Considerable uncertainty surrounds projections of climate change and its ecological consequences. We surveyed 2329 environmental biologists and found that greater expertise is associated with projections of greater climatic change and more severe consequences. The opinions of scientists with greater expertise converge, and they expect larger temperature increases, higher percentages of species extinctions, and a high percentage of species' ranges will change in response to climate change over the next 100 years. Importantly, even the highest of these estimates is at the lower bounds of many published projections of climate change and threats to biodiversity. These findings suggest that experts are relatively conservative and discerning about the magnitude of climate change and its biodiversity effects, but even their conservative estimates are substantial. We suggest that policymakers consult environmental biologists on emerging and controversial issues such as climate change and use transparent, standardized metrics of expertise when deciding which scientists to consult.