"Greenhouse gases are accumulating in Earth’s atmosphere as a result of human activities, causing surface air temperatures and subsurface ocean temperatures to rise." -Climate Chance Science, U.S. National Academy of Sciences, 2001 "The scientific understanding of climate change is now sufficiently clear to justify nations taking prompt action." -Joint [11] Science Academies’ Statement: Global Response to Climate Change, 2005

The earth is warming. Temperatures at the Earth's surface increased by an estimated 1.4°F (0.8°C) between 1900 and 2005.

The past decade was the hottest of the past 150 years and perhaps the past millennium. The hottest 22 years on record have occurred since 1980, and 2005 was the hottest on record.

The growing scientific consensus is that this warming is largely the result of emissions of carbon dioxide and other greenhouse gases from human activities including industrial processes, fossil fuel combustion, and changes in land use, such as deforestation. Projections of future warming suggest a global increase of 2.5ºF (1.4ºC) to 10.4ºF (5.8ºC) by 2100, with warming in the United States expected to be even higher.

In addition to warming, increases in sea level and changes in precipitation, including more frequent floods and droughts, are likely. These changes, over time, are referred to broadly as "climate change".

Unaddressed, climate change will have significant impacts across the United States and around the world. For instance, sea-level rise will add to stresses coastal communities are already facing, including erosion, storms, and pressures from development. In the arid and semi-arid western United States, relatively modest changes in precipitation can have large impacts on already limited water supplies. Terrestrial, freshwater, and coastal ecosystems of the United States are particularly sensitive to climate change, threatening biodiversity and ecosystem goods and services such as fisheries and recreation.

Even human health may be threatened as heat waves, extreme weather, and vector-borne diseases become more prevalent. While some of the effects of climate change may be positive, such as longer growing seasons in the northern United States and Canada that increase productivity of agriculture and forests, these positive impacts are unlikely to be sustained as the globe continues to warm.

Furthermore, even while the nation as a whole benefits, certain regions or sectors, such as the southern United States, may suffer. Similarly, many developing countries are even more vulnerable to the adverse impacts of climate change and less able to adapt. As nations continue to grow more interdependent, the United States may not be immune from impacts experienced elsewhere.

Most projections of future impacts do not address what could happen if warming continues beyond 2100, which is inevitable if steps to reduce emissions are not taken, or if the rate of change accelerates.

Furthermore, the longer warming persists and the greater its magnitude, the greater the risk of climate “surprises” such as abrupt or catastrophic changes in the global climate. Even if we are able to reduce emissions of greenhouse gases, some further warming is unavoidable. We must plan and take action now to adapt to the changes we will face as our climate changes.

http://www.pewclimate.org/global-warming-basics/basic_science/