Do we owe it to future generations, as a requirement of justice, to take action to mitigate anthropogenic climate change? This paper examines the implications of Derek Parfit’s notorious non-identity problem for that question. An argument from Jörg Tremmel that the non-identity effect of climate policy is “insignificant” is examined and found wanting, and a contrastive, difference-making approach for comparing different choices’ non-identity effects is developed. Using the approach, it is argued that the non-identity effect of a given policy response to climate change depends on the contrasting policy. Compared to a baseline scenario without further mitigation, the non-identity effect of choosing to limit climate change to 1.5°C would be highly significant.