As another year begins without any significant progress toward containing global warming, I find myself wondering if this isn't the perfect real-life example of the Tragedy of the Commons.

In the parable, as told by the ecologist Garret Hardin in his influential 1968 essay of the same name, herders sharing free use of a pasture invariably end up destroying it. Each finds that it pays to increase his or her herd as much as possible because s/he will get the full benefit of the additional animals, while bearing only a fraction of the cost of overgrazing, which is shared by all. Eventually, as each herder reaches the same conclusion, and continues to add animals, the pasture is ruined.

The reason the outcome is inevitable is that it is in the herders' rational self-interest to do what they do. It doesn't matter if one or two far-sighted herders refrain from acquiring additional animals. The others will only pick up the slack, so to speak, and the pasture will still be destroyed, though possibly over a longer period of time.

Of course, in real life, the outcome is open, not inevitable, because people can manage the pasture through rules or payment schemes. The challenge is coming up with an effective system and getting enough herders to agree to it. Hardin called the solution "mutual coercion, mutually agreed upon."

In addition to resource exploitation, the parable also applies to pollution. In the pollution scenario, instead of withdrawing resources from the commons, the rational company discharges harmful things into it. Again, this happens because the benefit (cheap disposal of wastes) goes all to the polluter, while the cost (fouled air or water) is shared by all.

Now, if instead of companies, we think in terms of countries, we have a pretty good explanation for the current impasse in international efforts to deal with global warming. The United States and China are the herders who insist on acting in their own rational self-interest. As long as they stick to their guns, there's nothing other countries can do. Mutual coercion, mutually agreed upon, is impossible because the United States and China don't agree and are too strong to be coerced. That leaves other countries with the voluntary self-restraint option, which can slow the process, but not change the outcome -- or can it?

It helps to consider the Prisoner's Dilemma game, to which the Tragedy of the Commons is related. In this game, two prisoners under arrest for the same crime are each encouraged to implicate the other. If only one "defects," as the choice of telling on the other is called in game theory, the defector goes free while the other prisoner gets a long sentence. But if both prisoners defect, both get medium sentences. On the other hand, if both keep silent, or "cooperate," as it is called, both receive short sentences. The prisoners are not allowed to consult so they don't know what the other will do (though even if they could communicate, there would still be a question of trust). The dilemma is that self-interest dictates defecting, but if both defect, both will do worse than if they had cooperated. Still, defecting is the better choice if the game is played only once.

However, when the game is replayed repeatedly against the same player in the Iterated Prisoner's Dilemma, players have a chance to figure out a better strategy based on what they learn about the other player over time. Indeed, in a complex series of computer simulations performed and described by Robert Axelrod in his seminal work, The Evolution of Cooperation, most players end up cooperating -- and doing better overall as a result.

Does that provide reason to hope for an optimal outcome in the global warming game? Maybe.

While the game is not played iteratively, with an outcome after each round, it does allow each of us to see -- and learn from -- other players' choices along the way, both on the macro and micro levels. That is, countries, companies and citizens have the chance to learn that cooperation rather than defection will yield a better long-term result -- not better in the sense of being morally superior or even more prudent, but more profitable. Imagine, for instance, that other countries start benefiting economically from greater use of clean energy technologies. You can bet the United States would take notice and change its own policies in response.

If you want to do your own part to change the outcome, use your purchasing power to demonstrate to companies that bringing clean alternatives to market will be lucrative. Buy a hybrid car or energy-efficient conventional car. Switch to green power. Get Energy Star appliances. Conserving energy (by shutting lights, unplugging infrequently used appliances, etc.) will help, too, not by changing the cost-benefit ratio for players, but by giving players a little more time to figure out how to win.

Lastly, when it comes to elections, think about "voting the environment," not just your pocketbook. Politicians need to learn the benefits of cooperating, too.

—Sheryl Eisenberg

Other tragedies of the commons. There are many kinds of commons -- and potential tragedies. In the email sphere, for instance, spam threatens to ruin the commons while anti-spam software is the chief means used to manage it.

ONLINE RESOURCES SCIENCE MAGAZINE The Tragedy Of The Commons WIKIPEDIA The Prisoner's Dilemma ROBERT AXELROD Evolving New Strategies SYNTHESIS/REGENERATION 24 Global Tragedy of the Commons at COP 6 BOSTON REVIEW Reclaiming the Commons FRIENDS OF THE COMMONS Homepage NRDC Global Warming SEATTLE.GOV US Mayors Climate Protection Agreement

Prisoner's Dilemma payoff. This matrix shows the payoff structure when the game is stripped of its storyline, players are awarded points instead of prison sentences, and the goal is scoring as many points as possible. On a single play, defecting is the best strategy because it guarantees 1 point and makes possible 5, whereas cooperating brings 3 at most and maybe none. With repeated play, however, defecting by one player will likely breed defecting by the other and low scores for both, while cooperating will do the reverse.

Play! Try your hand at the Tragedy of the Bunnies, a simulation of the Tragedy of the Commons, and The Prisoner's Dilemma.

Not really a game. In real life, the range of moves and players is not circumscribed by arbitrary rules. If the United States as a whole defects on the Kyoto agreement to bring greenhouse gas emissions down, localities in the United States can still cooperate -- and do. So, for instance, the mayors of nearly 200 cities have signed an agreement to "meet or beat" the Kyoto targets, California has set its own reduction targets, and seven northeastern states have agreed to a joint plan for bringing power plant emissions down.