Stable Water Isotopes as Evaluation Tools for Global Climate Model Simulations of the Amazon Basin Circulation

The isotopic abundances of O-18 and Deuterium over the Amazon are used to constrain simulations of the water cycle in this, the largest river basin in the world. Based on data in the Global Network on Isotopes in Precipitation (GNIP) database, we analyze the simulation of the land surface hydrology and water cycling. Temporal changes between 1965 and 2000 in stable water isotopic signatures in the Amazon have been used to evaluate global climate model (GCM) predictions revealing notable anomalies. For example, the differences in the wet season deuterium excess between Belem and Manaus are consistent with recent GCM simulations only if there has been a relative increase in evaporation from non-fractionating water sources over this period. Despite earlier predictions that land-use change signals would be found, late twentieth century data reveal no significant change in dry season isotopic characteristics. On the other hand, more recent isotopic data do show trends at stations in the Andes, where as much as 88% of the rainfall is thought to be derived from recycled moisture. These data might be linked to land-use change impacts. Simultaneous tracking of the two primary isotopes in water (HDO and H₂¹⁸O) makes it possible to trace the history of evaporative and condensation processes. Results of GCM simulations of Amazonian deforestation suggest that the recent stable isotope record is more consistent with the predicted effects of greenhouse warming, possibly combined with forest removal, than with the predicted effects of deforestation alone. At a minimum, large-scale simulations of South American climate ought to be tested against these isotopic data in any validation effort.