Any change in the balance between the quantity of energy absorbed compared to the amount emitted affects climate. The "greenhouse effect" is concerned with the infrared radiation given off by the earth. Part of this radiation is absorbed by the atmosphere, rather than being lost to space. The gases in the atmosphere that absorb infrared light primarily are water (H2), carbon dioxide (CO2), ozone (O3), nitrous oxide (NO2) and methane (CH4). The gases act as a sort of insulating blanket for the earth, in the same way they would act to lessen heat loss from a greenhouse, hence the name 'greenhouse effect'. It is estimated that the mean global surface temperature of the earth would be -25°C (-13°F) if not for the absorption of energy by carbon dioxide and water.
The concentration of water vapor in the atmosphere is higher than that of carbon dioxide. Consequently, most of this energy conservation is attributable to water. You can see this effect when you look at how temperature drops less on nights with heavy cloud cover as opposed to clear skies or when you consider how large the temperature difference between day and night is in places with lower relative humidity, like the desert.
Although the concentration of carbon dioxide in the atmosphere is low (~375 ppm in 2005), it has been increasing appreciably over time. A century ago, the carbon dioxide concentration was less than 300 ppm. Human activites are accountable for this increase, including consumption of fossil fuels and extensive clearing of land (less carbon dioxide can be consumed by photosynthesis). Changes in the levels of carbon dioxide in the atmosphere are associated with changes in the earth's climate.