History of Climate Change [Athena] [Curriculum] [Earth Resources]

History of Climate Change

Table of Contents


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 (59° F). 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. This map from the The Illinois State Museum exhibit on ice ages shows the extent of that ice. Retreat of the Laurentide Ice is an animation made by the Museum.

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.

Europeans first settled in America during these centuries. They had a hard time as a result of the cold conditions in the northern hemisphere, colder than they expected.

How do we know about past climate?

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. (Atmospheric Greenhouse Gases explains why.) 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.)

But consider these important ideas!

What are humans doing that might change the climate?

Humans are putting the greenhouse gas carbon dioxide into the air by burning fossil fuel or wood. We also add CFC (that is chlorofluorocarbons) compounds like Freon, used in refrigeration systems. CFC's are greenhouse gases as well as destroyers of ozone. We measure that carbon dioxide is currently increasing, and it seems to be getting warmer the last few decades. Although this suggests that increased greenhouse gases are causing the warming, it is not certain. Atmospheric Greenhouse Gases discusses these topics and offers data and student activities.

Changes in Alaska

If climate changes, the amount of frozen water is expected to change greatly. Because of all its sea-ice, glaciers, and permafrost (permanently frozen ground), Alaska is a great place to study changes in climate. Peat, tundra and boreal forests also respond sensitively to climate change.

Recordings show that Alaska has been warming at 0.7 - 0.8° C every 10 years, according to Alaska's Center for Global Change. If the trend s, the temperature could be 7 - 8° C warmer in 100 years. This is a big difference. In the last ice age it was only about 5 - 8° C cooler.

The Tables from Preparing for an Uncertain Future show details of observed and predicted climate trends. The Alaska Center for Global Change provides further information.

Return to Global Change.


Image Credits

Map from The Illinois State Museum exhibit on ice ages.
Graphs from Foland et al.; see reference below.

References


Written by: Hugh Anderson and Bernard Walter.

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Last Modified Friday 28 March 1997