Previous periods of rapid warming millions of years ago drastically altered plants and forests on Earth. Now, scientists see the beginnings of a more sudden, disruptive rearrangement of the world’s flora — a trend that will intensify if greenhouse gas emissions are not reined in. When the climate changes, so does the arrangement of the world’s plants. Species move back and forth toward the poles, up and downslope. Some species grow more common, others rarer. Species arrange themselves together in new combinations. The fossil record reveals plants for what they are, as mobile beings. For plant species, migrating in response to climate change is often a matter of survival.