What Causes Temperature Change Over Geologic Time?

Human civilization advanced over the past 8,000 years during a relatively constant and favorable climate. However, Earth's climate has not always been so constant and favorable for humans. Global mean temperature (GMT) has been 8° to 15°C warmer than today with polar areas free of ice, and GMT has been 5° to 15°C (40 to 60°F) cooler in mid-latitudes with continental glaciers – some as thick as 1 mile covering areas as far south as New York City.

The Discovery of Glacial Periods

Louis Agassiz
Louis Agassiz. Image Credit: Wikipedia

Nearly two centuries ago, naturalists wondered how large boulders could end up in odd places, like river valleys and plains. They knew that the boulders were too massive to be transported by rivers. Louis Agassiz (1807-1983) was a young professor who studied fossil fish. He proposed that a giant ice sheet once covered large areas of Earth. His classic Studies on Glaciers (1840) gave rise to a new field of research.