Over its 4.6 billion-year history, Earth has changed many times, in many ways. Homo sapiens, the name we give ourselves, evolved a few million years ago, a tiny point in time when Earth’s conditions were just so. A peculiar trait of a certain subgroup of Homo sapiens, aka humans, put itself at the center of the universe, creating myths, laws, and codes of behavior in support of that belief. And so Earth became ours. We became Earth’s dominant species, and by what we call the 21st century, a miniscule bit of geologic time, we had such effect on Earth that we have altered is atmospheric, geologic, hydrologic, and biospheric processes—and not in our favor.
Millions of years before humans evolved, Earth was covered by large, carbon-rich forests. When those plants died, they created large pools of petroleum and rich layers of coal. Those fossil fuels, as we call them, burn easily, releasing carbon dioxide into the atmosphere. Though Earth’s carbon and atmospheric cycles are complex, the understanding that carbon dioxide traps heat in the atmosphere is simple and crucial to understanding the largest of Earth’s five mass extinctions.
The geologic cycle is the story of plate tectonics, another of Earth’s complex process has repeated itself over time. Pieces of Earth’s crust move over its surface and create mountains, oceans, and volcanoes. About 250 million years ago, Earth’s geologic cycle created enormous volcanoes. Those volcanoes released carbon dioxide; moreover, they ignited those large deposits of fossil fuels. And Earth burned, overwhelming the young atmosphere with carbon dioxide. Yada, yada, yada, more than 90 percent of Earth’s species perished at the end of what we now call the Permian Period, became extinct as a result of the runaway greenhouse effect.