Researchers studying a small Canadian ice cap — the last remnant of a sheet that once covered nearly 30 percent of North America — found it began growing about 2,000 years ago, marking a new ice age. It started shrinking in the 1960s.
The ice cap's growth was Earth's natural cooling trend, building for two millennia. Human-caused warming reversed it within decades. That contrast offers a clear, physical baseline for measuring how strongly human activity now shapes the planet's climate, in a place anyone can see from satellite images.
✦ Earth was slowly entering a new ice age. We overtook it.