The gradual increase over time of the average temperature of the entire earth's lower atmosphere. Global warming has occurred in the distant past as the result of natural influences, but the term is most often used to refer to the warming predicted to occur as a result of emissions of greenhouse gases directly and indirectly caused from the burning of fossil fuels. An increase of the earth's temperature by only a few degrees results in dramatic climate change, loss of habitat and species, sea-level rise, and alteration of the oceans' currents.