Cooling the planet with a giant solar umbrella

Solar geoengineering would lower global temperatures. Are we ready to test it?
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Solar geoengineering research was once considered fringe, even taboo — an absurd hail-mary attempt to cool down the planet. Now, however, a small-but-growing number of scientists are saying it may be our only hope at reversing climate change.

Why It Matters

Scientists believe that, no matter how fast we cut the rate of new emissions, it’s already too late to stop significant warming. Meeting the Paris Agreement’s ambitious goal of holding warming below 2°C won’t be possible without also removing some carbon from the air. But such “negative emission” technologies are expensive and unproven, so it’s not clear they will work, either.

“If we can’t find a way to pull carbon out of the atmosphere quickly enough, then solar geoengineering would be the only option in the fight against climate change,” Andy Parker, a geoengineering governance and policy researcher, said.

That’s why scientists like Harvard’s David Keith say we should look at all options — no matter how radical — to reduce the risks associated with a warming planet. So far, researchers have been using computer simulations to study the effects of solar geoengineering, but pretty soon they want to take it outside.

“Theory alone doesn’t tell you what will happen in the atmosphere,” Keith told MIT Tech Review. “You can fool yourself if you don’t go out and make direct measurements.”

Solar geoengineering would be a truly global technology — whoever chooses to use it would be affecting everyone else on the planet.

Are we ready to test this in the wild?

Types of Solar Geoengineering

This is an umbrella term for several different approaches to reflecting sunlight before it hits the Earth, for example, sun shields in space, making ocean clouds more reflective, dissolving heat-trapping cirrus clouds — and, most famously, scattering aerosol particles into the stratosphere. This is the approach being pursued by Keith and his team at Harvard.

The Science

The stratosphere may be the perfect place to make the Earth’s atmosphere more reflective — particles injected there could spread around the globe and stay there for two or more years. If the particles are strategically placed, the aerosols would create a thin screen that reflects a small amount of sunlight back into space.

Most solar geoengineering research has focused on releasing sulfur dioxide. We know that this reflects sunlight back into space thanks to Mount Pinatubo’s massive eruption in 1991, which spewed 20 million tons of sulfur into the atmosphere and almost immediately lowered global temperatures by 0.5°C.

However, sulfur dioxide could also deplete the ozone layer, which shields the Earth from the sun’s ultraviolet radiation — that would be problematic, to say the least.

How the Harvard Experiment Would Work:

The researchers want to launch a high-altitude balloon — decked out with sensors and propellers — over New Mexico and spray a tiny amount of calcium carbonate into the stratosphere. (Calcium carbonate isn’t normally found in the stratosphere, but researchers think it’s safer for the ozone.)

The instruments would then record any changes that occur up there — they want to measure how the calcium carbonate interacts with gases in the ozone layer, how well the particles reflect sunlight, and if they disperse easily or merge together. (The tiny amount used in a would-be experiment is too small to actually alter global temperatures, according to the Harvard researchers.)

More Than We Bargained for?

Scientists generally agree that solar geoengineering would cool global temperatures. What they don’t know is what else it will do.

Alan Robock, a professor of environmental sciences at Rutgers, has come up with a list of 27 risks associated with the technology, including depleting the ozone layer, a drought in Africa and Asia, and turning the sky white.

More abstract concerns include: who gets to deploy something that affects literally everyone on the planet? The technology is fairly cheap and widely accessible — it’s thought that basically anyone with a few billion dollars could do it, at least for a couple of years.

“We know the problem is greenhouse gas, so the solution is you take the greenhouse gas out,” Daniel Cziczo, the head of the department of earth, atmospheric and planetary sciences at Purdue University, told MIT Tech Review.

“You don’t try to do something that we completely don’t understand.”

Harvard’s David Keith argues that our limited understanding is precisely why we need to do scientific research. “If you choose not to research, that means we may later make decisions in ignorance and in a rush, and that’s typically how you make bad decisions,” he told The Washington Post.

We Have To Cut

Still, there’s something that both opponents like Cziczo and researchers like Keith can agree on: we need to cut greenhouse gas emissions. If we did try solar geoengineering, it would be complementary to emissions reductions, to avoid severe and rapid warming, not in place of a long-term solution.

“Nothing changes the fact that in the long run, the only way to manage carbon risk is to stop emitting carbon-dioxide,” said Keith. “But, similarly, nothing we know about cutting carbon-dioxide emissions says that’s going to help us deal with the risk of CO2 that’s already in the atmosphere, or deal with climate risks in the very short term.”

For Keith and other geoengineering researchers — solar geoengineering could be the short-term solution we need.

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