History of Climate Change During earth’s history, climate has generally been warmer than it is today, but is periodically interrupted by short cooler periods. Our climate today exists in one of those cooler periods. (last 2 million years) Table 12.1 The long-term carbon cycle What happens when spreading rates at mid-ocean ridges increase? Late Paleocene torrid age (65 – 55 million years ago) Continental position 1. Ice age formation needs a continent over one of the poles. 2. North America must be at a high latitude location for ice to form on it. Deflection of Equatorial currents poleward. Example: Closing of the Isthmus of Panama ~3 million years ago. Intermediate-Term Changes in Climate 1. 2. Operates on time scales of 10,000’s to 100,000’s of years. During the Pleistocene Epoch, coming and going of ice ages and interglacial periods. The last ice age was called the Wisconsin in North America. Wisconsin lasted from ~80,000 – 10,000 years before present. Changes in earth/sun relationships. 1. The Milankovitch cycles A. Eccentricity – changes in the shape of the earth’s orbit around the sun. B. Axial tilt – changes in the inclination of the earth’s axis. Range of inclination from 21.5 to 24.5 degrees from perpendicular. C. Precession – the wobbling of the earth on it’s axis. Effect of Milankovitch Cycles When insolation levels result in cooler summers in the high latitudes of the Northern Hemisphere, glaciers advance. When predictions based on the cycles are compared actual timing of glacial advances and retreats, the correlation is very strong. Short-term climate change Cycles that last a few hundred to a few thousand years. Examples: The “little climatic optimum” (~900 – 1200 A.D.) and the “little ice age” (16th to mid-19th century). Causes of Short-TermClimate Change Volcanic eruptions Causes short-term climate cooling 2. Sunspot cycles and solar output 3. Disruption of the Ocean Conveyor Belt Heinrich Events- armadas of icebergs adding cold, fresh water to the ocean surface.