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Geoengineering is in the wind more and more these days, particularly the use of sun-blocking aerosols as a cheap, temporary counterweight to greenhouse-gas-driven global warming.

In pondering the plausibility or desirability of such a tool, it might be useful to start with a thought experiment:

1) Suppose humans are not heating the climate and oceans through the buildup of heat-trapping carbon dioxide. (This is only a thought experiment.)

2) Presume our capacity to understand Earth systems and devise sophisticated technologies continues to build. (Keep in mind this isn’t a given if budget priorities don’t shift.)

3) Consider the cost, in lives and money, exacted by today’s climatic extremes, let alone those worsened by warming. Many such costs can be reduced by developing suitable crops and water systems or building resilient communities. But not all. Then, on a very long time scale, consider the prospect of an inevitable new ice age.

Sifting these notions, it’s hard to avoid the conclusion that there will almost certainly come a moment when humans will start designing our climate and not simply perpetually adapt to its vagaries.

On a small scale, there’s already weather modification, from the Midwest to China. Researchers are even examining ways to stifle hurricanes. In the long run, you can be sure that humanity will do everything it can to avert an ice age, given the challenge of sustaining civilization with advancing miles-high ice sheets. (Revisit Thornton Wilder’s “Skin of Our Teeth” for a surreal view of this challenge.)

So far, humanity’s main climate intervention, through emissions of vast amounts of greenhouse gases, has been an unintended consequence of pursuing the most convenient energy choice — fossil fuels.

Efforts to curb that pulse of gases haven’t amounted to much, even with the Paris Agreement taking legal force on Nov. 4.

With all of this in mind, it could be argued that the momentum driving global warming is simply speeding the journey toward an inevitable juncture when we will start engineering the climate.

We’ve been terrible at managing emissions. Can we shift from unintended global warming to managing climate by design?



Welcome to the geoengineering debate.

I encourage anyone interested in climate change science and policy to read on for the rich discussion of geoengineering that follows, involving some of the analysts and scientists most involved in examining next steps. They include Oliver Morton of The Economist, Raymond Pierrehumbert of Oxford University, and Gernot Wagner and David Keith of Harvard.

The specific focus here is whether recent scientific conclusions about the inevitability of thousands of years of global warming have left out possible interventions involving brightening the planet to reflect some incoming solar energy.

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Geoengineering concepts have been discussed for decades. But a 2006 essay in Climatic Change* by the chemistry Nobelist Paul J. Crutzen kicked the community of atmospheric scientists into higher gear, proposing tests while noting that efforts to curb greenhouse gases appeared to be mostly “a pious wish.”

I have long supported investing in research on this possible response to global warming but questioned its real-world prospects. I still can’t imagine a scenario in which a single actor would initiate some sun-blocking initiative or, in contrast, a global consensus could be reached on its deployment (unless things get truly unhinged on the up side).

In many past posts, to illustrate that point, I’ve asked, “Who gets to set the global thermostat?”

Look at how hard it’s been gaining global agreement on a path to limiting humanity’s largely unintended warming influence through the buildup of heat-trapping emissions produced by our growth spurt.

But I’ve been shifting my thinking based on recent conversations with some of the analysts below pointing to a reasonable, testable, incremental path to managing sun-blocking aerosols as the world tackles the far tougher and costlier effort to decarbonize a growing economy that remains deeply dependent on fossil fuels.

We’ve begun managing the sooty particulate pollution that is linked to millions of premature deaths and also some warming.

With a bit more tailoring, there could be management of those particulates, high in the stratosphere, that have a volcano-like capacity to cool things a bit. See, for example, this 2015 paper by David Keith and Douglas G. MacMartin of the California Institute of Technology: “A temporary, moderate and responsive scenario for solar geoengineering.” (Their work is part of a suite of research projects paid for through a fund established by Bill Gates.)

I still see scant prospects for action, but the conversation is vital. Walling off this arena makes as little sense as talking about feeding some nine billion people on a still-biodiverse planet without technology, including genetic engineering.

The context of this chat was a paper published early in the year in Nature Climate Change on the long commitment to warming, as explored on Dot Earth. (Pierrehumbert was one of many authors.) The exchange was prompted by an interrogatory tweet from me, responding to Gernot Wagner of Harvard.

Wagner, the co-author of a great book on global warming risk and economics, “Climate Shock,” moved from the Environmental Defense Fund to Harvard recently to focus full-time on geoengineering policy. His piece for Mashable last month has a headline that says much: “ It’s time to take solar geoengineering seriously, even though it seems outlandish.”

Here’s the tweet that started this discussion back in February:

.@Revkin talks very long run with Pierrehumbert: https://t.co/5zPnQFijEd Does geoengineering change that picture? https://t.co/QBZgzGu6qK — Gernot Wagner (@GernotWagner) 15 Feb 16

My reply initially elicited a deeply informed, thorough and provocative email from Oliver Morton, who is a senior editor at The Economist and author of a vital book on geo-engineering, “The Planet Remade: How Geoengineering Could Change the World.”

The resulting discussion (also including Ken Caldeira of the Carnegie Institution) was sufficiently thorough that I set the exchange up on Medium. I’ve been in eternal “slow blogging” mode of late sadly, so this Dot Earth post is very tardy.

Here’s the first part of Morton’s provocation and a link to the rest:

Oliver Morton | You were kind enough to ask on Twitter what I thought, in the context of geoengineering, of the Clark et al. Nature Climate Change perspective on deep-time consequences of 21st century climate policy and the Dot Earth post to which it led. I think the paper is fascinating but also somewhat frustrating, so I thought I’d take the opportunity your inquiry provided to think through some of the issues in rather more detail than twitter allows…. I agree with the authors that taking a view measured in millennia is an appropriate, and underappreciated, part of assessing the impacts of anthropogenic climate change and the need for climate action; I also agree with their conclusion (and starting point) that a zero-net-emissions human world is a desirable target. And it is inspiring to see such progress being made in the detail with which models of ice sheet dynamics and other forms of change can be applied to the moderately far future. But I have a problem with the way in which, while discussing some plausible — if far from inevitable — forms of technological change, the authors choose to ignore others. They repeatedly describe change on the timescales they are looking at as “irreversible”. There is a potent sense in which this is true: once a change has happened it cannot be made not to have happened; once they have risen, neither carbon-dioxide concentrations nor sea levels can be made not to have risen. But that’s not what most people mean by “irreversible”: the term normally means pretty straightforwardly “can’t be reversed”. And at a couple of points in their paper the authors make it clear that that is indeed the sense in which they mean it. Specifically, they say: “The implication is that, in the absence of efficient, large-scale capture and storage of airborne carbon (emphasis mine), carbon emissions that have already occurred or will occur in the near future result in a commitment to climate change that will be irreversible on timescales of centuries to millennia and longer.” Which is to say: in the absence of technologies to reverse this, it is irreversible. Well yes. And in the absence of a reverse gear, a car is irreversible, too. That’s why cars have reverse gears.

That may read as a flippant trivialization. But I think the authors of the Nature Climate Change paper trivialize the issue, too; they just do it more subtly, through neglect. The sentence I just quoted implies pretty strongly that, in the presence of efficient (or for that matter inefficient) large-scale capture and storage of airborne carbon, carbon emissions that have already occurred or will occur in the near future might not result in a commitment to climate change that is irreversible on timescales of centuries to millennia and longer. That’s quite an important statement. But it receives almost no follow-up at all in the subsequent six pages. A paper that makes use of the concept of the Anthropocene — a concept predicated on the idea that human activity is a dominant factor in the state of the earth system — does not spend any time at all looking at what humans might try to do, or be able to do, about the problems it discusses over the periods it imagines. It is easy to understand why not. This is a paper by natural scientists, and the natural sciences give you no way of understanding what people will attempt to do in the time-scales under discussion. (Nor does anything else, which is one reason Rob Socolow’s notion of “Destiny studies” is, as you say on Dot Earth, a welcome one.) But rather than discuss the impossibility of predicting or modeling human decisions or capabilities, the authors choose not to address the issue at all, and this undercuts the seriousness of their undertaking. When there is a huge source of uncertainty in your analysis you should be explicit about it. You should try to assess the limits it puts on the salience of your results and what might be done about those limits. Instead the authors choose to proceed as though humans will do nothing other than what they are doing today, and make no effort to justify their decision to privilege that singular scenario. That is what I mean when I say I think they trivialize the issue.

I know that there are not yet any negative emissions technologies up to the task. But the possibility of such things is under active discussion. Indeed negative emissions are already being incorporated into the sort of integrated assessment models that inform discussion like those of COP 21 in Paris. Those scenarios typically demonstrate a lack of specificity about the costs and potentially enormous impacts of such technologies, which makes their ready and convenient acceptance of a currently hypothetical capability disturbing. But to ignore the potential capability completely does not redress that problem.

I urge you to read the rest of Morton’s essay and the resulting conversation on Medium.

There’s an additional layer of useful discussion on the Geoengineering Google Group.

A good separate starting point is this National Research Council report: “Climate Intervention: Reflecting Sunlight to Cool Earth.”