Since the Industrial Revolution, burning coal and oil has filled the air with sulfur, shortening the lives of billions of people. In response, countries passed stringent air pollution laws requiring coal plants to scrub out sulfur and ships to switch to cleaner fuels. Global sulfur emissions have fallen some 40 percent since 2006. China alone has slashed them by about 70 percent.
We should celebrate cleaner air, but we also have to reckon with an unintended consequence. It turns out that by reflecting sunlight back into space, tiny sulfur particles protected Earth from about a third of the warming caused by human emissions of carbon dioxide. Now more of the underlying greenhouse gas warming is showing through, accelerating climate change. As The Economist recently put it, “If India chokes less, it will fry more.”
For some of us in the world of climate science, this raises a thorny question: Should we explore replacing the inadvertent cooling effects of sulfur with a cleaner, deliberate version?
Geoengineering the climate in this way is not a new idea; it was in the first U.S. high-level climate report that reached President Lyndon Johnson’s desk in 1965. While several options have been proposed recently, the most plausible way to make Earth more reflective is to use a small fleet of high-altitude aircraft to increase the amount of sulfuric acid droplets in the upper atmosphere. We know this can work; when volcanic eruptions put large amounts of sulfur in the upper atmosphere, such as Pinatubo in 1991, Earth was noticeably cooler for a few years.
We should take such an idea seriously because the costs of losing accidental sulfur cooling were made painfully evident this year when heat waves pushed temperatures above 120 degrees Fahrenheit in the Middle East and North Africa. If sunlight reflection could save lives and protect the environment, it is at least worth discussing.
Because sulfur is much more effective at cooling the planet when put into the upper atmosphere compared with what’s released into the lower atmosphere when we burn fossil fuels, we’d have to add far less of it. And for the same amount of cooling produced from burning fossil fuels, sulfur in the upper atmosphere would cause at least 100 times smaller health impacts.
Sunlight reflection is no panacea. Putting sulfur in the upper atmosphere will damage the ozone layer, allowing more ultraviolet radiation through. Even if it reduces deaths from heat and extreme weather, large-scale deployment could exacerbate climate change in some locations, perhaps by shifting rainfall patterns.
There is also a more fundamental limitation to sunlight reflection. It is effectively a Band-Aid that treats the symptoms of climate change but not the underlying disease of greenhouse gases. And unlike a skinned knee, the Earth does not heal from climate change on any time scale that matters for human societies. Warming from carbon dioxide is astonishingly persistent; much of what we emit today will warm the planet for many thousands of years to come.
Even if the world drives emissions down to zero, the planet wouldn’t cool down for millenniums. The only durable way to return to cooler temperatures is to remove the excess carbon we have already added, and removing enough to reverse even 0.1 degree Celsius of warming would cost tens of trillions of dollars.
This means that the real risk of geoengineering is not some Hollywood-style catastrophe, but complacency. A cheap way to delay the effects of warming risks undermining the need to rapidly reduce emissions, and going down that path would risk locking our children into a dependency where even stopping the process becomes too expensive to contemplate.
Given all this, we are not advocating deploying geoengineering today. But if policymakers decide that it is needed, a more modest approach would be to run a small, carefully scaled program that slightly increases the upper atmosphere’s reflectivity to compensate for the loss of cooling as sulfur pollution is eliminated.
The goal would not be to dial the Earth to some preferred temperature, nor to offset all greenhouse warming. It would be to keep the total cooling from sulfur roughly constant for a period of time, reducing near-term climate risk while decarbonization efforts continue.
As world leaders gather in New York for the U.N. General Assembly and Climate Week, any discussions of sunlight reflection should have a clear, enforceable commitment to never cool the Earth more than today’s current sulfur emissions do. And it should come with a clear off-ramp: As the world reaches net-zero emissions and scales up carbon removal technologies later this century, the program should end.
Pacing matters as much as limits. If society ever chooses to test this approach, it should start small and move slowly. Tying it to reductions in air pollution allows a slow ramp-up, resulting in increments that are imperceptible to most of us but visible to satellites and sensors. This should be coupled with regular checkpoints to assess side effects on regional rainfall, the atmosphere and ozone. The intent is to buy a modest, temporary buffer, not to start a new arm of climate control.
If we stay focused on the cure of reducing emissions and consider bounded and temporary sunlight reflection, we could preserve cleaner air, avoid a near-term temperature surge and not betray the generations to come, who will live with the consequences of our choices today.
Zeke Hausfather is the climate research lead at Stripe and a research scientist with Berkeley Earth. David Keith is a professor of geophysical sciences and founding director of the Climate Systems Engineering Initiative at the University of Chicago.
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