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Published online on March 1, 2006, 10.1073/pnas.0504902103

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Geophysics
Stabilizing the Earth's climate is not a losing game: Supporting evidence from public goods experiments

( climate change | public goods game | reputation | tragedy of the commons )

Manfred Milinski *{dagger}, Dirk Semmann *, Hans-Jürgen Krambeck *, and Jochem Marotzke {dagger}{ddagger}

*Department of Evolutionary Ecology, Max Planck Institute of Limnology, 24306 Plön, Germany, and {ddagger}The Ocean in the Earth System, Max Planck Institute for Meteorology, 20146 Hamburg, Germany

Edited by Stephen H. Schneider, Stanford University, Stanford, CA, and approved January 13, 2006 (received for review June 13, 2005)

Maintaining the Earth's climate within habitable boundaries is probably the greatest "public goods game" played by humans. However, with >6 billion "players" taking part, the game seems to rule out individual altruistic behavior. Thus, climate protection is a problem of sustaining a public resource that everybody is free to overuse, a "tragedy of the commons" problem that emerges in many social dilemmas. We perform a previously undescribed type of public goods experiment with human subjects contributing to a public pool. In contrast to the standard protocol, here the common pool is not divided among the participants; instead, it is promised that the pool will be invested to encourage people to reduce their fossil fuel use. Our extensive experiments demonstrate that players can behave altruistically to maintain the Earth's climate given the right set of circumstances. We find a nonzero basic level of altruistic behavior, which is enhanced if the players are provided with expert information describing the state of knowledge in climate research. Furthermore, personal investments in climate protection increase substantially if players can invest publicly, thus gaining social reputation. This increase occurs because subjects reward other subjects' contributions to sustaining the climate, thus reinforcing their altruism. Therefore, altruism may convert to net personal benefit and to relaxing the dilemma if the gain in reputation is large enough. Our finding that people reward contributions to sustaining the climate of others is a surprising result. There are obvious ways these unexpected findings can be applied on a large scale.

Author contributions: M.M., D.S., and J.M. designed research; M.M. and D.S. performed research; H.-J.K. contributed new reagents/analytic tools; M.M. analyzed data; and M.M. and J.M. wrote the paper.

Conflict of interest statement: No conflicts declared.

{dagger}To whom correspondence may be addressed.

Manfred Milinski, E-mail: milinski{at}mpil-ploen.mpg.de
Jochem Marotzke, E-mail: marotzke{at}dkrz.de

www.pnas.org/cgi/doi/10.1073/pnas.0504902103
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