Early climate studies of modern tropical deforestation61 have focused attention on the importance of climate-vegetation interactions in governing the surface energy and moisture fluxes and hence the importance of climate-vegetation interactions. More recently,62 it was proposed that boreal forests, in addition to tropical vegetation, have an important climatic effect. A climate model was used to demonstrate that expansion of the boreal forest, associated with greenhouse warming, would enhance this warming through albedo feedback because trees mask the high reflectance of the high-latitude snow cover. In particular, tree cover, even with underlying snow cover, promotes springtime warming. These contributions introduce the potential of climate-vegetation feedbacks to substantially modify the sensitivity of the Earth's climate to a large number of different forcing factors. Earth history enables an assessment of the importance of vegetation-climate feedbacks for a diverse number of forcing factors (carbon dioxide, solar variations, orbital variations, sea level, changes in continental geometry). But most importantly, the combination of knowledge about the nature of the forcing factors, the paleobotanical record of vegetation distribution, and the record of temperatures presents a unique opportunity to critically assess climate-biosphere sensitivity.