But even if those computer forecasts did not exist, a huge amount of evidence suggests that scientists have the basic story right. The most important evidence comes from the study of past climate conditions, a field known as paleoclimate research. The amount of carbon dioxide in the air has fluctuated naturally in the past, and every time it rises, the Earth warms up, ice melts, and the ocean rises. A hundred miles inland from today’s East Coast, seashells can be dug from ancient beaches that are three million years old. These past conditions are not a perfect guide to the future, either, because humans are pumping carbon dioxide into the air far faster than nature has ever done.