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[font size=3]There is universal agreement among countries that global emissions should be limited so as to prevent “dangerous interference” with the climate system. Our research strongly suggests that if a threshold for catastrophic climate change could be identified with certainty, free-riding behavior would be disciplined; countries very likely would propose a collective target certain to avoid catastrophe, would pledge to contribute their fair share to the global effort, and would act so as to fulfill their promises. Scientists have endeavored to support this negotiation strategy by identifying a “red line” for collective action, but thresholds for “abrupt and catastrophic” climate change are inherently uncertain. Our research suggests that, under these circumstances, countries are very likely to propose to do less collectively than is needed to avert catastrophe, pledge to contribute less than their fair share of the amount proposed, and end up contributing even less than their pledge. The climate change game is a prisoners’ dilemma, but not for the reasons usually given. What makes it a prisoners’ dilemma is not just the need for collective action but uncertainty about the threshold for dangerous climate change.

Our analysis is consistent with how the climate negotiations have played out so far. Concern about climate thresholds has reinforced the need to limit emissions so as to reduce, if not eliminate, the risk of dangerous interference, without having any noticeable effect on how countries behave. As in our experiment, countries have pledged to do less than is needed to meet their stated collective goals. We will not know until 2020 if the Copenhagen Accord pledges will be met, but if our experimental results are a reliable guide, countries may end up emitting more than they pledged—with potentially profound and possibly irreversible consequences.

Our research thus underscores the need to pursue alternative negotiation strategies for transforming the prisoners’ dilemma. Collective action can succeed, we have shown, when the underlying prisoners’ dilemma game is transformed into a coordination game. Although threshold uncertainty spoils this transformation, previous research shows that strategic treaty design can bring about a similar transformation. One way is by the use of trade restrictions against nonparticipating countries. If the loss from the trade restrictions exceeds the gains from free riding, every country will want to participate in a treaty, so long as each is assured that others will participate; this is how the Montreal Protocol enforced restrictions on the production and consumption of chlorofluorocarbons to protect the ozone layer (3). Another way to make abatement a coordination game is by the use of technology standards when these exhibit network externalities—that is, when the returns to each country of adopting a standard increase with the number of other countries that adopt the standard (30); this is how the MARPOL treaty limited releases of oil into the sea by tankers (3). Climate change is a more complex challenge, but our research suggests that strategies like these will be more successful than relying exclusively on the fear of dangerous climate change.

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[font face=Serif][font size=3]… in 1985, British Antarctic Survey scientists Farman, Gardiner and Shanklin published results of abnormally low ozone concentrations above Halley Bay near the South Pole. They speculated that this was connected to increased levels of CFCs in the atmosphere. It took several other attempts to establish the Antarctic losses as real and significant, especially after NASA had retrieved matching data from its satellite recordings. The impact of these studies, the metaphor 'ozone hole', and the colourful visual representation in a time lapse animation proved shocking enough for negotiators in Montreal to take the issue seriously.

Also in 1985, 20 nations, including most of the major CFC producers, signed the Vienna Convention, which established a framework for negotiating international regulations on ozone-depleting substances. After the discovery of the ozone hole it only took 18 months to reach a binding agreement in Montreal.

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