From studies of geology, Earth is known to get gradually warmer and colder over time.
Geologists also know that — 55 million years ago — Earth underwent a unique warming event. In about 10 thousand years, Earth’s average global temperature went up seven degrees Celsius. Scientists think that during that warming, deposits of frozen methane on the ocean floor — called methane hydrates — began to melt.
This would have released methane gas — which would have converted to carbon dioxide as it bubbled to the ocean surface. Carbon dioxide is a greenhouse gas. Gerald Dickens — an associate professor of Earth Science at Rice University — says this increase in carbon would have accelerated Earth’s warming.
Gerald Dickens: Why it’s sort of significant is the minimum amount of carbon that we could have added to the ocean-atmosphere system back then, is on the sort of ballpark of what we think we’re going to add over the next couple hundred years through fossil fuel burning. And so where it’s become very interesting — why there’s a lot of research into this strange time frame, 55 million years ago — it appears to be the only natural analog for what we’re doing today.
The earlier warming took 10 thousand years. Now, because we’re adding so much greenhouse gas to the air, it might take only a couple of centuries for Earth to warm as much again.