2.6 million years of climate change - peaks represent a warm earth, troughs a cold earth

The Quaternary is a subdivision of geological time (the Quaternary Period) which covers the last two million years up to the present day. (The exact duration is a matter of debate with estimates of the onset of the Quaternary Period placed at between 1.8 million years and 2.6 million years by different authors) The Quaternary and the Tertiary Periods together form the Cenozoic Era. The Quaternary can be subdivided into two epochs; the Pleistocene (two million years to ten thousand years ago) and the Holocene (ten thousand years ago to the present day).

The Quaternary Period has been one of extraordinary changes in global environment as well as the period during which much of hominid evolution took place.

Since the middle of the l9th century geologists realised that during the most recent period of geological time (widely known as "The Ice-Age") there had been large changes in the landscape and the environment. Indeed it was widely accepted by the 1860s that the action of glaciers had profoundly altered the surface of the earth over wide areas, including most of northern Europe and North America. Modern scientific techniques have given us great insight into the scale and timing of the climatic changes that led to the expansion of the global ice caps to three times their present extent, causing glaciation in formerly unglaciated regions.

In the last 30 years evidence from deep sea drilling has shown that about 25 million years ago the earth's climate began to cool and the polar ice caps began to expand steadily. The cause of this slow, inexorable, change in global environment is still unknown but adjustment of atmospheric chemistry, possibly due to increasing volcanic activity may have been the trigger. We now know that both volcanic dust in the upper atmosphere and the chemical changes wrought by massive releases of sulphur dioxide from certain volcanoes can cause large climatic changes. It is possible that very large explosive eruptions threw dust into the atmosphere while gentler (but large), gas emitting, eruptions also began to occur more frequently. The two combined to bring the earth's climate close to a threshold of change and therefore more susceptible to external forcing by variables that were previously less important. With the global climate systems becoming less stable the stage was set for climatic change and this is what occurred.

The global cooling that began during the Tertiary culminated in a series of step-like, sudden, changes in climatic conditions over the last two and a half million years as the climate began to "seesaw" from cool to warm and the massive polar ice caps expanded and contracted. This period includes the Quaternary Period and the latter is therefore characterised by very variable climatic conditions. At the latitude of northwestern Europe the warmer periods are known as temperate stages (or interglacials -periods between glaciations) and the colder periods as cold stages (or glacials).

The overriding controls on such vast variation in climate that has occurred during the Quaternary are three predominant characteristics of the earth's astronomical position relative to the sun, which vary cyclically over time on different wavelengths (e.g. see the readable account in Imbrie, J. and Imbrie, K.P. 1979. Ice Ages: Solving the mystery. Macmillan, London and Basingstoke). This theory, first adequately proposed by a Serbian mathematician, Milutin Milankovitch, who spent over 30 years perfecting it -publishing detailed accounts in the 1930s and 40s, was not confirmed (or widely accepted) until the 1970s. It is these astronomical variables that have been forcing our climate during the Quaternary and the sudden reversals in trend and the step like pattern is the product of vast positive and negative feedback systems operating in the climate system.

The effect of these climatic changes is spatially dissimilar (i.e. in different parts of the World the climate has reacted differently -perhaps becoming wetter or drier rather than colder and warmer) and also their periodicity has altered over time during the Quaternary, but in northwestern Europe the last 750,000 years have been characterised by long periods (c.100,000 years) of cold climates interspersed with shorter periods (c.10-15,000 years) of warmer conditions. Viewed against current fears of global warming (currently believed to have been less than 1oC in the last 100 years) then it gives us a chilling insight into the actual geological evidence which suggests that over 90% of the last 750ka has been very cold at our latitude (with mean annual air temperatures dropping well below zero). It is thought by some that any trend in global warming will be easily outweighed by much more important overlying trends and climatic feedback systems. Of course we cannot tell what our climate will do due to gross human disturbance of atmospheric chemistry but we must remember that human interference may also trigger cooling, a possibly less pleasant scenario, as a long term consequence of initial warming -this possibility appears more likely on the evidence of the last 750,000 years.

Quaternary climate change has produced a geological record dominated by sediments deposited under glacial, periglacial and temperate environmental conditions and this is, by its very nature, a fragmentary one. So we do not find neatly ordered deposits of alternating glacial (cold stage) and interglacial (temperate stage) sediments but a partial record that is very hard to piece together and which is the subject of much debate. The repeated and extensive cold stages have included some intense glaciations and these have wiped much of the preceding record leaving us with a heavily glaciated landscape with many features of indeterminate age.

Understanding the issues outlined above is of great importance to many branches of science concerned with topics as diverse as the possible future impact of global climate change to construction on Quaternary sediments or understanding and being able to reconstruct the environments of the past.

The Quaternary Research Association exists to promote understanding of the Quaternary Period by publishing fieldguides, technical guides and an international journal as well as holding field meetings and speaker meetings on a regular basis. Our WWW page outlines our planned meetings and publications.
You could also visit: Illinois State Museum's exhibit on Ice Ages