BACK in July a California businessman dumped 100 tonnes of an iron-containing chemical into the Pacific Ocean as an experiment in geoengineering. The aim of the project, which seems to have succeeded, was to generate a massive algae bloom. Algae sucks carbon dioxide out of the air as it grows, and may then sequester it away for centuries as it dies and sinks to the bottom of the ocean. The experiment was widely condemned by scientists and environmentalists who pointed out that the perpetrator violated international law. It seems probable, however, that the rogue bit of atmospheric tinkering is just the first of many unilateral geoengineering gambits.

In a new paper, economist Martin Weitzman outlines the nature of the problem:

This paper begins with the realization that there are really two diﬀerent externalities involved in the climate change problem, that they have near-opposite properties, that they interact, and that it seems diﬃcult to say oﬀhand which one is more threatening than the other. The ﬁrst externality, described by the above quotes, comes in the usual familiar form of a public goods problem whose challenge is enormous because so much is at stake and it is so diﬃcult to reach an international governing agreement that divides up the relatively expensive sacriﬁces that would be required by each nation to really make much of a dent in greenhouse gas (GHG) concentrations. The classic governance problem here is to limit the underprovision of a public good from free riding.

A second less-familiar externality shows up in the scary form of geoengineering the stratosphere with reﬂective particles to block incoming solar radiation. This geoengineeringtype externality is so relatively cheap to enact that it might in principle eﬀectively be undertaken unilaterally by one nation feeling itself under climate siege, to the detriment of other nations. The challenge with this second global externality also appears to be enormous, because here too so much is at stake and it also seems diﬃcult to reach an international governing agreement. If the ﬁrst externality founders on the “free rider” problem of underprovision, then the second externality founders on what might be called the “free driver” problem of overprovision. If the ﬁrst externality is the “mother of all externalities,” then the second externality might be called the “father of all externalities.” These two powerful externalities appear to be almost polar opposites, between which the world is trapped.

There are several arguments made against the strategy of simply reducing emissions as a primary method of combating climate change. One is that reducing emissions by the amount necessary to prevent significant climate change would simply be too costly to win public support (in the absence of major research breakthroughs). Another, however, is that coordinating emission reductions across the whole of the world's nearly 200 countries is critical to the success of the policy—big reductions in America and Europe won't matter much if emissions grow relentlessly across the emerging world, for instance—and such international coordination is very difficult to imagine. It would require agreement on targets and mechanisms for penalising shirking countries. The world has had some success with coordination of this nature, in averting wars, for example, and in policing and liberalising trade. But those efforts have been decades in the making—time we lack.

What seems increasingly important to understand, however, is that the need for international cooperation will be if anything more serious in a world that doesn't act to control emissions (or control emissions enough to prevent substantial warming). From a pure adaptation standpoint, migration flows are sure to be enormous and potentially destabilising, but also represent one of the very best ways to reduce the human and economic costs of climate change. In the absence of coordination, relatively open countries may be swamped and may restrict immigration even more as a result, intensifying flows to other relatively open countries until openness to migrants across the world is significantly reduced.

Just as serious a concern, however, is that pressure for geoengineering solutions will grow as the effects of warming intensify. Large, northerly countries like Canada and Russia have an almost unchecked ability to adapt but smaller and more equatorial places will quickly run out of options. It is unrealistic to suppose that unilateral geoengineering schemes won't be an inevitable result.

Such schemes could pose huge risks. Successful, precisely deployed efforts might nonetheless have unpredictable and substantial side effects or unpleasant distributional costs. Without a forum to address such effects, geopolitical tensions could worsen in a hurry. Even more frightening, uncoordinated efforts could be too successful, flipping earth from a warming scenario to a dangerously cold one.

People have been engineering the climate in very aggressive fashion for more than a century now. We have made ourselves atmospheric managers, and there is no going back. If the world can't create a functional international forum for addressing atmospheric management—one with teeth—then the costs of global warming are going to be far higher than they ought to be, whatever the mix of policies used to attack it.