One carbon removal technology is planting trees, in turn sometimes subsumed under a broader umbrella of “natural climate solutions.” That is surely part of the overall solution, but it can indeed only be one part of it. Planting trees might sound more innocuous than building large industrial facilities to take CO2 out of the atmosphere; however, it also comes with significant limitations. One of these is the time and space needed to plant the billions of trees needed to make a dent in atmospheric CO2 concentrations. Another is permanence. Trees decay, releasing CO2 in the process. In technical terms, trees help take CO2 out of the atmosphere, but they keep the carbon in the biosphere instead of returning it to the geosphere. Other carbon removal techniques do, in fact, remove CO2 from the biosphere entirely.
Meanwhile, even planting trees has now been used as a delaying tactic to avoid doing what’s necessary. U.S. Republicans under President Donald Trump, for example, have used their “One Trillion Trees” initiative as a way to detract from the need to cut CO2 – moral hazard in action, or perhaps better: moral hazard inaction. None of this, of course, means that we should not be planting more trees. We should. However, we must not use it as an excuse to delay CO2 emissions cuts.
A possible role for carbon removal and solar geoengineering
Most importantly, we must stop burning fossil fuels and putting CO2 into the atmosphere. Nothing else will do. There are indeed other, even more potent, and thus important greenhouse gases. Methane (CH4), for example, might be more important than CO2 for the rate of global warming – something solar geoengineering, too, has a direct role in affecting (see Chapter 2).9 Nitrous oxide (N2O) is similarly more potent than CO2, around 300 times so on a 100-year timescale. And yes, technically water (H2O) is the most important greenhouse gas of them all. However, human CO2 emissions stand alone in their long-term influence on the changing climate.
Cutting CO2, even to zero, will only stop the further increase in climate impacts. It won’t stop them altogether. That immediately leads to another important step: coping with what’s already in store. Not unlike both carbon removal and especially solar geoengineering today, mentioning climate adaptation was once considered taboo among many committed environmentalists, and for similar reasons. “Let’s stop climate change first,” the refrain went in the 1990s, “only then can we start talking about adapting to warming already in store.” Even Vice President Al Gore believed as much at the time, considering adaptation a mere distraction. He has long since publicly changed his mind on the topic.10
Adaptation, of course, can only go so far. For one, there are the usual endemic inequalities. It’s the rich who adapt. The poor suffer. Then there are limits to adaptation. Building a seawall to protect against extreme storm surges is one thing; adapting to one or two meters of sea-level rise by century’s end by moving entire cities to higher land within decades is quite another. Parts of Miami are flooding today, on sunny days.11
Enter carbon removal, taking excess CO2 out of the atmosphere and, ideally, putting it back underground, into the geosphere. Carbon removal, meanwhile, comes with important caveats of its own, not least the same kind of moral hazard that beset earlier adaptation conversations. Equally important, much like cutting CO2 emissions in the first place, removing it from the atmosphere is both slow and, for the most part, relatively expensive.
Solar geoengineering, by contrast, is fast, cheap, and imperfect.12 These three characteristics make solar geoengineering unique among possible climate policy interventions. They also go to the heart of the solar geoengineering gamble. Little is fully known and, thus, certain. Lots depends on details yet to be worked out, and some may never be known for sure. Governance is key. Each of the three core characteristics figures in this assessment.
Fast, cheap, and imperfect
Fast means that solar geoengineering, fully deployed, could help lower global average temperatures within weeks and months – rather than the years and decades that it would take for CO2 reductions. For example, Mt. Pinatubo’s eruption in June 1992 in the Philippines lowered global average temperatures by around 0.5°C within a year. A year later, temperatures were back to normal and have been rising ever since (see Chapter 2).
Cheap is relative, but most estimates put the direct engineering costs for deploying stratospheric aerosols at a scale somewhere in the single-digit billions of dollars per year. Think of several dozen newly designed planes with large fuselages and enormous wingspans flying missions into the stratosphere around the clock.13 That’s not exactly free, but it might as well be. The direct deployment costs are in the single-digit billions of dollars, compared to cutting CO2 emissions or removing carbon ex post, both typically measured in trillions of dollars. It is cheap enough to ensure that the direct costs do not matter meaningfully in a deployment decision made by the world’s governments.
Imperfect is just that: solar geoengineering does not address the root cause of excess CO2 in the atmosphere. It comes with plenty of potential risks. It might be a really bad idea to contemplate, and worse to actually go through with. Equally important, none of that might matter in light of the first two characteristics, all but pushing the world toward deploying solar geoengineering sooner than most of us might deem possible – or desirable – today.
The combination of fast and cheap puts solar geoengineering at the exact opposite end of the spectrum from cutting CO2 emissions in the first place. Whereas cutting CO2 is all about motivating more people, companies, and countries to do more, solar geoengineering governance is largely about stopping premature deployment – doing it too fast, too much, stupidly.
A gamble worth exploring
One does not need to like solar geoengineering to take the idea seriously. I don’t like it. The mere thought of it is scary, as I believe it should be. Somebody somewhere will surely find a way to abuse it. Conceptually, as a foil for ambitious CO2 cuts, people already have. In 2008, at the height of the most significant U.S. federal climate policy push to that date, Newt Gingrich wrote an op-ed saying how solar geoengineering shows that we don’t need to cut CO2 emissions.14 If only.
I remember shaking hands with David Keith on Saturday, December 12, 2015 in my living room in Cambridge, MA, agreeing to work on what would turn into Harvard’s Solar Geoengineering Research Program. The day is significant for indeed a much more significant reason. It was the same day that the Paris Climate Agreement was gaveled into place across the Atlantic. The irony of the moment was not lost on either of us.
The Paris Agreement has been widely hailed for breathing new life into sluggish global climate negotiations. Nobody thought it would solve climate change. Nothing can, by itself. But the Agreement clearly did show some momentum in the right direction and, after a four-year hiatus here in the United States, the pendulum is once again swinging hard in the right direction, hopefully without avoiding the swing back. All of that momentum toward more ambitious emissions cuts is clearly good, and nothing should take away from it!
While somewhat ironic then, it is precisely against this backdrop of increased global ambition to cut CO2 emissions in the first place, and a broader understanding of the importance of serious climate action, that solar geoengineering should be discussed.
It must not be either–or. The best approach is a balanced portfolio, where solar geoengineering might have some, at most temporary, role in mitigating the worst effects of climate change, while the world cuts CO2 emissions rapidly – to zero, and then some.15
Such a balanced