Paleontological information was used to evaluate and compare how Rocky Mountain mammalian communities changed during past global warming events characterized by different durations (350, ∼10,000–20,000, and 4 million years) and different per–100-year warming rates (1.0°C, 0.1°C, 0.06–0.08°C, 0.0002–0.0003°C per 100 years). Our goals were to determine whether biotic changes observed today are characteristic of or accelerated relative to what took place during past global warming events and to clarify the possible trajectory of mammalian faunal change that climate change may initiate. This determination is complicated because actual warming rates scale inversely with the time during which temperature is measured, and species with different life-history strategies respond (or do not) in different ways. Nevertheless, examination of past global warming episodes suggested that approximately concurrent with warming, a predictable sequence of biotic events occurs at the regional scale of the central and northern United States Rocky Mountains. First, phenotypic and density changes in populations are detectable within 100 years. Extinction of some species, noticeable changes in taxonomic composition of communities, and possibly reduction in species richness follow as warming extends to a few thousand years. Faunal turnover nears 100% and species diversity may increase when warm temperatures last hundreds of thousands to millions of years, because speciation takes place and faunal changes initiated by a variety of shorter-term processes accumulate. Climate-induced faunal changes reported for the current global warming episode probably do not yet exceed the normal background rate, but continued warming during the next few decades, especially combined with the many other pressures of humans on natural ecosystems, has a high probability of producing effects that have not been experienced often, if ever, in mammalian history.
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