The leading greenhouse gas is carbon dioxide, a pollutant emitted from the burning of fossil fuels, such as coal, oil, and natural gas. While it is true that there has always been some natural climate variability, record levels of carbon dioxide are having a far reaching change over our weather, sea levels, and climate. Global warming is one of the most serious challenges facing us today. To protect the health and economic well-being of current and future generations, we must reduce our emissions of heat-trapping gases by using the technology。 The Earth is only 5 to 9 degrees warmer today than it was 10,000 years ago during the last ice age. Throughout history, major shifts in temperature occurred at a rate of a few degrees over thousands of years. They were accompanied by radical ecological changes, including the extinction of many species. Manmade global warming is occurring much faster — faster, in fact, than at any time in the past 10,000 years. Unless we slow and ultimately reverse the buildup of greenhouse gases, we will have decades, not millennia, to try to adapt to radical changes in weather patterns, sea levels and serious threats to human health. Increased flooding, storms and agricultural losses could devastate our economy. Plants and animals that cannot adapt to new conditions will be come extinct. 

Resources: http://www.sierraclub.org/globalwarming/dangerousexperiment/ http://www.sierraclub.org/energy/overview/