A British academic group has published a major report on geoengineering technologies that could be used to mitigate climate change. These technologies come in two flavors: carbon dioxide removal and solar radiation management (reflecting the sun's energy back into outer space). The bottom line is that many of these technologies have potential, but also pose significant risks and are likely to be extremely costly. For example, capturing carbon dioxide from ambient air would be the most precise and accurate method of addressing climate change, since it would remove the source of the problem; but the know technologies are not cost-effective. Stratospheric aerosols (sending particles into the air to reflect solar energy back into space, as occurs after volcanic eruptions) are cheaper but could have potentially catastrophic effects on other aspects of the climate, such as the ozone layer.

It is not hard to anticipate that, despite these problems, geoengineering will become the rallying cry for skeptics of a climate treaty. Indeed, the British scientists clearly anticipated this reaction, and prepared for it with a rather gloomy title for their press release: "Stop emitting CO2 or geoengineering could be our only hope"—an otherwise puzzling start to what might seem like a pretty optimistic take on an approach to the climate problem which seemed out of the realm of possibility not many years ago. The first sentence of the press release drives the message home: "The future of the Earth could rest on potentially dangerous and unproven geoengineering technologies unless emissions of carbon dioxide can be greatly reduced, the latest Royal Society report has found." Yet the conclusion of the report is that we should invest heavily in researching these technologies.

But the authors of the report are right to worry about climate-treaty skepticism. The right way to approach the climate change problem is first to force people and business to internalize the costs of their emissions. This can be done only through a climate treaty that imposes a tax or a (more-or-less) equivalent quantity restriction with tradable permits. If geoengineering can remove carbon from the atmosphere at an economical cost, then taxes can be reduced or quantities limits raised as necessary. But it is unrealistic to expect that geoengineering will simply eliminate the problem; much more likely, it will mitigate the problem at the margins, so that over the long term greenhouse gas emissions will not need to be lowered to zero, but they will still need to be curtailed, and only a climate treaty can accomplish that goal.

The most worrying implication of the report is that it injects an enormous dose of uncertainty into an already extraordinarily complex problem. It is not hard to imagine countries like China and India deciding that they would rather gamble with geoengineering than risk the short-term economic costs that would be imposed by any meaningful climate treaty. Indeed, they might reasonably expect to free ride on geoengineering initiatives undertaken by the United States and Europe, which would benefit everyone in the world. Ideally, international cooperation would ensure a fair distribution of costs. But the United States and Europe probably cannot credibly tell China that they will not fund geoengineering projects unless China participates, whereas they can credibly tell China that they will not cut emissions unless China does as well. The strategic settings are different. With respect to cutting emissions, the United States and Europe cannot solve the climate problem without Chinese (and Indian and Russian and Brazilian…) participation because of their enormous contributions to the climate problem (China is the leading emitter of greenhouse gases) and because of the "leakage" problem—industry will migrate from regulated countries to unregulated countries. With respect to geoengineering, it is at least possible that the United States and Europe can solve this problem, or at least make great inroads. If so, and if China and other countries respond rationally by free riding, then geoengineering may end up costing the United States a lot more than a reasonable climate treaty would.

A number of people writing in the comments thread asked what would happen under international law if the United States undertook a massive geoengineering project that went horribly awry and wiped out Bangladesh or some other country. The short answer is—nothing. Bangladesh could complain until it is blue in its face but it would have no legal claim against the United States. There is nothing like tort law in international law; tort principles have to be put together from the ground up in treaties. Those treaties are few and far between; Bangladesh and the United States belong to no treaty that would create liability for a geoengineering failure. Domestic remedies would be unavailable because of sovereign immunity.

This is not to say that the United States would not pay compensation of some sort. Americans would have to deal with world opinion and their own consciences. But suppose, as I suggested in my earlier post, that the United States alone engaged in geoengineering while the rest of the world merrily free rode. One can imagine Americans believing that if other countries are not paying for the benefits, then they should not complain if they end up bearing some of the costs of failure.

All of this underscores the point I made in my first post: the potential for geoengineering does not eliminate the need for a climate treaty, and instead just complicates negotiations. Ideally, negotiators would resolve in advance how the costs of geoengineering would be shared, and who would be responsible for harms caused by failure.

A few people asked how I could be so sure that geoengineering doesn't eliminate the need for limits on emissions. The answer is: that is what scientists think. But common sense suggests this as well. The question is like asking why we don't just eliminate all environmental and nuisance law with the expectation that the government will come up with a device that extracts all pollution from the air, rendering regulation of the polluting activities of individuals and businesses unnecessary. Geoengineering will take place at a scale that only governments can afford, and will require close coordination among the different governments that engage in it. It is hard to understand why people think that geoengineering would avoid top-down government regulation, or cooperation among governments, of the sort that they find so distasteful about limits on emissions.