In 1991, Mount Pinatubo erupted in the Philippines, injecting nearly 20 million tons of sulfur dioxide into the sky. This sudden burst of gas in the atmosphere reflected away sunlight from Earth and lowered the planet’s average temperature by about 1 degree Fahrenheit for a little longer than a year. If humanity wants to turn down the worst effects of global warming, then it simply needs to do that, with planes, indefinitely.

That is the idea, at least, of solar geoengineering. To its supporters, mimicking a large volcanic eruption is an ethical necessity, a way to soften the blow of climate change. “Perhaps the best reason to take cooling seriously is that benefits seem likely to go to the poorest countries,” David Keith, a Harvard professor of applied physics, wrote in The New York Times last year. “Poor people tend to live in hot places.”

The technology necessary for geoengineering has never been successfully demonstrated, but already many economists are sure that it will happen at some point this century. That’s because its theoretical costs are little more than a few flights, a spraying device, and a couple million tons of sulfuric acid—expensive, to be sure, but not out of the reach of your average island nation or climate-concerned billionaire.

But geoengineering is far from a sure bet. To its critics, the technology runs the risk of re-creating some of the worst mass extinctions in the planet’s history, which have been the consequence of boosting both the atmosphere’s carbon-dioxide level and its sulfate level. We’re already pumping CO2 into the sky, they argue. Why are we so sure that adding sulfates won’t just backfire?

A new study hints that solar geoengineering is going to be even more complicated than its proponents realize: It could expose tens or even hundreds of millions of people to malaria every year. “There’s a lot of research on the potential of geoengineering to reduce hazards, like extreme heat and sea-level rise,” Christopher Trisos, a co-author of the study and the director of the Climate Risk Lab at the University of Cape Town, in South Africa, told me. “But you can’t just assume that geoengineering universally reduces risks to people.”

In an extreme, worst-case scenario, in which humanity raises global temperatures by several degrees by 2070 before trying to bring them back to a 2020 level, geoengineering could put perhaps 1 billion people at an increased risk of malaria. The study found that geoengineering can reshuffle major risks around the world even in less extreme scenarios. In a more moderate simulation, where humanity achieves net-zero carbon pollution by the middle of the century, but still seeks to restore temperatures to 2020 levels, the tricky questions remain. In this world, countries in West Africa, such as Nigeria and Ghana, would see increased malaria risk for about 100 million people total. But in East African countries, such as Ethiopia and Kenya, geoengineering would reduce malaria risk for about 100 million people.

Many scientists worry that climate change will worsen the spread of some mosquito-borne diseases, such as dengue fever and yellow fever. But malaria is in a category by itself. In 2020, more than 627,000 people died of malaria, and some 241 million people contracted it. Malaria remains the sixth-highest cause of death in low-income countries, where it kills more people than car crashes, HIV/AIDS, and tuberculosis. In the words of the study’s authors, no other mosquito-borne virus has a toll even “remotely comparable” to the sickness and death caused by malaria.

The trade-off between geoengineering and malaria emerges for two reasons. First, solar geoengineering doesn’t perfectly turn back the clock on Earth’s climate. Even if you add enough sulfate aerosols to perfectly counteract the amount of warming from carbon pollution, you’re still changing the climate’s physics, not restoring what once was. Many geoengineering simulations produce strange phenomena, such as “tropical overcooling,” wherein land near the equator is cooler than you would expect, even while territory near the poles remains much hotter. The most recent Intergovernmental Panel on Climate Change reports found that these regional and seasonal distortions appear “with high confidence” in many types of geoengineering models.

And that’s a problem, because malaria does not have a linear relationship with temperature. The malaria parasite is spread by mosquitoes, which are cold-blooded and depend on the ambient air temperature to set the pace of their metabolism; malaria risk, then, tends to increase as the temperature gets hotter. It peaks at an average of 25 degrees Celsius, or 77 degrees Fahrenheit, Colin Carlson, a Georgetown University professor and co-author of the paper, told me. But as the temperature keeps rising, mosquitoes become less and less able to function, and at about 34 degrees Celsius, or 93 degrees Fahrenheit, they start “dropping out of the air.” That means there is a hard thermal peak to mosquito survival and, with it, malaria transmission.

The study found that tropical overcooling and the ideal temperature for malaria transmission can interact in troubling ways. In some parts of the world, geoengineering took a place that would have been too hot to allow mosquito survival and brought it back into a survivable range. In others, it restored the close-to-25-degree-Celsius temperatures that mosquitoes need to thrive.

Of course, mosquitoes are not the only animals that struggle to cope with extreme heat. Humans do too. If it’s so hot out that people and animals cannot survive, then geoengineering may make sense anyway. But the reality is likely to be far more nuanced. “I think it’s a gamble to say that when you add up the catastrophic biological risk [of high temperatures], what you have is not outweighed by other factors,” Carlson said.

One of those other factors might be meningitis, a dangerous and sometimes fatal infection of the tissue surrounding the brain and spinal cord. Because tiny particles of dust can increase the risk of meningitis, the disease is most prevalent in a swath of Africa called the Meningitis Belt, where people are regularly exposed to airborne sand and dust from the Sahara. Seasonal monsoons often rinse the dust out of the air. But in some simulations of geoengineering, the monsoon season weakens, Carlson said, exposing more people in sub-Saharan Africa to dust, and thus perhaps increasing their meningitis risk.

Even if geoengineering reshuffles the geography of malaria while not boosting the overall numbers, increasing transmission in new areas but extinguishing it in the old, that would create a public-health problem. Certain parts of the world have prepared their health and medical infrastructure for malaria; if the burden of the disease shifts to new places, those places won’t have the same built-up expertise and institutional factors designed for it. The disease will exact a higher toll, at least at the beginning.

Keith, the Harvard professor, who has called geoengineering the “least worst” way to cool the planet, told me that this study—and the broader effort it represented to quantify the health impacts of warming—was worthwhile. But he doubted that the math would come out against geoengineering. “It’s good [that] people are looking at it,” he said. “But we shouldn’t be at all surprised that solar geoengineering doesn’t uniformly improve health outcomes, because warming doesn’t uniformly harm health outcomes.” He did a little mental math: The raw destructive power of heat is expected to cause about 5 million deaths a year by the end of the century, he said. Malaria, by comparison, causes about half a million deaths a year, a number that has fallen since the turn of the century. Malaria has a lot of room to get worse before it rivals heat’s end-of-century burden.

More broadly, the study shows that solar geoengineering could worsen people’s lives even in the poor countries that it is supposed to help most. “What vexes me about geoengineering trade-offs is that they’re discussed as if there is one thing called ‘the global South’ that is on the front lines of climate change and that will benefit from ‘solutions,’” Carlson said. “This is the first trolley problem we’ve seen” in studying climate change’s impacts, he said. “There are winners and losers of this.”


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