From palaeoclimatic records scientists now know that during the last 2 million years the Earth's climate has fluctuated between periods of relative warmth and relative cold, with global average surface temperature changing by as much a 5°C between the two climatic regimes. Although over the longer term (50 million years) the Earth has become much colder since the age of the dinosaurs, with permanent ice cover at both the North and South poles, the size of the polar ice caps has repeatedly grown and shrunk roughly every 100,000 years. The colder periods, called glacials or Ice Ages, have usually lasted for 80,000 to 100,000 years, whilst the intervening warmer intervals have been much shorter, lasting about 10,000 years. The last Ice Age or glacial period on Earth ended roughly 14,000 years ago. At that time, much of Northern Europe and North America lay under huge ice sheets that today remain only in Greenland. At present we may be coming towards the end of the latest warmer interglacial period, although mankind's alteration of the atmosphere through greenhouse gas pollution makes predicting the long term future of our global climate difficult.
A climate forcing mechanism which may explain the reconstructed climate changes of glacial-interglacial transitions over tens of thousands of years must exhibit variations over similar time scales. Such a mechanism, the Yugoslav mathematician Milankovitch believed, could be found in the subtle, yet significant variations in the Earth orbit around the Sun. Since Milankovitch first proposed his orbital theory of climate change in 1941, many palaeoclimatic records from ice and sea sediment cores have been used to confirm these ideas.
More recently, scientists realised that the changes in the amount of energy received by the Earth from the Sun as a result of orbital variations would not be enough to explain the significant shifts in climate between Ice Age and interglacial conditions on Earth. To account for the large global temperature change between warm and cold periods, additional feedback effects were proposed that would augment the initial climatic response to the orbital variations. Such feedback involved the greenhouse gas carbon dioxide.
At the same time as scientists were using ice cores to decipher past changes in temperature, they were also analysing the gaseous composition of trapped air bubbles in the ice. Such analysis revealed that during the last Ice Age, levels of carbon dioxide in the atmosphere were significantly lower than they were before mankind began polluting the atmosphere 200 years ago. In fact, reconstructed records of temperature and atmospheric carbon dioxide were found to match almost exactly. Since carbon dioxide is a significant greenhouse gas, it was believed that initial climatic changes resulting from orbital variations were somehow affecting the composition of the atmosphere, to the extent that a lowering of carbon dioxide concentrations was increasing the global cooling.