Air pollution has a cooling effect. Does clean air cause more warming?
Behind this graph is an awkward question. These aerosols are known as air pollution. And that’s something you want to reduce. Because it benefits the health of people and animals. But at the same time, that masking, cooling effect diminishes. For the climate this means: an acceleration of warming. “Then we will have to do even more than we already think to keep global warming below 1.5 degrees,” says Jos Lelieveld, director of the Max Planck Institute for Chemistry in Mainz. So the question is, should we or shouldn’t we reduce air pollution?
Reflect or absorb
Climate scientists find this difficult to explain, says Lelieveld. “They are concerned about being misunderstood.” The idea can arise that it is better not to tackle air pollution. At the same time, however, 3.3 million people worldwide die prematurely every year due to air pollution, Lelieveld and colleagues calculated in 2015. These people mainly die from heart, brain and lung diseases. Most deaths occur in China (1.36 million per year) and India (645,000), the scientists calculated at the time in their publication in Nature. “Air pollution is almost as harmful as smoking.” In view of the health benefits, it is crystal clear to Lelieveld: air pollution must be tackled.
“It’s a difficult split,” says Maarten Krol, professor of air quality and atmospheric chemistry at Wageningen University & Research. According to him, what also makes it difficult is that there are several aerosols, with different effects. Some reflect sunlight, others absorb it. Their effects also vary. Some aerosols promote cloud formation. Others cause clouds to dissolve. And clouds, in turn, provide additional complexity. Because they usually reflect incident sunlight and inhibit warming. But it is also possible that clouds block the heat that radiates from the earth, which amplifies warming. In addition, aerosols in the atmosphere can react with each other and create new compounds in all kinds of ways that are not always understood.
You can no longer see the forest for the trees. Nevertheless, aerosols can be roughly divided into two groups, say Krol and Lelieveld. The ones that contribute to the warming, and the ones that cool. This can also be seen in that figure 2 in the summary of the IPCC report. There is another graph on the right side of the page. The most important greenhouse gases and aerosols are listed here, together with their contribution to global warming. The aerosol that provides by far the most cooling is sulfur dioxide. Provides the most warming black carbon. This is popularly called soot, but according to Lelieveld that is not entirely correct. “Soot consists of balls of pure carbon. With black carbon there is also a lot of junk around those balls.”
From a climate point of view, you might think: tackle black carbon of all air polluting substances first. Then you avoid that warm-up. Then reduce sulfur dioxide. But that is practically impossible, says Lelieveld. Because both aerosols are mainly released during the combustion of coal and biomass. “You can hardly deal with them separately.”
This also applies in a broader sense, says Detlef van Vuuren, senior researcher at the Netherlands Environmental Assessment Agency. Many of the aerosols produced by man are released during the combustion of fossil fuels. To stop global warming, we will need to quickly reduce greenhouse gas emissions to zero. This means that humans have to get rid of fossil fuels. “This automatically reduces air pollution,” says Van Vuuren. And therefore also that net cooling effect.
Van Vuuren expects that aerosol emissions will decrease in the coming decades. Because many countries already have policies to specifically tackle air pollution. And because more and more countries want to reduce their greenhouse gas emissions. Reducing aerosol emissions worldwide will amplify global warming. “The IPCC already takes this into account in its scenarios,” says Van Vuuren. According to him, it does not mean that we will no longer achieve the Paris agreement – a maximum warming of 2 degrees Celsius, preferably 1.5.
Lelieveld suspects that we have already seen the effect of aerosol decrease in Europe. Because the air has become cleaner since 1980, Dutch people now live an average of six years longer, the RIVM calculated two years ago. At the same time, Europe has been one of the fastest warming regions in recent decades. “It was 0.4 degrees per decade,” says Lelieveld. He suspects that the approach to air pollution played a role in this. “But that has yet to be investigated.”