After the end of the last ice age — when sea levels rose, glaciers receded and global average temperatures soared as much as seven degrees Celsius — the Earth’s ecosystems were utterly transformed. Forests evolved from once barren, ice-covered ground. Dark, cool stands of pine were replaced by thickets of hickory and oak. Woodlands gave way to scrub, and savanna turned to desert. The more temperatures increased in a particular landscape, the more dramatic the ecological shifts. Researchers are reporting Thursday in the journal Science that it is happening again. A sweeping survey of global fossil and temperature records from the past 20,000 years suggests that Earth’s terrestrial ecosystems are at risk of another, even faster transformation unless aggressive action is taken against climate change.