CREDIT: MAURICE WEISS/OSTKREUZ

Climate change is a tragedy of the commons of existential importance. At the annual United Nations climate summit that concludes this week, parties will affirm the necessity to avoid dangerous climate change. But between now and next year's summit, these same countries will in many ways act so as to hasten the outcome that they say must be avoided. This disjunction between what countries say and what they do has been repeated every year since the first summit in 1995. It is a pattern of behavior that seems irrational, but that can be explained. American ecologist Garrett Hardin's classic article, “The Tragedy of the Commons,” published in Science 50 years ago this week, vividly describes the dilemma that causes this behavior (see page 1236).

CREDIT: ADAPTED FROM ISTOCK.COM/ERHUI1979

Herders, wrote Hardin, are motivated by private gain, so have incentives to add animals to their shared pasture “without limit.” As in the climate change game, all herders also want their pasture to be saved, but none is willing to bear the personal sacrifice needed to prevent its destruction. Saving the pasture requires collective action. Hardin's proposed corrective is “mutual coercion.” Writing in 1651, British philosopher Thomas Hobbes similarly concluded that a sovereign is needed to tie people “by fear of punishment to the performance of their covenants.”

However, a critical difference between climate change and Hardin's parable is that the players in the climate game are nation states. Although individuals can be subjected to coercion by a higher authority, human organization has not evolved to give any institution sovereignty over the nation state. Solutions to global collective action problems must involve covenants (treaties) among states that are self-enforcing.

To stabilize the climate, a treaty must get all states to (i) participate in and (ii) comply with an agreement that (iii) drives emissions to zero. The Paris Agreement, adopted at the 2015 summit, secures the first requirement, and possibly the second, but only because it is a voluntary agreement and will fall short of meeting the third requirement. The Montreal Protocol, negotiated in 1987 to protect the stratospheric ozone layer, meets all three requirements, thanks partially to a ban on trade in chlorofluorocarbons between parties to the protocol and nonparties. Because of the ban, once the vast majority of countries joined the agreement, all others wanted to join. William Nordhaus, a recipient of this year's Nobel Memorial Prize in Economic Sciences, has recently analyzed a similar cure for climate change in which members of a “climate club” who agree to curb emissions impose a tariff on imports from nonmembers to encourage their participation. Unfortunately, his analysis shows that as the carbon tax rises to the level needed to stabilize the climate, participation in the club collapses.

Breaking up the problem may provide more leverage for enforcement. The Kigali Amendment to the Montreal Protocol, adopted in December 2016, phases down hydrofluorocarbons, a group of greenhouse gases, and this will be effective in addressing this particular cause of climate change for the same reasons that the Montreal Protocol has been effective in protecting the ozone layer. Other climate agreements, adopted in parallel with the Paris Agreement, should be negotiated for individual sectors, such as aluminum and steel and international aviation and shipping, all linked to trade.

However, the time has come to contemplate other, more radical solutions. The October 2018 Intergovernmental Panel on Climate Change special report concluded that limiting temperature change to 1.5°C cannot be achieved by simply curbing emissions, but requires removing CO 2 from the atmosphere. The only true “backstop” for limiting climate change is removal of CO 2 by industrial processes, which converts the problem from one of changing behavior into one of joint financing of a large-scale project. Another option, solar geoengineering, acts directly on global mean temperature, but is considered risky. Of course, not using it could also be risky. In the end, regardless of pathways forward, we will have to choose between risks to address the scale of this problem and achieve, rather than merely aspire to, global collective action on climate change.