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The response of tropical forest carbon balance to global change is highly dependent on the factors limiting net primary productivity (NPP) in this biome. Current empirical global NPP–climate relationships predict that the response of NPP to climate diminishes at higher levels of mean annual precipitation (MAP) and mean annual temperature (MAT), but data have been relatively scarce in warm and wet tropical ecosystems. By integrating data from a new comprehensive global survey of NPP from tropical forests and a climate gradient from Maui, Hawaii, along with data previously used to develop NPP–climate relationships, I show that there is a strong negative relationship between MAP and NPP in humid ecosystems. The relationships derived here clearly demonstrate that NPP in wet tropical forests is sensitive to climate, and that future forest growth may be limited by increased precipitation forecast by global climate …