Global Warming 101
Global warming is already under way. The urgency of taking action becomes clearer with each new scientific study.
Global warming is caused primarily by carbon dioxide and other heat-trapping emissions from our burning of fossil fuels (when we drive cars or generate electricity) and our clearing of forests. These emissions remain in our atmosphere for decades or even centuries.
Although heat-trapping emissions result from both human and natural causes, there is concrete evidence that most of the warming over the last 50 years is due to our own actions. Recent studies have confirmed that current carbon dioxide levels in the atmosphere are the highest they have been in the last 650,000 years.  
Atmospheric carbon dioxide record data sources: Keeling and Whorf (2004),
Petit et al. (1999), IPCC (2001), Ahn et al. (2004).
Art credits: (astronaut, pyramid) © Photos.com; (car, mammoth) © Clipart.com.
Some of the most obvious signs of global warming are visible in the Arctic, where rising temperatures and melting ice are dramatically affecting the region’s unique landscapes and wildlife—as well as people’s lives and livelihoods. Across the globe, other early warning signs include melting glaciers, more extreme heat, shifting ranges of plants and animals, and the earlier onset of spring.
The profound impact rising temperatures have had in the Arctic provides a window into a future we may all experience. With continued warming, we can expect more extreme heat and drought, rising sea levels, and higher-intensity tropical storms. At risk are our coastal property and resources, the livability of our cities in summer, and the productivity of our farms, forests, and fisheries. The strength of the evidence prompted 11 national academies of science, including the U.S. National Academy of Science, to declare in 2005 that, "the scientific understanding of climate change is now sufficiently clear to justify nations taking prompt action."
We cannot avoid all the consequences of global warming, but practical solutions for minimizing the consequences exist today. As a nation, and as individuals, we must utilize available and affordable technologies to increase our energy efficiency, reduce emissions from our vehicles, and end our dependency on fossil fuels by shifting to renewable energy sources. By committing ourselves to action today and supporting policies that adopt these solutions, we can help ensure that our children and grandchildren inherit a healthy world full of opportunity.
For more details on the science of global warming, visit the Union of Concerned Scientists website at http://www.ucsusa.org/global_warming/science.
 Intergovernmental Panel on Climate Change (IPCC). 2001. Climate change 2001: The scientific basis. Cambridge, UK: Cambridge University Press.
 EPICA. 2004. Eight glacial cycles from an Antarctic ice core. Nature 429:623-628.
 Petit, J.R., J. Jouzel, D. Raynaud, N.I. Barkov, J.-M. Barnola, I. Basile, M. Bender, J. Chappellaz, M. Davis, G. Delaygue, M. Delmotte, V.M. Kotlyakov, M. Legrand, V.Y. Lipenkov, C. Lorius, L. Pépin, C. Ritz, E. Saltzman, and M. Stievenard. 1999. Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica. Nature 399:429-436.
 Siegenthaler, U., T.F. Stocker, E. Monnin, D. Lüthi, J. Schwander, B. Stauffer, D. Raynaud, J.-M. Barnola, H. Fischer, V. Masson-Delmotte, and J. Jouzel. 2005. Stable carbon cycle-climate relationship during the late Pleistocene. Science 310:1313-1316.