For nearly 50 years, the image of the Earth, as seen by Apollo astronauts circling the moon, has been iconic among environmentalists.
Suppose there were some aliens out there and they’d been watching our planet for its entire 4.5 billion year history. What would they have seen? Over nearly all that immense time, Earth’s appearance would have altered very gradually. The continents drifted; the ice cover waxed and waned; successive species emerged, evolved and became extinct.
But, in just a tiny sliver of the Earth’s history — the last one-millionth part, a few thousand years — the patterns of vegetation altered much faster than before. This signaled the start of agriculture. The pace of change accelerated as human populations rose. The world entered a new geological era: the Anthropocene.
Then there were other changes, even more abrupt. Within just 50 years, the carbon dioxide in the atmosphere began to rise anomalously fast. And something else unprecedented happened: rockets launched from the planet’s surface escaped the biosphere completely. Some were propelled into orbits around the Earth; some journeyed to the moon and planets.