  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.
