Although no one was around to measure the atmosphere's CO₂ concentration millions of years ago, paleoclimatologists can reconstruct past temperature and carbon dioxide levels using ice cores, tree rings, corals, ancient pollen, and sedimentary rocks. These natural recorders of climate fluctuations can also reveal how various animals and plants thrived or perished during different geological periods.

While studying the Age of Dinosaurs, for example, some researchers dissect leaves that got trapped in sediment layers. "The little holes in the skin of leaves are more common when there are lower carbon dioxide levels," explained Paul Olsen, a geologist and paleontologist at Columbia Climate School's Lamont-Doherty Earth Observatory.

Scientists like Olsen have repeatedly found that during several periods of Earth's history, organisms have experienced radically higher concentrations of carbon dioxide and hotter average temperatures than today. However, that doesn't mean everything will be fine if we keep heating the planet by burning fossil fuels.

"The problem today is not higher global temperature or carbon dioxide levels alone. The problem is the rate of change," explained Olsen. "Throughout most of the Earth's history, carbon dioxide levels have generally changed very slowly. That gave organisms and their ecosystems sufficient time to adapt to climate change through both evolution and migration."