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Climate change

All in the game

Nature volume 441, pages 583584 (01 June 2006) | Download Citation


It is in the public interest to keep Earth's climate on an even keel — the public, in this case, being all the world's population. Are you prepared to stake your own reputation on helping to improve matters?

Earlier this year, the German newspaper Hamburger Abendblatt published an advertisement1 drawing attention to the impact of human activities on global climate:

Human activities have already demonstrably changed global climate, and further, much greater changes must be expected throughout this century. The emissions of CO2 and other greenhouse gases will further accelerate global warming... Some future climatic consequences of human-induced CO2 emissions, for example some warming and sea-level rise, cannot be prevented, and human societies will have to adapt to these changes. Other consequences can perhaps be prevented by reducing CO2 emissions. Everyday measures can contribute to climate protection.

Nothing odd about that, you might think. But there was. Not only was the advert written by the director of the Max Planck Institute for Meteorology in Hamburg, Jochem Marotzke, but it was paid for by the proceeds of a ‘climate game’ that was designed to explore the willingness of individuals to invest in protecting Earth's climate (Fig. 1, overleaf). A paper2 describing the climate game by Manfred Milinski, along with Marotzke and two other colleagues, was published simultaneously with the advert. What's the story behind that paper and the advert?

Figure 1: That ad — and how it was paid for.
Figure 1

The Earth's climate is a ‘public good’. It is shared by the entire human population, so it apparently does not pay for a single individual to invest in protecting it: the direct benefit that an individual gains from his or her investment is much smaller than the costs. The Earth's climate is therefore vulnerable to overexploitation — it faces a ‘tragedy of the commons’3. Experiments with public goods games are one way of exploring how such a tragedy might be avoided.

In a typical public goods game4,5,6,7, members of a group can simultaneously invest money in a group project. The group's investment is increased by the experimenter, and the return is shared equally among all members of the group. In such a setting, it typically does not pay for a player to invest, because the player's direct return will tend to be a fraction of his or her investment. Assume, for example, that the group consists of three players, and the experimenter doubles the group's investment. Then, when one player invests (say) $1, and the others invest nothing, that player gets back only two-thirds from his or her own investment, because that investment is doubled by the experimenter but then divided among all three players. Making an investment is an altruistic act in favour of the other players. The best strategy is to invest nothing in the group project but to benefit from the others' investment. It is therefore not surprising that, if played repeatedly, the amount of money invested in the group project declines rapidly3. But if all the members of a group instead continue to invest, the entire group is better off.

Such cooperation between group members can be achieved by changing the rules of the game. For example, if players can punish or reward other players based on their behaviour in the public goods game, a high level of investment is maintained4,5,7. Players who are known to make investments in the public good gain a good reputation; and that reputation translates into direct benefits of either receiving rewards or avoiding punishment.

Maintaining cooperation in these games seems far removed from addressing such problems as global warming. But Milinski and colleagues' climate game2 is a step in that direction. As in previous studies, rounds of a public goods game were alternated with rounds of an ‘indirect reciprocity game’8,9,10,11 in which players could reward other players based on their reputation. But in contrast to previous versions of the game, the yield from the public goods game was not given back to the group. Instead, it was paid into a ‘climate fund’ that was used to pay for the advert. Amazingly, the players still maintained a high level of cooperation in the public goods game, and in the indirect reciprocity game they preferentially rewarded individuals who invested in the climate fund.

The experiments revealed that reputation is essential in maintaining a high level of investment. In rounds where the investment was anonymous, the degree of cooperation was much smaller than in rounds where the investment was made public. Apparently, individuals invested in the climate game to maintain a good reputation and thereby increase the chances of being rewarded by the other players in the indirect reciprocity game. The main message of this study, then, is that whenever individual behaviour is relevant to the public good, it should be made public. Only then can an individual's regard for his or her own reputation be fully exploited.

How might these principles work in practice? Energy costs of individual households could, for example, be published by local authorities. Companies could be ranked according to their emissions and their investments in climate protection. The large costs that may arise from global climate change have to be communicated, even if the details are not yet fully understood. And even small investments in climate protection may help, such as a slight reduction of room temperatures in winter or making the effort to use public rather than private transport, where available.

Let no one underestimate the difficulty in persuading individuals around the world, let alone businesses and entire nations, about the necessity of putting the brakes on the human impact on climate. And cynics may sneer at the prospect of applying the principles of an academic exercise on a world stage. But like it or not, we are all involved in the very real game of climate change. It is one of the few games we cannot afford to lose.


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  1. Thomas Pfeiffer and Martin A. Nowak are in the Program for Evolutionary Dynamics, Department of Organismic and Evolutionary Biology, and Department of Mathematics, Harvard University, Cambridge, Massachusetts 02138, USA.

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