How Do We Know What Earth's Climate Was Like Long

Ice cores are scientists’ best source for historical climate data. Every winter, some snow coating Arctic and Antarctic ice sheets is left behind and compressed into a layer of ice. By extracting cylinders of ice from sheets thousands of meters thick, scientists can analyze dust, ash, pollen and bubbles of atmospheric gas trapped inside. The deepest discovered ice cores are an estimated 800,000 years old. The particles trapped inside give scientists clues about volcanic eruptions, desert extent and forest fires. The presence of certain ions indicates past ocean activity, levels of sea ice and even the intensity of the Sun. The bubbles can be released to reveal the make-up of the ancient atmosphere, including greenhouse gas levels.