Small changes in the Earth¿s heat balance can lead to large climatic changes. For example, the ice ages during the last several million years--and the warmer periods in between--appear to have been triggered by no more than a different seasonal and latitudinal distribution of the solar energy absorbed by the Earth, not by a change in output from the sun. The geologic record shows that the differences in ice cover, sea level and precipitation as well as in plant and animal populations were quite dramatic between the ice ages and the warm interglacials. Yet the global average temperature differences corresponding to these radically different climates were only about 5 degrees C in the tropics and 8 degrees C in polar regions.
At this point, the Earth is probably on the threshold of an entirely new epoch in which the global climate and the distribution of life will be strongly influenced by a single species: humans. Some are calling this new epoch the anthropocene and it is all thanks to our increasing the relatively small amount of carbon dioxide in the atmosphere by burning the vast stores of carbon trapped inside of the fossil fuels that power our modern lives.