AbstractRates of change in intrinsic water use efficiency (W) of trees relative to those in atmospheric [CO₂] (c_a) have been mostly assessed via short-term studies (e.g., leaf analysis, flux analysis) and/or step increases in c_a (e.g., FACE studies). Here we use compiled data for abundances of carbon isotopes in tree stems to show that on decadal scales, rates of change (dW/dc_a) vary with location and rainfall within the global tropics. For the period 1915–1995, and including corrections for mesophyll conductance and photorespiration, dW/dc_a for drier tropical forests (receiving ~ 1000 mm rainfall) were at least twice that of the wettest (receiving ~ 4000 mm). The data also empirically confirm theorized roles of tropical forests in changes in atmospheric ¹³C/¹²C ratios (the ¹³C Suess Effect). Further formal analysis of geographic variation in decade-to-century scale dW/dc_a will be needed to refine current models that predict increases in carbon uptake by forests without hydrological cost.