Despite widespread scientific consensus that the major cause of climate change is an increase in greenhouse gases from human action, a global treaty to stop the emissions seems impossible to attain. A major obstacle has been an incapacity to agree on the obligations of poor versus rich nations. In fact, while the 1997 Kyoto Protocol only set emissions targets for developed countries, the present mood is different, with every country being asked to play its part, even if nobody seems to be able to reach a consensus on how to do it.
But a study about to be published in the journal PNAS has now managed to identify a key obstacle for the agreements – researchers from the Portuguese group ATP, Universities of Minho and Lisbon, in collaboration with Simon A. Levin, from Princeton University found that in discussions between differently wealthy nations there is tendency for individuals to imitate and bond with those similar to themselves (a phenomenon called homophily). And in climate change discussions, homophily can increase the reluctance of poorer nations to contribute towards emission control.
But once identified a problem, it is easier to find solutions and Vítor Vasconcelos, Jorge Pacheco and colleagues also discovered that if a few unconditional co-operators can be found among the poorer nations, these, properly supported, can become “role models” that effectively push the balance towards an agreement.
The Portuguese team - which is internationally known by their studies on cooperation and climate control – has previously shown that cooperation for climate control can only be possible if approached at regional or domestic level, with local institutions sanctioning those that do not collaborate. Those results, together with the new findings, help to understand better the complex social interactions behind the negotiations for climate control and, if listen to, can actually enhance our real life ability to reach an agreement.
But how did the researchers reached these results? Their study used game theory, a branch of mathematics that studies human social interactions and has became a crucial tool to understand the interactions behind the efforts to reach a climate control agreement.
 Vítor V. Vasconcelos, Francisco C. Santos, Jorge M. Pacheco & Simon A. Levin, Climate Policies under Wealth Inequality, Proceedings of the National Academy of Sciences U S A (in press).
 Francisco C. Santos and Jorge M. Pacheco “Risk of collective failure provides an escape from the tragedy of the commons”, Proceedings of the National Academy of Sciences U S A 108 (2011) 10421-10425
 Vítor V. Vasconcelos, Francisco C. Santos and Jorge M. Pacheco “A bottom-up institutional approach to cooperative governance of risky commons”, Nature Climate Change 3 (9) pp. 797-801 (2013)
The idea is to design a “public goods game” (using a series of mathematic equations) that describes the strategies behind climate control negotiations, where the welfare of the planet is the “public good” (a global good from which everyone benefits, whether they contribute to it or not). The game can then be used, not only to understand specific parameters influencing the negotiations, but also to determine how to make the partners move towards cooperation/climate control.
In their new work Vasconcelos and colleagues developed a model (Ref.  below) to analyse the impact of wealth inequality on the process of cooperation in climate agreements (they did this by adapting a previously used game, see Refs.  and ). In the new game, the population of those attending a climate summit is split into 2 classes: the rich, comprising 20% of the population, and the poor, accounting for the remaining 80%. As the rich have more wealth, they contribute more than the poor.
What the researchers found, though, was that this asymmetry led to significant changes in the social dynamics of decision-making, in particular when combined with homophily (literally love of self) which blocks the contributions from poor nations that are indispensable to overcome Global Warming. Rich nations in general contribute more, and often compensate for the lower contribution of poor, but only to a certain limit.
But this negative impact of homophily can be overturned if there are a few unconditional poor cooperators. In fact, the cooperative potential that the researchers call “obstinate behavior” is stronger among the poor than among the rich.
Remarkably, when homophily is absent - which means that all individuals share the same network of influence - wealth inequality actually leads to more global cooperation than a scenario where all individuals are equally wealth.