History has a way of repeating itself. Humans are currently conducting a grand experiment with Earth’s climate, but the outcome of that experiment may be foretold. According to Penn State climate scientist Richard Alley, the future—or a somewhat diluted version of it—happened 55.9 million years ago.
Geologists have long considered the Paleocene-Eocene Thermal Maximum (PETM) the most direct analog to modern climate change. During the PETM, some 2,000 and 4,500 billion tons of carbon were offloaded into the atmosphere over thousands of years, for reasons that are still being debated. Today, carbon concentrations are rising even faster, and the scientific debate over who’s responsible ended long ago.