The mitigation of the effects of climate change on humankind is one of the most pressing and important collective governance problems nowadays$^{1-4}$. To explore different solutions and scenarios, previous works have framed this problem into a Public Goods Game (PGG), where a dilemma between short-term interests and long-term sustainability arises$^{5-9}$. In such a context, subjects are placed in groups and play a PGG with the aim of avoiding dangerous climate change impact. Here we report on a lab experiment designed to explore two important ingredients: costly punishment to free-riders and group size. Our results show that for high punishment risk, more groups succeed in achieving the global target, this finding being robust against group size. Interestingly enough, we also find a non-trivial effect of the size of the groups: the larger the size of the groups facing the dilemmas, the higher the punishment risk should be to achieve the desired goal. Overall, the results of the present study shed more light into possible deterrent effects of plausible measures that can be put in place when negotiating climate treaties and provide more hints regarding climate-related policies and strategies.
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