Ice Ages & Past Climates
The chemistry of seafloor sediments full of fossilized microscopic shells can reveal ocean temperatures that existed tens of thousands of years ago. (Photo by Tom Kleindinst, Woods Hole Oceanographic Institution)
Earth’s climate has undergone many changes over the course of geologic history, but the past one million years or so have been among the most dynamic. During that time, the planet has experienced repeated cycles of glacial (cold) and interglacial (warm) periods lasting about 80,000 years on average.
These were most likely driven by regular changes in Earth’s orbit and rotation known as the Milankovich Cycles that govern the seasonal timing and intensity of solar energy entering the atmosphere. Other factors that may have contributed to the formation and cessation of ice ages are the amount of greenhouse gases (mainly carbon dioxide, methane, and water vapor) in Earth’s atmosphere, the extent of sea and land-based ice across the northern hemisphere, and shifts in patterns of wind and ocean currents.
During ice ages, the most characteristic change to the planet has been the formation and spread large ice sheets and glaciers across much the Northern Hemisphere. The sheer weight of the ice at the height of the last ice age depressed Earth’s crust to such an extent that many areas are still slowly but noticeably rebounding to this day, 18,000 years after the retreat of the glaciers.
The formation of the ice also removed so much water from the global ocean that sea levels during ice ages were notably lower than interglacial periods such as the present day—as much as 400 feet lower during some periods. The movement of the ice across the surface of the planet also scoured deep valleys, created extensive chains of hills known as moraines, and created extensive lakes, including the Great Lakes.
Understanding the onset and termination of glacial and interglacial cycles is a key part of efforts to understand how Earth’s climate system works and how it responds to changes and disruptions. The steady rise of greenhouse gases in Earth’s atmosphere caused by human activity is a primary cause for concern because of its ability to potentially bring about additional, larger changes.
Scientists are also looking for ways to match changes in Earth’s past environmental conditions with the timing and speed of changes to past climate in order to understand how sensitive the climate system is to disruption and what chain of events might contribute to forcing large-scale changes or, conversely, to helping moderate changes.