Solar Geoengineering May Be Our Last Resort for Climate Change. What If It Doesn’t Work?

Photo: Pierre Longnus/Getty Images

Last week, at a dinner organized by the New York Times to promote Nathaniel Rich’s magazine-length global-warming history “Losing Earth,” the legendary climate scientist Wallace Smith Broecker — who coined the term “global warming” and is, miraculously, still with us and, at 86, still working — raised his hand and asked to speak. The room had been chattering about the importance of hopefulness in fighting climate change, and in writing about it — a common refrain among advocates battling the possibility of burnout. Broecker wanted to offer a dose of perspective — to cut against the hope a bit, and, he said, give just a sense of how big the problem really is.

There are now devices, he said, capable of extracting carbon out of the atmosphere — that’s the good news. They have about the mechanical complexity of a car, he said, and, at roughly $30,000, cost about as much — that’s the first part of the bad news. To merely match the amount of carbon we are putting into the atmosphere every day, he said, we’d need 100 million of them. To take carbon down by just 3 parts per million (ppm) per year — we are at about 410 ppm right now, and rising, already about 60 ppm above the 350 ppm threshold most scientists believe is the tipping point of problematic change — we’d need 100 million more. To take it down by 20 parts per million, we’d need a billion of the machines, distributed optimally around the planet. Each of them, he reminded the audience, would cost $30,000 — and that was just to build them. Then we’d have to run them.

Of course, Broecker continued, there was another option: solar geoengineering. We could shoot sulfur dioxide into the atmosphere, where it would turn into droplets of sulfuric acid capable of reflecting at least some of the sunlight coming toward Earth — which could counterbalance the planet-warming effects of more greenhouse gases in the atmosphere. It would also be much cheaper. He told me a similar thing when I visited him last year, while researching a long feature surveying worst-case scenarios for climate change. He said then that he doubted he’d live to see the experiment undertaken. “But in your lifetime …”

“Is there any reason not to do it?,” another guest at the dinner interjected. Well, there are risks and downsides in any environmental intervention, Broecker replied, but in considering whether to do anything you need to also consider the scale of the problem you are dealing with, and not forgo something that will help a lot, with a very big problem, just because it might also hurt a little. In the big picture, he suggested, the cost-benefit analysis was mathematically clear. But even the trivial considerations have pluses and minuses: an atmosphere full of sulfur would make our sunsets exquisitely beautiful, Broecker said, but of course it would mean the end of blue skies, too.

Over the last few years, as the climate news has grown increasingly bleak — not just in terms of what used to be called natural disasters but from academic research surveying likely future effects — this has become a somewhat common feeling among climate scientists and others most concerned about the fate of the planet: Worse comes to worst, we can pump some SO2 into the air. Geoengineering used to be dismissed as sci-fi fantasy, an obviously foolish strategy with so many drawbacks it would be silly, if not suicidal, to consider as an alternative to dramatically cutting back on emissions. More and more, it is being talked about as a last-resort backstop, to be deployed when all our other interventions fail. And, more and more, that backstop seems at least like a real possibility, considering that, climate awareness and green-energy revolution notwithstanding, globally our emissions are not only not shrinking dramatically, as they need to, they are not shrinking at all. In fact, they are growing.

Today, those counting on that backstop got some bad news. In a new paper, published in Nature, Jonathan Proctor, Solomon Hsiang, Jennifer Burney, Marshall Burke, and Wolfram Schlenker offer what is the first study of geoengineering ever to appear in the journal. Their study is not a complete assessment of the risks of suspending sulfur dioxide in the atmosphere, focusing only on the likely agricultural effects. But their findings are not encouraging: The negative impact on plant growth, they say, would almost entirely cancel out the positive impact of cooling. In other words, at least in terms of agriculture, solar geoengineering promises no net benefit in addressing the impacts of global warming.

When you think about it, the finding is not too surprising: This kind of geoengineering would shade the planet from sunlight, and sunlight, of course, is a major driver of plant growth. Which also suggests the findings may have some dispiriting implications for using geoengineering with other strategies of climate-change mitigation and adaptation, namely the hope that reforestation and what is called “bio-energy with carbon capture and storage” — or BECCS — could meaningfully minimize global warming. Both of these strategies would work, theoretically, by deploying plants’ natural photosynthetic processes, which draws carbon out of the air and produces oxygen, at grand scale. Unfortunately, photosynthesis requires sunlight, so a planet receiving less of it would also suck less carbon.

This is especially distressing coming on the heels of several bits of recent analysis on the “solutions side” of the climate crisis: a report of the European Academies’ Science Advisory Council found that negative-emissions technologies have “limited realistic potential” to even slow the increase in concentration of greenhouse gases in the atmosphere; a letter in Nature Climate Change described the forestry and agricultural technologies behind BECCS as “difficult to reconcile with planetary boundaries”; and one calculation measuring the effectiveness of BECCS found that keeping the world on track for Paris goals “would require plantations covering two to three times the size of India — a third of the planet’s arable land,” or more than double that which is currently used to produce all the world’s agriculture. The climate world is so desperate for good news that when, in June, an encouraging but fairly modest new carbon-capture technology was announced, it was breathlessly reported as a major breakthrough capable of “stopping” or even “reversing” climate change. In fact, the new technology cut costs, but only enough to make it still the most expensive of all existing forms of carbon capture, and the paper’s lead author had to come out publicly and clarify. “No one sensible is advocating that we bet on this Tech to stop climate change,” he wrote, adding (ominously for a technologist): “Climate change will never be stopped by a technology.” Which is all to say that, as immediate news from the climate has grown somewhat dramatically bleaker just over the last year, the news from the laboratories working on last-ditch strategies hasn’t been much better.

Notably, the scientists behind the new paper — two of them, Solomon Hsiang and Marshall Burke, are in an emerging field of scholarship studying not just how climate change is unfolding but how it is likely to affect human life on the planet — do not believe their new research represents a decisive argument against geoengineering. The agricultural costs may cancel out the agricultural benefits, but that does not mean the same will hold for other climate impacts — the effect on arctic ice melt, for instance, would surely be positive, and what solar geoengineering would mean for the economy beyond agriculture and for public health is not yet clear (though some studies do show sulfur sufficient to cool the planet by a degree would cost tens of thousands of lives each year, at least). And so, the authors of the new paper suggest, more research is needed; in the meantime, they say, we should not disregard SO2 as a possible, partial solution, no matter how dramatic its impact on the world’s food production. Which is, really, quite remarkable logic — and a sign, as Wally Broecker might say, that the problem is just that big.