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According to general circula-tion models of future climate in a world with double the preindustrial carbon dioxide (CO2) concentrations, the rate of warming in the lower troposphere will increase with altitude. Thus, temperatures will rise more in the high mountains than at lower elevations (see the figure)(1). Maximum temperature increases are predicted to occur in the high mountains of Ecuador, Peru, Bolivia, and northern Chile. If the models are correct, the changes will have important consequences for mountain glaciers and for communities that rely on glacier-fed water supplies. Is there evidence that temperatures are changing more at higher than at lower elevations? Although surface temperatures may not be the same as in the free air, in high mountain regions the differences are small (2), and changes in temperature should thus be similar at the surface and in the adjacent free air. Unfortunately, few …