types, track plumes, and identify particular sources could both inform mitigation and help pin down some of the uncertainties about aerosol impacts.
I think it also makes sense to look at regional priorities. For example, reducing the sources of dark aerosols that fall on Arctic ice and accelerate melting is a no-brainer. Policies should be tailored with this sort of regional impact in mind.
Are you optimistic we can balance the tradeoff?
Seddon: I do think that the increase in global warming is beginning to draw attention to the importance of aerosols and the relevance of managing human impacts on aerosols. That’s a good first step. The next step, though, is not so much to “balance” as to compensate for the tradeoff.
We must keep moving on cleaning up the air. It’s the single biggest environmental health risk in the world. And doing that will lead to additional “unmasking” [allowing more solar energy into the atmosphere], which will lead to higher temperatures in the coming decades. So, we need to actually do more, faster: cut the methane, tackle the black carbon, and develop the institutional ability to make legitimate, ethical, sound decisions about whether and how to use other approaches like stratospheric aerosol injection.
I’m confident that we can and I’m doing my part to create the conditions that ensure that we will.
