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C L I MAT E C H A NG E A N D T H E P O L A R R E G I O N S

the sun. Cool periods of the past produced what are called ice ages,whenmassive glaciers like those in the polar regions covered muchmore of Earth’s NorthernHemisphere.About 20,000 years ago, as the last IceAge was ending, the glaciers startedmelting in North America and other parts of the world. The process lasted for about 10,000 years. Before the glaciers melted, ice sheets in North America reached as far south as what is now Illinois. Scientists believe that the gas carbon dioxide (CO 2 ) and the subsequent changes in Earth’s orbit played a part in ending the last ice age. As the planet slowly warmed, CO 2 stored deep in the Southern Ocean around Antarctica was released into the atmosphere . This gas has been called a greenhouse gas, because when it goes into the atmosphere it traps heat close to Earth’s surface while still letting in light. A greenhouse used to raise plants does the same thing—it lets in sunlight and keeps the sun’s heat inside the building. Many greenhouse gases occur naturally, and during the 19th century, scientists began to understand that a“greenhouse effect” explained some rise in temperature on Earth. The greenhouse

effect has a positive side, as the warmth it generates allowed early humans to spread out around the globe and for plants to grow. Problems began to arise,however, over the last few centuries. Inventors created such things as the steam engine, gas-powered vehicles,and electrical power plants,which

Greenhouse gases