In every region of the world, the study found, temperatures grew less extreme with geo-engineering. And while geo-engineering disturbed the climate of some regions in new but small ways, it overall reduced the effects of climate change in places it was worst. “Those regions experiencing the greatest climate change are the most likely to see it reduced by [solar geo-engineering],” the study says.
There are many asterisks here. The study focuses on very vast regions: One of its areas encompasses the entire Pacific coast of South America; another includes almost all of Pakistan, India, and Bangladesh. It also does not study the lines on the planet that may matter far more: national borders. Even if rainfall remained constant in each of the studied regions, it could still cause conflict by shifting from one country to another, disrupting agriculture and water supplies. The study cannot examine that possibility.
The study also examines a form of unadulterated climate change that will likely be easier than what we are on track for: It uses a scenario that assumes carbon-dioxide levels will sit at about 560 parts per million, but even some moderate scenarios assume they will pass that point in the middle of the century. (The absolute worst-case scenario projects 1370 parts per million by 2100.)
“I am not saying that we know solar geo-engineering reduces risk,” Keith said. He acknowledged that the paper presents idealized risks, and those may be wrong. But the paper also, he said, makes “the most important case that solar geo-engineering could be really useful.”
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“It’s important to keep in mind that this study actually tells us very little about the feasibility of the idea of geo-engineering,” Flegal said. “It is reasonable to ask whether we expect the real world to behave like these models. [And] even if the real world behaved like these models, it is not clear to me that we should expect that this research will inform ‘rational’ decision making in this domain.”
“People’s experience of climate is not entirely driven by physical climate variables,” she added. “It is mediated by all sorts of other cultural, political, social, and economic factors.” In other words, even if solar geo-engineering didn’t cause a particularly bad weather outcome, people may still blame it.
While scientists have long hypothesized about the role of solar geo-engineering, serious researchers avoided the topic for all of the 20th century. That changed 13 years ago, when the Dutch chemist Paul Crutzen, a recipient of the Nobel Prize, called for a new program of “active scientific research” into the technique. Since then, governments and philanthropists have begun funding programs on the topic.
“There’s much more interest among really senior science-policy leaders than there used to be,” said Keith. In 2017, during President Barack Obama’s final days in office, the White House proposed a multiyear, comprehensive investigation into the topic. The National Academies of Science, Engineering, and Medicine also recently formed a committee to study it.