For over the past 200 years, the burning of fossil fuels, such as coal and oil, and deforestation have caused the concentrations of heat-trapping "greenhouse gases" to increase significantly in our atmosphere. These gases prevent heat from escaping to space, somewhat like the glass panels of a greenhouse. Greenhouse gases are necessary to life as we know it, because they keep the planet's surface warmer than it otherwise would be. But, as the concentrations of these gases continue to increase in the atmosphere, the Earth's temperature is climbing above past levels. According to NOAA and NASA data, the Earth's average surface temperature has increased by about 1.2 to 1.4ºF in the last 100 years. The eight warmest years on record (since 1850) have all occurred since 1998.
If greenhouse gases continue to increase, climate models predict that the average temperature at the Earth's surface could increase from 3.2 to 7.2ºF above 1990 levels. Scientists are certain that human activities are changing the composition of the atmosphere, and that increasing the concentration of greenhouse gases will change the planet's climate. But they are not sure by how much it will change, at what rate it will change, or what the exact effects will be. 
Dr. Charles David Keeling was the first scientist to study carbon dioxide and its effects on the atmosphere. Previously, scientists did not know the fate of the excess carbon dioxide in the atmosphere that is produced by the burning of fossil fuels and other industrial activities. Keeling's research, however, has shown that atmospheric carbon dioxide levels are increasing. They have risen more than 14 percent since 1957. 
Main Greenhouse Gases
The main greenhouse gases include carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), a number of fluorinated gases, and water vapor (H2O). Some greenhouse gases occur naturally, such as water vapor and carbon dioxide, while others (such as chlorofluorocarbons) are produced only through human activities. Greenhouse gas inventories account for only those gases whose effects are well-understood (e.g., they include CO2, CH4, N2O, and fluorinated gases, and they include only anthropogenic (human-controlled) sources.