Human activities contribute emissions of four principal greenhouse gases: carbon dioxide, methane, nitrous oxide and the halocarbons (a group of gases containing fluorine, chlorine and bromine).
Starting from the first industrial revolution in the 1880s, societies have consumed vast amounts of fossil fuels. Burning of fossil fuels emits carbon dioxide gases (main greenhouse gas), thereby raising the amount of CO2 levels in the atmosphere.
Methane is released from human activities such as agriculture, natural gas distributions and landfills. Nitrous oxide is emitted from fertilizer usage and fossil fuel burning, too. Principal halocarbons, such as chlorofluorocarbons, were used intensively as refrigeration agents. Such chemicals cannot be broken down and consequently cause the depletion of the Earth's stratospheric ozone layer, a barrier that shields the Earth from excessive ultra-violet radiation from the Sun.
Figure 2: Electricity generation is one of the biggest contributor to climate change (Global Warming).
State of Global Warming
As a result of the Greenhouse Effect, the average global temperature has been climbing:
According to the World Meteorological Organization (WMO), the decade of 1998-2007 is the warmest on record. The global mean surface temperature for 2007 is currently estimated at 0.41°C/0.74°F above the 1961-1990 annual average of 14.00°C/57.20°F. - Global Warming - Encyclopedia of Earth
Figure 3: Global Mean Temperature over Land and Ocean (Jan-Dec). (Source: NCDC/NESDIS/NOAA)