Results/Conclusions Field observations in boreal, temperate, and tropical ecosystems have detected biome changes in the 20th century in 15 of 19 identified cases. In the 21st century, one-tenth to one-half of the world's land area may be highly (confidence ~0.80) to very highly (confidence ~0.95) vulnerable to vegetation shifts at the biome level. Temperate mixed forest, boreal conifer, and tundra and alpine biomes show the highest vulnerability, whereas tropical evergreen broadleaf forest and desert biomes show the lowest vulnerability. A mismatch between the patterns of vulnerability that we found and the geographic priorities that natural resource organizations have established suggests the need for adaptation of management plans. Approximately a billion people live in highly vulnerable areas.