Scientists have uncovered a potentially lethal paradox at the heart of efforts to slow human-caused climate change.
A series of new studies suggest a stark truth.
One the one hand, cutting fossil fuel pollution is necessary for avoiding severe destruction over the long term. But such cuts will make the earth much hotter in the short term.
One recent study cast the well-known declines in air pollution during the COVID-19 pandemic in a darker light.
These cuts remain one of the only examples of successful cuts to climate-warming pollution, but the new study found that those pandemic-era cuts in air pollution led to a rise in global temperatures.
The findings, published on Wednesday in the journal NPJ Climate and Atmospheric Science, unveil a stark paradox at the heart of human-caused climate change.
It suggests that while cutting fossil fuel pollution is necessary for avoiding severe destruction over the long term — such cuts will make things noticeably worse in the short term.
The pandemic-era economic slowdown led to “a large-scale geophysical experiment,” study leader Örjan Gustafsson of Stockholm University said in a statement.
That’s because the shuttered factories and power plants led to a corresponding crash in emissions.
Even so, not all emissions fell in the same way.
From a research station in the Maldives, an island archipelago off the coast of India, Gustafsson’s team detected that when pollution from smokestacks fell, so did concentrations of aerosols — tiny floating particles that hang in the atmosphere.
That fall was an unmistakable boon to public health. According to Our World in Data, these contaminants — like tiny floating particles of soot or sulfates — cause millions of global deaths per year.
But for all the damage they do to human lungs, aerosols also help shade the earth by scattering light particles from the sun that would otherwise warm the planet.
After the cuts, the study found that light reaching the surface increased by 7 percent.
“While the sky became bluer and the air cleaner, climate warming increased when these cooling air particles were removed,” Gustafsson said.
While aerosol concentrations fell as the smokestacks shut off, other gases remained stubbornly high.
In particular, levels of the most potent climate-warming gases — like carbon dioxide — barely changed.
Carbon dioxide and other greenhouse gases warm the planet by trapping heat. The combination of more heat hitting the same amount of carbon dioxide meant a straightforward rise in temperatures.
The sudden rise in temperatures led by the pandemic reduction is a stark example of a more general problem — one that has long haunted the drive to cut pollution.
A draft study led by Columbia University climate scientist James Hansen suggests that the recent rise in temperatures doesn’t come from greenhouse gases at all, but from the reduction in sulfate aerosols since the early-2000s.
Hansen has an esteemed pedigree on this issue. He is the former NASA scientist who in 1988 warned Congress about the dangers posed by burning fossil fuels, which he explained were causing climate change by releasing carbon dioxide.
But by 2021, Hansen was troubled: the Earth was warming too fast.
In part, that was because the U.S. and world governments had largely ignored his calls to cut carbon emissions. Atmospheric carbon dioxide levels increased by more than 40 percent between 1990 and 2021.
But even that surge in carbon dioxide levels wasn’t enough to account for how fast the climate was warming, Hansen and fellow scientist Makiko Sato warned in 2021.
“Something is going on in addition to greenhouse warming,” they wrote.
Their culprit: The fact that the immediate aerosols released by fossil fuels temporarily hid their worse impacts, meaning that cutting them would make things worse before it made them better.
The two warned that declines in aerosols could lead the rate of global warming to double by 2040.
Last week, Hansen and colleagues reiterated those concerns. Under current emissions reduction policies, they predicted rapid warming.
Average temperature increase “will likely pierce the 1.5°C ceiling in the 2020s and 2°C before 2050,” they wrote.
One coauthor on the paper pointed to another troubling natural experiment of the COVID-19 era: the sudden surge in temperatures above ocean shipping lanes that were suddenly devoid of ships.
“For decades, this area has been kept relatively cool by sulfur emissions from ships,” climate entrepreneur Leon Simons wrote on Twitter.
“But this changed in 2020,” he added. “More extreme weather is likely.”
Not all climate scientists accept Hansen and company’s conclusion.
“I have nothing but respect & reverence for [Hansen] … but I think he is wrong on this one,” University of Pennsylvania climate scientist Michael Mann tweeted.
But Mann said they agreed on something important: We don’t understand the earth’s atmosphere as well as we need to. “And where there is uncertainty, we should weigh in on the side of precaution,” Mann tweeted.
Like the larger threat of climate change, this threat has loomed for all long time.
The thorny double-edged relationship between aerosol and carbon dioxide emissions is something Hansen had warned of as early as 1990.
In their 2021 paper, Sato and Hansen described the problem — that the longer we burn fossil fuels, the hotter it becomes when we finally stop — not simply in practical terms but moral ones.
Their point wasn’t subtle. The paper was subtitled “Faustian Payment Comes Due,” about the legendary doctor who makes a deadly deal with the Devil in exchange for an enviable life — at least for a while.
But there was one difference, they noted: “Dr. Faustus had to pay the debt himself. We have willed it to our children and grandchildren.”