5. Documenting Past Climate Change

To eliminate the variability inherent in the weather, ‘Climate’ is taken to be a 30-year average of temperature, precipitation and other factors. As the old saying goes, ‘The climate is what you expect; the weather is what you get.’ Earth’s climate has varied between two major states, ‘greenhouse,’ where there is no ice in the polar regions and equator–pole temperature gradients are small, and ‘icehouse,’ where the polar regions are covered by ice and the latitudinal temperature gradient is large. During the icehouse state, the climate oscillates between glacials when the polar ice expands down to the mid-latitudes, and interglacials when it is restricted to high latitudes. The last change from greenhouse to icehouse occurred about 35 million years ago. For the past 800,000 years, glacials have lasted about 80,000 years, followed by a deglaciation lasting 10,000 years and an interglacial lasting 10,000 years. Determination of changing geologic process rates has become possible through the development of more sophisticated techniques involving potassium–argon dating, reversals of Earth’s magnetic field, fission track dating, astronomical dating, and tritium, carbon-14, and beryllium-10 dating techniques. Human activities have greatly accelerated the rates of geologic change. The climate of the past 6,000 years, during which civilization has developed, is the longest period of stability in the past few million years.