Global warming is a natural phenomenon in which greenhouse gases -- largely methane and carbon dioxide -- begin to build up in the Earth's atmosphere. As these gases accumulate, they form a layer that traps the sun's heat as it moves toward the Earth, resulting in large-scale climate change all over the world. An example of a naturally caused greenhouse gas is the emission from volcanic eruptions.
Global warming also refers to the combination of natural and mostly manmade events that are causing this large-scale climate change. Most notably, trapping of greenhouse gases released by human activities is causing a warming of the planet. Though global warming is hotly debated, there is scientific proof that people continuing to burn fossil fuels -- such as coal, oil and gas -- at higher rates has caused most of our planet's warming. The atmosphere above Earth has warmed by about 1.5 degrees Fahrenheit (about 16.5 Celsius) since 1900 [source: National Wildlife Federation]. In fact, the decade from 2000 to 2010 was the warmest on the planet, and surface temperatures in 2010 matched those of 2005 as the warmest ever recorded [source: NASA].
The reason global warming is important is that warmer temperatures affect Arctic sea ice, causing it to melt. That in itself might not seem so important, except that the melting ice affects thousands of years of habitats, and in turn adds to changing sea levels and temperatures. Already, scientists have noted that some river and lake ice has broken up sooner than it should, changes in plant and animal ranges are appearing, trees are flowering sooner than normal and more intense heat waves are occurring.
Even though one degree change sounds miniscule, it takes very little to upset the normal course of the Earth's fragile environment. The record of the Earth's climate is preserved in structures such as coral reefs, ice cores and tree rings. This record shows that at the end of the last ice age, the average temperatures were only about 5 to 9 degrees cooler than they are today, yet the northeastern portion of the U.S. was covered in thousands of feet of ice [source: Global Climate Change].
And if it's hard to picture the effect from a temperature measurement, consider this: Between 1990 and 2010, there was a 29 percent increase in the Earth's warming effect simply from greenhouse gases, according to the World Meteorological Organization [source: Waugh]. That's much worse than any of the scenarios predicted at a United Nations expert panel held nearly 10 years earlier.
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