The Faustian Bargain of Reducing Air Pollution
Newsletter Opinion Writer This summer of extremes has been a summer of mystery, debate and even some confusion for climate scientists, whove been watching the news with the rest of us and asking, What, exactly, is going on? Is it just baseline global warming, trending upward, that explains the extreme temperatures on land and over sea? The arrival of a planet-warming El Nino in the Pacific? How much of the oceans extra warming should we chalk up to the fact that relatively little Saharan dust has blown westward over the Atlantic this year, reflecting less sunlight into the atmosphere than usual? How much to unusual wind patterns? And how much temperature rise is attributable to the eruption of the Hunga Tonga underwater volcano in the South Pacific, which vaporized miles of ocean in January 2022, blasting into the atmosphere 58,000 Olympic swimming pools , an often-overlooked greenhouse gas? When considering anomalies like the temperature in Phoenix, where it was 110 degrees for , how much should we attribute to whats called the heat island effect, by which the built infrastructure of cities absorbs and radiates heat and so adds to the extremes? And when considering sea-surface temperatures, what role is being played by recent regulations designed to significantly reduce the sulfur emissions of ships, since less pollution in the air means more heat making its way to the water below? The debates showcase the complexity of the climate system but also arrive packaged in a kind of climate partisanship, with alarmists typically fearing that warming may have intensified beyond human control, establishment scientists mostly emphasizing that we remain in charge of our fate and can trust existing science and climate minimizers more likely attributing these startling disruptions to a complex system full of variability and to factors other than narrowly defined warming the heat-island effect, for instance, or that underwater volcano. But these are not merely matters of mood affiliation. And science does already offer some preliminary insights, many of them summarized in a prepared by Robert Rohde of Berkeley Earth to illustrate the relative contributions of various factors in warming over the past decade. The El Nino effect has been relatively weak to this point, for instance, though it comes on the heels of a cooling La Nina and is over the coming months. The contribution of that volcano probably explains only a few of global warming, and the reduction of sulfur from shipping emissions will probably contribute about globally by 2050 though that is still a significant effect, when climate scientists routinely warn that every tenth of a degree matters. And almost certainly, the sulfur effect has been larger locally, along particular shipping routes in the worlds oceans, where some anomalies have been observed. The story of sulfur emissions is also powered to some degree by motivated reasoning, since it appeals to those who would at least consider pumping more sulfur into the sky to cool the planet (this geoengineering project is known as stratospheric aerosol injection, a form of something called solar radiation management). But it also points to one fact about future warming that is quite underappreciated outside the scientific community: Air pollution is going to play a huge and complicated role in shaping the climate of the next few decades. It is already. By reflecting sunlight, industrial aerosol pollutants a group of particulates that includes sulfur dioxide, nitrogen dioxide and black carbon or soot cool the planet, masking some share of the warming we might otherwise have seen. How much? Estimates vary, and the uncertainties are indeed larger than those around the estimated effect of carbon emissions. But while overall, the world has warmed about 1.2 degrees Celsius above the preindustrial average, the Intergovernmental Panel on Climate Change estimates that aerosols are simultaneously cooling the planet by about . The high end of the estimated range is almost a full degree. And some have given higher ranges. Overall, vast emissions of aerosols since the start of the industrial age have had a profound cooling effect, wrote Geeta Persad, Bjorn Samset and Laura Wilcox in published late last year. Without them, the global warming we see today would be 30 to 50 percent greater. Nevertheless, they went on, the impacts of aerosols on climate risk are often ignored. For the most part, carbon emissions and aerosol pollution are produced by the same processes: the burning of fossil fuels (although some measures, like industrial scrubbers, reduce aerosols without a similar reduction in greenhouse gases). And impacts of both emissions and pollution are very strong arguments for ending that burning to limit temperature rise, on the one hand, and to reduce the human toll of air pollution, which is today estimated to kill perhaps 10 million people each year and reduce average life expectancy by . But undoing the processes has opposite impacts on global temperature: Reducing carbon emissions slows warming, but reducing air pollution accelerates it. This is not to say that because the world is set to embark on fossil drawdown it is due for a big warming bump, or that airborne particulate matter shouldnt be reduced or that conventional modeling hasnt incorporated aerosols impact. In relatively ambitious climate scenarios, where both aerosols and carbon emissions are aggressively drawn down, the effects mostly offset each other over the next few decades, said Dukes Drew Shindell, among the worlds leading experts on air pollution. But if pollution is tackled more aggressively than carbon emissions through air-quality measures like those introduced in the United States and Europe 50 years ago or more recently in China the effects wouldnt quite cancel. And you could see more warming from aerosol drawdowns than from carbon emissions over the next few decades. The pioneering climate scientist James Hansen, who has long called this challenge a Faustian bargain, recently suggested that aerosol drawdown may double rates of warming. The calculation, which was published as a preprint and has not yet been peer-reviewed, is a controversial one, attracting a fair amount of criticism. But while his estimate of the effect is larger than most others, the basic dynamic he describes is almost a climate-science truism: If emissions stay flat while aerosols decline, warming should accelerate. That is all just the effect on global average temperature, which, scientists say, is not even the most significant way aerosols will shape the climatological future. Thats because while carbon dioxide mixes so well in the atmosphere that its effects are global no matter where it comes from, aerosols dont have only a global impact. They were the main reason temperatures in Europe the authors of the Nature commentary wrote. They drove a decline in the South Asian monsoon during the second half of the last century. And they were a major driver of the late-20th-century , which triggered a famine that killed 100,000 people. But it also isnt exactly true that the impacts are entirely localized. Some pollution stays local, with local effects. Other pollution travels, carrying effects far from the responsible source. And in many cases, aerosol perturbations in one place can have ripple effects elsewhere. European aerosol pollution has produced , for instance. Those droughts in the Sahel, and the 100,000 deaths that followed, were driven not by West African aerosols but by North American and European pollution. To me, Persad emphasized the complications of all this. A very naive approach to aerosols from a climate perspective is that theyve been a good thing over the historical period, since theyve been masking all this warming and buying us time, she told me. But when you take all of those effects into consideration and go beyond the global mean temperature, you start to recognize that the aerosols are not actually good from a climate perspective, either. Theyre definitely not good from an air quality perspective, but theyre actually not good at a local level on the climate side, either. Still, she said, in certain regions, warming the climate by a certain amount through the reduction of aerosols will have a bigger impact than heating up the same region by the same amount through greenhouse warming in the case of precipitation, the effect is , thanks to the different way the warming transpires in the atmosphere. Warm the world by removing aerosol emissions, and this will create more extremely hot days, more extreme precipitation events and more consecutive dry days over highly populated regions, Persad and her co-authors wrote in Nature. We talk a lot about global average forcing from aerosols, said Zeke Hausfather, a Berkeley Earth colleague of Rodhes and a climate scientist at the payments company Stripe. But if global estimates are for aerosol effects of about a half degree, you could be talking about a degree or more of additional warming, regionally. This is especially concerning, he said, because the parts of the world that are the most populous are also the ones with the highest aerosol concentrations. In terms of climate impacts on humanity, the effects of getting rid of aerosol cooling is much more concentrated where people live than the effects of global warming more broadly. In particular, he continued, we could have some of the most populated places in the world, like India, that have historically warmed more slowly than the rest of the world, suddenly experiencing much more rapid warming. Persad recently bought a house, she told me, and took out a 30-year mortgage. Over the lifetime of my mortgage, she said, the biggest source of climate uncertainty is going to be aerosols.