Abstract
There is a growing literature describing responses in the abundance, and distribution of species to recent climate change and a heightened concern about what will happen if global temperatures rise to 4°C and beyond. There is an urgent need for clear guidance on where and how fast biodiversity is most likely to be affected so that management plans can accommodate expected consequences. One approach to providing rapid assessment, is the velocity of climate change which describes the speed and direction of shifting climate contours. We apply the velocity of climate change to derive trajectories for climatic niches. We use the collective properties of trajectories to classify areas with different implications for biodiversity. Coastlines act as barriers and locally cooler areas act as attractors, influencing the flow of trajectories and creating distinct climate source and sink areas. Climate migrants leaving source areas may not be replaced: sources lack climatic connections to environments from which new migrants may arrive. The non-replacement of climate migrants in thermal source regions, where the local climate departs from historical conditions of that and surrounding areas, may result in net losses of indigenous biodiversity, and facilitate the establishment of new species into the abandoned niches. Converging temperature isotherms in sinks may concentrate climate migrants, increasing local diversity, while certain classes of sinks such as those occurring where trajectories are blocked by coasts represent climatic dead-ends where species have nowhere to go to avoid warming, creating potential for high local extinction rates. The congruency in patterns of future projections with past reconstructions suggests that management actions for conserving biodiversity that are informed by current climate trajectories could remain effective into the future, and that the relative extent of source and sink areas depends on the rate of warming, with intense warming resulting in larger source areas.