The earth's climate varies naturally. Each component of this complex system evolves on a different timescale. The atmosphere changes in hours, and its detailed behaviour is impossible to predict beyond a few days. The upper layers of the oceans adjust in the course of a few seasons, while changes in the deep oceans can take centuries. The animal and plant life of the biosphere (which influences rainfall and temperature) normally varies over decades. The cryosphere (snow and ice) is slower still: changes in thick ice sheets take centuries. The geosphere (the solid earth itself) varies slowest of all - mountain-building and continental drift (which influence winds and ocean currents) take place over millions of years.

Past natural climate changes offer vital insights into human-induced climate change. Studies of past climates ("paleoclimatology") give a sense of the scale of future changes projected by climate models. They also provide a crucial check on scientists' understanding of key climate processes and their ability to model them.

Systematic global temperature records are available only since 1860. These include land-based air temperature measurements and sea-surface temperature measurements. Such data need to be checked carefully for any biases that may be introduced by changes in observation methods or sites. For example, many meteorological stations have been located in or near cities. As cities grow, they can have a significant warming effect on the local climate. Such effects must be – and are – taken into account in estimating recent changes in global temperature.

Studies of earlier climates are based on indirect evidence. Changing lake levels, for example, can reveal the past balance between rainfall and evaporation. Tree-rings, coral, ice-caps, or ocean sediments can all preserve information about the past. Using a combination of measurements, models, and "detective work", scientists convert the quantities they can measure (such as the chemical composition of an ice-core sample) into the physical variables they wish to investigate (such as the Antarctic temperature of 100,000 years ago).

The earth's climate has been dominated by ice ages for the past few million years. Ice ages are almost certainly triggered by slow "wobbles" in the earth's axis and its orbit around the sun. These wobbles affect the total amount of energy the planet receives from the sun and in particular its geographic distribution. During an ice age, global temperatures fall by 5^oC and ice-sheets advance over much of Europe and North America. Ice ages are separated by warmer "interglacial" periods.

Changes in greenhouse gas concentrations may have helped to amplify ice-age cycles. The small fluctuations in energy arriving from the sun due to the earth's orbital wobbles are not large enough to account for the size of global temperature changes during the ice age cycles. Ice-core samples show that greenhouse gas levels also varied significantly and may have played an important role in amplifying temperature fluctuations.

Reconstructions of past climates can be used as a check on climate model projections. Comparing a model "prediction" of ice-age climate with the evidence from paleoclimatology provides a crucial check on the model's representation of processes relevant for future climate change. But the paleoclimatic evidence can be ambiguous: some sources suggest that, compared with today, tropical seas were some 5^oC colder at the peak of the last ice age, while others suggest only 1-2^oC. As a result, separating model errors from uncertainties in the evidence can be difficult.

The climate seems to have been remarkably stable since the last ice age ended 10,000 years ago. As far as scientists can tell, global temperatures have varied by less than one degree since the dawn of human civilisation. Against the apparently extreme and sometimes rapid climate fluctuations of the preceding 100,000 years, this stands out as a relatively peaceful interglacial period.

Models predict that the climate could be warmer by the end of the 21st century than it was during any previous inter-glacial period. In a period between two ice ages about 125,000 years ago, much of Europe and Asia appear to have been about 2^oC warmer than they are today. However, models are predicting that temperatures could rise by much more than this over large stretches of this region during the 21st century if greenhouse gas emissions continue as projected.

Abrupt climate variations in the distant past appear to have been traumatic for life on earth. The earth's biological history is punctuated by so-called "mass extinction events" during which a large fraction of the world's species are wiped out. There are many possible reasons for mass extinctions, but the records suggest that some of these events coincided with relatively abrupt changes in climate – similar in magnitude to the kind of change now forecast for the 21st century. Over the next 100 years we may experience conditions unknown since before the ice ages began many millions of years ago.