1. A method comprising: generating values of thermal couplings between a plurality of layers of a three-dimensional processor stack that includes a plurality of processor cores, wherein at least one of the plurality of processor cores is implemented in each of the plurality of layers, and wherein the values of the thermal couplings indicate temperature changes in each of the plurality of layers as a function of temperature changes in each of the other layers; and in response to a thermal event in one of the plurality of layers, selectively throttling at least one of the plurality of processor cores implemented in the plurality of layers based on the values of the thermal couplings and measures of criticality of threads executing on the plurality of processor cores.