Terrestrial ecosystems have encountered substantial warming over the past century, with temperatures increasing about twice as rapidly over the oceans. Here, we review the likehood of continued changes in terrestrial climate, including analysis of the Coupled Model Intercomparison Project global climate model ensemble. Inertia toward continued emissions creates potential 21st-century global warming that is comparable in magnitude to that of the largest global changes in the past 65 million years but is orders of magnitude more rapid. The rate of warming implies a velocity of climate change and required range shifts of up to several kilometers per year, raising the prospect of daunting challenges for ecosystems, especially in the context of extensive land use and degradation,cahnges in frequency and severity of extreme events, and interactions with other stresses.