Geoengineering the planet in the face of daunting challenges posed by a changing climate. It’s a crazy idea. Harvard researcher Gernot Wagner clearly acknowledges that and devoutly hopes things never get to that point.

But Wagner is just as emphatic in supporting our seriously researching the issue and learning all we can in case, just in case, it becomes the last best option.

It’s time to start thinking seriously about flooding the stratosphere with sulfate particles to cool the planet, says Wagner. A former energy expert with the Environmental Defense Fund, Wagner is a research associate at Harvard’s School of Engineering and Applied Sciences. He is co-author with Harvard economist Martin L. Weitzman of the 2015 book Climate Shock. To address the challenges of a rapidly heating planet, solar geoengineering has to be on the table, right up there with global efforts to cut carbon emissions, he says.

“If the world is serious about limiting global average warming to 2 degrees Celsius, and especially 1.5 degrees, we must seriously look at solar geoengineering as a possible option in the climate policy toolkit,” Wagner said by e-mail recently.

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‘Simple’ idea?

The idea seems simple enough: sulfate particles reflect light, so disperse planeloads of them into the stratosphere. That will reduce the amount of sunlight that reaches the ground – helping to cool the planet, or at least minimize the excessive warming.

That is just what happened naturally with the 1991 eruption of Mount Pinatubo, which dumped about 17 million tons of sulfur dioxide into the stratosphere – lowering average global surface temperatures in the northern hemisphere by nearly 1 degree F in 1992-1993.

Of course, replicating the effects of a volcano by artificially pumping megatons of sulfate particles into the stratosphere is rife with risks, some known, most unknown.

Wagner isn’t alone in calling for a serious look at solar geoengineering, as he did recently in Mashable, Wired magazine (with Harvard colleague David Keith), and elsewhere. And he’s hardly a lone voice in the wilderness. Keith too has also argued for governance that’s needed to guide research into solar geoengineering.

It’s true that progress is being made to curtail greenhouse gas emissions: There is the Paris climate accord of 2015; a historic United Nations agreement on October 6 to reduce international aviation emissions; an October 15 agreement in Kigali, Rwanda, by 170 countries to eliminate heat-trapping hydrofluorocarbons (HFCs); Justin Trudeau’s carbon tax plans in Canada; and more.

But that’s simply not enough, many say, and carbon emissions continue to rise globally. Last April, the Senate’s appropriations committee expressed a desire to have the Department of Energy research albedo modification.

We’ll need more than solar geoengineering

In September, the National Center for Atmospheric Research released a study that found that geoengineering with sulfate particles will require a sustained effort of artificially pumping 18 megatons of sulfate particles into the stratosphere every year for 160 years – 160 years! – to slow the rate of warming.

And geoengineering alone won’t do it. The NCAR study assumes that the globe will also drastically cut carbon emissions beginning in 2040. But even in that best-case scenario, we’ll still see the consequences of elevated levels of CO2 already built into the climate system: more episodes of extreme heat in North America, more retreating sea ice in the Arctic, changing patterns of precipitation globally, and more. It just won’t be as severe as doing nothing.

Wagner says he is an optimist and also a realist. “Solar geoengineering is not a replacement for cutting emissions,” he says. “Whatever analogy you prefer — a Band-Aid, a fire extinguisher, chemotherapy for the planet, etc. – all point to the fact that we must treat the underlying condition. That means cutting CO2 and other greenhouse gases.”

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In Climate Shock, Wagner and Weitzman frame the climate problem in economic terms: Carbon pollution is a “free-rider” problem. “It’s in no one’s narrow self-interest to do enough” to cut their own carbon pollution. On the other hand, solar “geoengineering is so cheap to do crudely, and it has such high leverage, that it almost has the exact opposite properties of carbon pollution…. it’s the ‘free-driver’ effect that may push us to geoengineer our way out of [global warming].”

And that’s precisely the danger, Wagner and Weitzman acknowledged in Climate Shock, and Wagner has been emphasizing repeatedly. Despite the daunting scale that the NCAR science team outlined in their recent study, throwing a bunch of sulfate particles into the stratosphere would not be prohibitively expensive or complicated.

Wagner and Weitzman wrote in Climate Shock that it could cost as little as $1 billion to $10 billion annually to get average global temperatures back down to pre-industrial levels, and it could be accomplished with “a fleet of a few dozen planes flying around the clock.”

While the NCAR study suggests that the effort must be sustained over a century and a half and coupled with massive cuts in CO2 emissions, the point that Wagner and Weitzman make is that the geoengineering part is doable economically. It’s no pittance, but clearly within reach at a global scale given the cost of damages avoided.

“Geoengineering is too cheap to dismiss as a fringe cause developed by sinister scientists looking for the next big sensational issue to attract some attention and grant money, as some pundits would have it,” Wagner and Weitzman wrote in Climate Shock. “If anything, it’s the most experienced of climate scientists who take the issue most seriously. And not because they like to.”

Potential downsides are substantial

The potential downsides of solar geoengineering clearly are enormous.

In February 2015, the National Academy of Sciences released “Climate Intervention: Reflecting Sunlight to Cool Earth,” a report that warned against artificially modifying the globe’s albedo – its reflectivity – until more is known about the risks and benefits.

Wagner and Weitzman also warn that resorting to geoengineering may just be too easy, lulling the world into wrongly thinking it can get away without having to cut carbon emissions. They see that cop-out as a nonstarter.

Meanwhile, there are likely to be unintended consequences – some known and many unknown. We know, for example, that sulfur dioxide damages the ozone layer. But we don’t know how reducing the amount of sunlight that reaches the ground will alter rainfall patterns.

And what about nonlinear responses? What if an agreed-upon international effort cools the planet by orders of magnitude more than expected? “It’s not hard to imagine a scenario where a large decrease in total solar radiation could result in temperature decreases to below pre-industrial levels,” Wagner and Weitzman write. “Runaway global warming is bad. Creating an artificial ice age would not be great, either.”

Yet another negative consequence Wagner and Weitzman point to: Sulfate particles would eventually wash out of the stratosphere and rain back to the ground, where they could cause major public health problems. Many countries for years have worked hard to reduce sulfur-dioxide pollution in the lower atmosphere, which has improved the health of millions of people. Intentionally spreading it in the atmosphere to combat global warming would come with a cost to public health.

Then there are the unknowns related to human social behavior. Consider the threat of a rogue actor, whether a nation, corporation or individual, deciding to geoengineer on its own. “What if optimizing geoengineering for India hurts China, and vice versa?” Wagner and Weitzman write in Climate Shock. Would we want a geoengineering match between two nuclear powers, each with over a billion people?”

But Wagner isn’t as worried about rogue actors as one might think.

“Unilateral deployment is scary. It is also rather unlikely,” he says. He thinks it very unlikely that a rogue actor could act undetected. Furthermore, “why would anyone attempt that at the danger of being found out, without at least coordinating among some allies?”

A way forward: governance and oversight of research

More research into solar geoengineering is vital, Wagner insists, and now is the time for that effort to get seriously under way. But also vital is governance over how that research is to proceed. Climate Shock cites work by Keith, from Harvard, and UCLA’s Ted Parson, who proposed in 2013 a way forward.

In short, Keith and Parson wrote that research into solar geoengineering should be guided by key principles. Among them:

1. A moratorium should be declared for geoengineering experiments above a certain size.
2. Below a certain small-scale threshold, high-value research may proceed.
3. Projects should meet strong transparency requirements.

Wagner says concentrated research point in unexpected directions, conceivably making solar geoengineering safer than it may seem today. A study published in July examined other substances, including calcium carbonate, that when dispersed in the stratosphere may be more effective and less risky than sulfur dioxide. Keith was one of that study’s authors.

“The first/early result? Using calcite appears to have better properties than sulfate aerosols when it comes to reflecting back sunlight, and it helps restore the ozone layer,” Wagner says.

“I’m not saying that one paper proves it. It doesn’t. Much more work needs to be done before we can make any such statement. But that’s the point: Much more research must be done.”

There’s been a lot of work to see where a relentless rise in greenhouse gas emissions may take the planet. It’s time now to look more closely at what people can do in addition to burning less carbon.

“We must finally allow the research to proceed to figure out what the benefits and risks truly are,” Wagner says.