In 1991, the Philippine volcano Mt. Pinatubo erupted, sending millions of tons of sulfur dioxide into the stratosphere — and kicking off the modern era of thinking about geoengineering. Solar radiation management, the process of modifying the amount of sunlight that warms the Earth, has been an ongoing topic of early-stage scientific investigation and debate.
But a paper published today in PLOS Climate argues that the best place to start looking at cooling the Earth might not be on the Earth at all. It might be the moon — at least that’s what a team led by Benjamin Bromley, an astrophysicist at the University of Utah, argues.
“We’re not the first to consider putting something between us and the sun,” Bromley tells Inverse. But the amount of energy required to launch a swarm of satellites or a thin-film solar shield that could block out about 1.8 percent of the Sun’s energy — enough to offset anthropogenic climate change — would be the equivalent of 25,000 Saturn V rockets.
How much dust? About 100 million tons a year, which is thousands of times more than all the mass humanity has ever launched into space combined.
In the scale of the solar system, “a small amount of dust,” he says, “can really intercept a lot of sunlight.” This is at least on the scale of Bromley’s usual subjects, which are the dust clouds that consolidate to form planets. “We were just looking to see what the possibilities were, and we learned very quickly that it takes a lot of dust to make a difference.”
How much dust? About 100 million tons a year, which is thousands of times more than all the mass humanity has ever launched into space combined.
By launching dust from the Moon, rather than satellites or solar shields from Earth, humanity could take advantage of the Moon’s lower gravity and abundant, well, dust. The study proposes that lunar dust could be launched towards the Earth’s first Lagrange point, or L1, a spot in space where the gravity of the Earth and Sun stabilize.
Lunar dust makes for a very good reflector, allowing for what Bromley calls “the simplest possible mining operation on the Moon.” It is difficult to get the dust to stay in place — the pressure of sunlight on the tiny grains pushes them away over the course of a few months, so it would need to be replenished constantly. You would “need a precisely milled dust grain to stay at the Lagrange point,” he says.
“We went without consideration of feasibility,” Bromley explains. “We were playing with ideas; reality will rein them in.”
An undertaking on this scale would be far and away the biggest engineering project in human history. While the amount of moon dust that would be needed to cool the Earth is small compared to cosmic bodies or even, say, the 8 billion tons of coal mined annually, a solar radiation management program would be the most complicated engineering and political project ever seen.
But the vast amount of equipment needed to mine, refine, power, and launch even the simplest moon dust would “require a huge effort on the part of humanity to implement,” says Bromley. And all that would be a drop in the bucket compared to the 94 million tons of moon dust it would fire at the Lagrange point between the Earth and the Sun every year. That’s thousands of times more than the total mass humanity has launched into space since Sputnik first went beeping around the Earth 66 years ago, and it would need to be repeated annually for the foreseeable future to avoid catastrophic termination shock.
How much energy would it require? According to the team’s calculations, launching enough moon dust sunward to shade the Earth — just shy of 100 million tons — would take 166 terawatt hours of energy a year. It would be, the team notes in their paper, the equivalent of launching 2000 Saturn V rockets.
That would be 150 percent of the output of the 112 terawatt-hours generated by the largest power-generating complex ever built on Earth, the Three Gorges Dam, which is powered by a 400-mile-long reservoir. It’s enough to put the project as the world’s 25th-highest consumer of energy, just behind Thailand’s 70 million people.
Last year, NASA announced a set of proposals to put nuclear power plants on the moon by the end of the 2020s. But those reactors would generate 40 kilowatts. That’s more than enough for a small lunar base but about 4 billion times less than it would take to power a solar radiation management system. Put another way, it’s about 1.4 billion times more than the International Space Station, which sports the largest set of solar arrays in space.
Dimming the sun’s light would affect every corner of the globe equally and would have to be continued over the course of generations.
What is still less clear is how a project like this, either in space or in the Earth’s atmosphere, would be governed. Dimming the sun’s light would affect every corner of the globe equally and would have to be continued over the course of generations.
If the Earth were to be cooled by blocking out some small fraction of the Sun’s light, that doesn’t mean it would halt the processes underlying global warming. If human society used managing sunlight to mask global warming while continuing to pump out carbon dioxide, a 2021 report by the National Academies of Science, Engineering, and Medicine warns, it would entail an “unacceptable risk of catastrophically rapid warming” if it were to be ended or interrupted, the “termination shock” caused as Earth warms to the level it would have experienced otherwise.
Even though solar radiation management projects like this would be the most massive technological undertaking in human history, Bromley emphasizes that “this is not a pass for our work on climate change, despite the huge effort — this is a temporary fix.”
