Metabolic theory uses first principles of physics, chemistry, and biology to understand more mechanistically and quantitatively the well-known effects of body size and temperature on metabolic rate and on the structure and dynamics of ecological systems. Metabolic theory predicts that most biologically-mediated ecological processes - including photosynthesis, respiration, population growth and turnover, competition, predation, herbivory, parasitism, disease, and mutualism – and most evolutionary processes – including nucleotide substitution, responses to selection, speciation, and extinction - should decrease as a power function of body size and increase as an exponential function of temperature. So with global warming, all of these rate processes would be predicted to increase, and the increases in productivity and biological kinetics would be predicted to generate and maintain higher species diversity. A major caveat, however, is that these predicted effects will be diminished or reversed when temperatures become stressfully high or when water, nutrients, or dispersal become increasingly limiting. Additional caveats are that individual species may exhibit individualistic responses, and particular habitat types may exhibit idiosyncratic responses. Finally, with increasing spatial scales from local sites to the globe, there may be differential responses and increasingly long time lags.