Because carbon emissions create externalities across countries and generations, climate policy requires international cooperation and intergenerational altruism. A differential game using overlapping generations with intergenerational altruism shows how altruism and cooperation interact, and provides estimates of their relative importance in determining equilibrium steady-state carbon levels. A small increase in cooperation has a larger equilibrium effect than a small increase in altruism, beginning at empirically plausible levels. A large increase in altruism may have a larger equilibrium effect, compared to a large increase in cooperation. Climate investments may be dynamic strategic complements, reducing but not eliminating incentives to free ride.