Past Climate

Climate change, meaning a change in average air temperature and precipitation, is a significant aspect of global change. As a guide to the future consider the past climate; we know most about temperatures.

These two figures show former temperatures with major periods of glaciation labeled. The dashed lines are the present global average temperature of about 15 ˚C. Thus the solid curves show small changes from this average; note that the temperature drops only about 5 ˚C during a glaciation. This has occurred about every 100,000 years, with smaller wiggles in between. That is, there has been a 100,000 year glaciation cycle for the past million years or so, and there may be shorter cycles as well.

The most recent glaciation, 20,000 years ago, is called the Laurentide, and Earth is still recovering from it.

http://www.museum.state.il.us/exhibits/ice_ages/images/laurentide.mpg

The most recent small drop in average temperature caused the Little Ice Age of 1500-1700 AD, which history describes. Mountain glaciers advanced in Europe and rivers like the Thames in England froze solid, which doesn't happen now.

How do we know about past climate?

･ The geological record of carved mountain valleys, scratched bedrock, and glacial debris and moraines gives evidence of the past several million years.

･ Recently, cores have been removed from the ice at Vostok Station in Antarctica. The longest cores are about 2000 meters, sampling layers of ice deposited as early as 160,000 years ago. The ice trapped bubbles of air when it froze. The ratio of oxygen isotopes in this air indicates the average air temperature at the time the bubble was trapped in ice. The bubbles also trap atmospheric greenhouse gases that can be measured.

Fossil plants and the distribution of pollen show that vegetation has changed, consistent with changing climate.

Pollen from plants, buried in shallow deposits of earth, indicate the distribution of vegetation since the last glaciation, about 20,000 years ago.

Tree rings provide a record of the weather back 3,000 years in some cases, and hundreds of years in many areas.

What Caused the Ice Ages?

No one is certain what caused the lower temperatures of the Ice Ages, and several different causes probably worked together. The 100,000 year long cycle is likely caused by variations in the Earth's orbit and tilt that change the amount of heat reaching Earth from the Sun. A small change can make the few degrees difference between a temperate climate and ice. Also, the Sun's total output of heat may vary a little.

The amounts of the so-called atmospheric greenhouse gases, primarily carbon dioxide and methane, affect temperature. If nothing else changes, more greenhouse gases would make the temperature rise. Data from the air bubbles in ice cores show that in the past when the temperature was higher there was more carbon dioxide. Thus the temperature and the amount of the greenhouse gas carbon dioxide were correlated. (That is they changed together, and in an understandable way.)

･ The observation that past temperature and the amount of carbon dioxide in the atmosphere changed together doesn't tell us that one effect caused the other.

･ Variations in solar energy could have changed the temperature. Could temperature differences alter the amount of carbon dioxide?

･ Or did some unknown cause change the carbon dioxide, and this altered the temperature? Or did something unknown change both temperature and carbon dioxide?

･ The cause and effect relationship might be different in different geological eras. Specifically man-made carbon dioxide is changing temperature, but in the past it was the other way around.

http://vathena.arc.nasa.gov/curric/land/global/index.html