Two periods of geologic time, the Pleistocene and the Holocene epochs, have encompassed the past two million years in the Earth's history. The Pleistocene in particular is very important because it is the period in which most of the Ice Ages occurred. It began 1.8 million years ago and lasted until 10,000 years ago. The Earth's climate during this time period was marked by cycles of warming and cooling, which each lasted from centuries to perhaps a couple of millennia. One of these was a cold period known as the Younger Dryas period, which was 1300 years long and began 12,900 years ago. It interrupted a warming period known as the Bolling-Allerod Period, and after it was over the climate returned to its previous warmth. These cycles were probably caused by the melting of glaciers into the Atlantic Ocean during the warmer intervals, which lowered the salinity of the ocean water, which consequently caused the melting to stop. Another probable cause is the collapse of ice sheets, which is caused by the melting of the bottom layers of the glacial deposits due to the rise of built-up heat in the Earth. Once the ice sheet collapsed, the loss of the stored energy triggered another build-up of ice in the deposit.
During the Holocene epoch, the climate of the Earth became warmer during the Medieval Warm Period of the ninth and tenth centuries. This period was followed by the Little Ice Age, which lasted from the fourteenth to the nineteen centuries. Since 1861 the Earth has warmed by about 0.5 degrees Celsius. Most of this warming took place before 1940, but much of it has taken place in the last 15 years. The global temperature of the Earth is projected to rise anywhere from 0.8 to 2.6 degrees Celsius in the next thirty to forty years.
Scientists have discovered these facts by means of several different methods. A number of archaic records that contain meteorological information were found and they provided some information, but this was limited because the people who recorded the information were not interested in studying long-term climate. Studying the tree-growth patterns of certain areas also helps to derive information about that place's climate and the climate of the Earth in general. Scientists can gather information based on the width, make-up and density of the annual rings in the trunks of these trees. Also, drilling in ice caps in such places as Greenland and high-altitude areas of the tropics yielded a vast amount of information about changes in the Earth's climate.
Changes in climate can be caused by several different factors. Volcanic gases and dust, changes in ocean circulation, fluctuations in solar output, and increased concentrations of greenhouse gases in the atmosphere can all cause climate changes. In particular, carbon dioxide levels in the atmosphere and changes in the salinity of the ocean water are also important in causing these climate changes. The changes in the Earth's climate over the past two million years have been caused by a combination of these factors, and each of them has significantly affected the Earth's climate in some way.
Major Climatic Periods of the Past Two Million Years
More General Information about the Earth's Cooling Cycles
Magazine Articles Related to Climate Change
Jones, P.D., 1990, The Climate of the Past 1000 Years: Endeavour, v. 14, p. 129-136.
Jones (1990) discusses the major climactic eras of the past one thousand years, and the factors which caused them to change. He talks about how data about these eras was obtained from studying the rings of trees that are thousands of years old and by studying the cores of tropical ice caps. Then he describes the cool period of the ninth and tenth centuries, the Medieval Warm Period of the twelfth century, the Little Ice Age between the fourteenth and nineteenth centuries, and the warming of the Earth's global temperature by 0.5 degrees Celsius since 1861. Jones also lists volcanic ejections, changes in ocean circulation, solar output variations, and greater concentrations of greenhouse gases in the atmosphere as agents of these changes in the Earth's atmosphere. He finishes with a prediction of the future of the Earth's climate and an estimate of its future warming.
Kerr, Richard A., 1993, How Ice Age Climate Got the Shakes: Science, v. 260, p. 890-892.
In this article, Kerr relays information on theories pertaining to what may have caused the ends of the Ice Ages. He discusses at length the quick and random changes between warm and cold that the Earth's climate sustained, and the notion that some sort of "switch" may have controlled these changes. One theory attributes these changes to variations in the ocean currents, and evidence of iceberg growth and collapse that could have changed the climates of North America and Europe was found was connected with this theory. By determining whether or not there are "switches" that cause massive changes in the Earth's climate, scientists hope to ensure that mankind will not unwittingly cause such a transformation.
Berner, Robert A., 1990, Atmospheric Carbon Dioxide Levels Over Phanerozoic Time: Science, v. 249, p. 1382-1386.
Crowley, Thomas J., and North, Gerald R., 1988, Abrupt Climate Change and Extinction Events in Earth History: Science, v. 240, p. 996-1001.
Ice AgesAn online museum exhibit that explains what the Ice Ages were and when they occurred, and what caused them.
Ice Age ClimateA map that shows the annual mean difference between the temperatures in the Ice Age compared with the temperatures of today.
Earth's Natural Cooling CycleAn explanation of the Earth's climactic rhythms, which lead to its changes in climate.
Researching Climate Changes
When conducting research on the Earth's climate changes, many websites
provide basic and general information. This information is designed for a
wider audience and is easier to understand. More specific and technical
information can be obtained from articles in scientific journals. This
information is usually geared toward scientists and experts, however, and
can be hard to decipher. Both of these types of sources can be very
helpful; it simply depends on what type of information you are looking
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