The level of climate-change mitigation depends on how humans assess the risk arising from missing the 2°C target

摘要

The established international 2°C target stipulates that global warming should be limited to below 2°C compared with pre-industrial periods; this has emerged as the most prominent interpretation of how to avoid dangerous climate change. The 2°C target was confirmed and made legally binding in the Paris agreement at the “climate summit” (Conference of Parties 21, COP21) in December 2015. But despite agreement on the target, greenhouse-gas emissions are unlikely to fall soon and fast enough to meet the target, raising the question of whether this target needs to be revised or reinterpreted, and also of why there is insufficient cooperation toward emissions reduction despite the risk of dangerous climate change. Previous theoretical and experimental research has suggested that cooperation towards emissions reduction is undermined by uncertainty about the threshold marking the transition to dangerous climate change. However, even if the threshold and hence the location of the target are known precisely, uncertainty ensues because of an unknown risk that arises from missing the collective target. How humans deal with this risk has not been investigated experimentally. Here we investigate how individuals behave under different risk scenarios if a collective target is missed. We perform economics experiments framed as a collective-risk social dilemma and directly examine the extent to which human subjects trade pay-out reduction for risk. We show that a reduced assessed risk arising from missing the collective target leads to reduced contributions towards the target; but that risk reduction causes the subjects almost to maximize their individual pay-out by balancing the effort to reach the target against the risk posed by missing it. We provide quantitative support for the argument that group contributions toward the collective target can be interpreted as proportional to mitigated warming. We conclude that reinterpretation of the 2°C target as less strict causes additional warming. However, our subjects deal effectively with a risk of dangerous climate change that they assess to depend gradually on global warming. Our results suggest that, if the additional warming is judged to be acceptable, a less strict interpretation of the 2°C target might support finding a trade-off between the effort put into climate mitigation and the risk of dangerous climate change.