The period for which we have instrumental records of climate change, such as observational records of temperature and rainfall, spans only a tiny fraction of Earth History. Furthermore, although we are now concerned with global warming due to mankind's greenhouse gas pollution of the atmosphere, this contemporary climate change should be placed in the context of much longer term changes in climate that have taken place quite naturally. Prehistoric climate change is known to climatologists and Earth scientists as palaeoclimatic change.
The global climate has shifted and varied for billions of year, perhaps since the Earth first had an atmosphere. The oldest palaeoclimatic records have allowed us to reconstruct climate fairly reliably during the last 500 million years. Over this time, the global climate has moved from extensive periods of global warmth to periods of global cold several times, each lasting 100 million years or more. Although today we are concerned about global warming, we do in fact lie in the middle of global ice-house climate, which began 40 million years ago, when the first permanent ice sheets formed on Antarctica. The change from the much warmer global climate which existed during the age of the dinosaurs, when global average temperature was perhaps 10°C higher than at present, is thought to have been caused by changes in the distribution of landmasses and the associated changes to energy redistribution throughout the climate system.
Within the long-term global icehouse climate, much shorter-term fluctuations in global climate have occurred. Relatively cold periods known as Ice Ages or glacials, each lasting roughly 100,000 years, are interspersed with much shorter warmer episodes or interglacials, lasting only 10,000 years. We now have a relatively clear record of such climatic fluctuations over the last 2 million years. Currently, the global climate lies within an interglacial. Global average temperature 20,000 years ago towards the end of the last Ice Age was some 5°C lower than today, when the north polar ice sheets were expanded to cover a considerably greater area of the continental Northern Hemisphere than is the case today. These glacial-interglacial fluctuations are believed to be driven by changes in the position of the Earth in its orbit around the Sun, and enhanced by climatic feedback processes which involve changes in ocean circulation and the greenhouse gas composition of the atmosphere.
Within the latest present interglacial period, further fluctuations in the global climate can be seen in the palaeoclimatic records and more recently in the instrumental records. During the last 1000 years, the climate has moved from a period of Medieval warmth to a "Little Ice Age" between the 16th and 19th centuries, with changes of between 0.5 and 1°C in the global average surface temperature. Although it is not clear what has caused these climatic changes, variations in the Sun's energy output, ocean circulation and the occurrence of major volcanic eruptions are believed to play a part.
Most recently, we have entered a renewed period of global warming since the beginning of the 20th century that we suspect is the result of mankind's enhancement of the natural greenhouse effect through the pollution of the atmosphere. Global average temperature is now about the same as it was during the Medieval warm period, although still much lower than it was 100 million years ago during the age of the dinosaurs.