We introduce a novel game where a decaying atmospheric quality, modeled as a stock variable determining the payoff externality, can be counteracted through individual mitigation efforts. It encompasses three characteristics of climate change as a social dilemma: (a) the continuous nature of climate degradation, (b) the constant influx of emissions resulting from human economic activities, and (c) the greater efficacy of early mitigation actions. We report findings from an experiment where, across four treatments, we manipulate the starting atmospheric quality and introduce inequality in the endowments employed to mitigate. Results indicate that subjects fail to mitigate in early periods, an individually rational strategy. We do not find differences between treatments at the aggregate level. However, participants treat their groupmates’ past mitigation as a strategic substitute for their own mitigation (i.e., if others’ mitigation increase, participants reduce their mitigation). This substitution is less intense if the initial atmospheric quality is negative.