The greenhouse effect
The atmosphere has a natural supply of "greenhouse gases." They capture heat and keep the surface of the Earth warm enough for us to live on. Without the greenhouse effect, the planet would be an uninhabitable, frozen wasteland.
Before the Industrial Revolution, the amount of carbon dioxide (CO2) and other greenhouse gases released into the atmosphere was in a rough balance with what could be stored on Earth. Natural emissions of heat-trapping gases matched what could be absorbed in natural sinks. For example, plants take in CO2 when they grow in spring and summer, and release it back to the atmosphere when they decay and die in fall and winter.

Too much greenhouse effect
Industry took off in the mid-1700s, and people started emitting large amounts of greenhouse gases. Fossil fuels were burned more and more to run our cars, trucks, factories, planes and power plants, adding to the natural supply of greenhouse gases. The gases-which can stay in the atmosphere for at least fifty years and up to centuries-are building up beyond the Earth's capacity to remove them and, in effect, creating an extra-thick heat blanket around the Earth.
The result is that the globe has heated up by about one degree Fahrenheit over the past century-and it has heated up more intensely over the past two decades.
If one degree doesn't sound like a lot, consider this: the difference in global average temperatures between modern times and the last ice age-when much of Canada and the northern U.S. were covered with thick ice sheets-was only about 9 degrees Fahrenheit. So in fact one degree is very significant-especially since the unnatural warming will continue as long as we keep putting extra greenhouse gases in the atmosphere.

How much is too much?
Already, people have increased the amount of CO2, the chief global warming pollutant, in the atmosphere to 31 percent above pre-industrial levels. There is more CO2 in the atmosphere now than at any time in the last 650,000 years. Studies of the Earth's climate history show that even small changes in CO2 levels generally have come with significant shifts in the global average temperature.
Scientists expect that, in the absence of effective policies to reduce greenhouse gas pollution, the global average temperature will increase another 2.0 degrees Fahrenheit to 11.5 degrees Fahrenheit by 2100.
Even if the temperature change is at the small end of the predictions, the alterations to the climate are expected to be serious: more intense storms, more pronounced droughts, coastal areas more severely eroded by rising seas. At the high end of the predictions, the world could face abrupt, catastrophic and irreversible consequences..

The science is clear
Scientists are no longer debating the basic facts of climate change. In February 2007, the thousands of scientific experts collectively known as the Intergovernmental Panel on Climate Change (IPCC) concluded that there is greater than 90 percent likelihood that people are causing global warming. (IPCC, 2007)
These latest findings amplify what other highly respected science organizations say:
" In a joint statement with 10 other National Academies of Science, the U.S. National Academy of Sciences said:
"The scientific understanding of climate change is now sufficiently clear to justify nations taking prompt action. It is vital that all nations identify cost-effective steps that they can take now, to contribute to substantial and long-term reduction in net global greenhouse gas emissions."-Joint Statement of Science Academies: Global Response to Climate Change, 2005
" The American Geophysical Union, a respected organization comprising over 41,000 Earth and space scientists, wrote in its position on climate change that "natural influences cannot explain the rapid increase in global near-surface temperatures observed during the second half of the 20th century."