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) Long-Term changes in climate Long time scales refer to periods of many millions of years or more. 1. Changes in solar luminosity - Our young sun shone 25 – 30% less brightly than it does today. But, climate during early earth’s history was generally warmer than today. 2. The long-term carbon cycle Over long periods of time, the carbon cycle alters the amount of carbon dioxide in the atmosphere. Volcanic activity causes CO2 outgassing Outgassing increases when spreading rates at midocean ridges increases 3. Continental position and the role of plate tectonics. a. Ice ages have only occurred when a continent has been positioned over one of the poles. b. North America must be at a high latitude location for ice to form on it. c. Continental position can also affect ocean currents and force currents to either flow primarily north/south or east/west. Intermediate-Term Changes in Climate 1. Operates on time scales of 10,000’s to 100,000’s of years. 2. The coming and going of ice ages during the last 1.8 million years are operating on this time scale. 3. During this Pleistocene Epoch, ice ages and intervening interglacial periods have come and gone possibly as many as 15 times. The last ice age was called the Wisconsin in North America. Wisconsin lasted from ~80,000 – 10,000 years before present. Causes of intermediate-term climate change 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 1. When seasonal variations in radiation received in the Northern Hemisphere are the least; the result is a glacial advance. 2. When seasonal variations in radiation received in the Northern Hemisphere are the greatest; glaciers retreat and we enter into an interglacial period. • When predictions based on the cycles are compared actual timing of glacial advances and retreats, the correlation is very strong. Short-term climate change 1. Within ice ages or interglacial periods, there are much shorter term cycles of climate warming and cooling. 2. These cycles may last a few hundred to a few thousand years. 3. Examples would include the “little climatic optimum” (~900 – 1200 A.D.) and the “little ice age” (16th to mid19th century). Possible Causes of Short-Term Climate Change 1. Volcanic eruptions: example Tambora 1815 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.