Global Warming

Scientists know for certain that human activities are changing the composition of Earths atmosphere.

Increasing levels of greenhouse gases, like carbon dioxide (CO2), in the atmosphere since pre-industrial times have been well documented. There is no doubt this atmospheric buildup of carbon dioxide and other greenhouse gases is largely the result of human activities.

Its well accepted by scientists that greenhouse gases trap heat in the Earths atmosphere and tend to warm the planet. By increasing the levels of greenhouse gases in the atmosphere, human activities are strengthening Earths natural greenhouse effect. The key greenhouse gases emitted by human activities remain in the atmosphere for periods ranging from decades to centuries.

A warming trend of about 1 F has been recorded since the late 19th century. Warming has occurred in both the northern and southern hemispheres, and over the oceans.

Confirmation of 20th-century global warming is further substantiated by melting glaciers, decreased snow cover in the northern hemisphere and even warming below ground.

Figuring out to what extent the human-induced accumulation of greenhouse gases since pre-industrial times is responsible for the global warming trend is not easy. This is because other factors, both natural and human, affect our planets temperature. Scientific understanding of these other factors most notably natural climatic variations, changes in the suns energy, and the cooling effects of pollutant aerosols remains incomplete.

Nevertheless, the Intergovernmental Panel on Climate Change (IPCC) stated there was a discernible human influence on climate; and that the observed warming trend is unlikely to be entirely natural in origin. In the most recent Third Assessment Report (2001), IPCC wrote There is new and stronger evidence that most of the warming observed over the last 50 years is attributable to human activities.

In short, scientists think rising levels of greenhouse gases in the atmosphere are contributing to global warming, as would be expected; but to what extent is difficult to determine at the present time.

As atmospheric levels of greenhouse gases continue to rise, scientists estimate average global temperatures will continue to rise as a result. By how much and how fast remain uncertain. IPCC projects further global warming of 2. 2-10 F (1. 4-5. 8 C) by the year 2100. This range results from uncertainties in greenhouse gas emissions, the possible cooling effects of atmospheric particles such as sulfates, and the climates response to changes in the atmosphere.

The IPCC states that even the low end of this warming projection would probably be greater than any seen in the last 10, 000 years, but the actual annual to decadal changes would include considerable natural variability. What are the Big Unknowns? Scientists have identified that our health, agriculture, water resources, forests, wildlife and coastal areas are vulnerable to ...