The evidence from past climates
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 5oC 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
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 5oC colder at the peak of the last ice age, while others suggest only 1-2oC. 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 2oC 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