As concerns over future climate change crystallize, scientists are devoting more of their energies to understanding the climates of the past.
Loch Nagar (Scotland). A number of European projects (Alpe, Molar, Emerge, Eurolimpacs) are studying mountain lakes that, despite what one may imagine, do not escape pollution. This research has yielded a great deal of sedimentary, biological and chemical data. It has also permitted the study of diatomea, unicellular algae able to provide climatic indicators.
"The best way of testing climatic models is to test their ability to describe climates of the past. Paleoclimatological data (temperature, composition of the atmosphere, ocean currents, etc.) allow us to put models to the test and compare their validity,” explains Dominique Raynaud, Director of the Laboratory for Environmental Glaciology and Geophysics in Grenoble (FR). But finding out what the weather was like hundreds of thousands of years ago is no mean task. To do so, scientists have to become genuine sleuths, gleaning a maximum of information from the slightest indicator and then crosschecking the data to build up an overall picture.
Marine sediments Marine sediments are a rich source of clues to a distant past. Among other things, they contain remains of the abundant planktonic organisms knows as foraminifers. These minute creatures possess a calcium shell that contains the various oxygen isotopes in proportions that vary with temperature. The resulting data can be crosschecked against the metal trace content, whose penetration also depends on temperature. What is more, foraminifers are found in numerous species with a varying oxygen requirement – enabling researchers to form an idea of the oxygenation of oceans.
Scientists can also investigate the secrets of alkenones that are found in many varieties of surface algae. These tiny lipidic molecules – which conserve remarkably well in sediments – are unsaturated to a greater or lesser degree depending on the ambient temperature, as algae seem to use them to maintain a constant internal viscosity.
In some cases, marine sediments can yield information dating back as far as 50 million years. Beyond that, they are not much use, tending to be too degraded and very scarce. This is because the ocean bed is subject to constant renewal through the movement of the tectonic plates. Dating marine sediments is always a delicate matter and the subject of controversy.
Plunging into mountain lakes The paleoclimatologist’s toolbox has other treasures too. Sediments found in lakes, for example, can provide interesting, if more local, information. The EU has supported a number of projects (Alpe, Molar, Emerge, Eurolimpacs) to study mountain lakes. Lying at altitudes that one would have imagined would guarantee certain purity, even these show the unmistakeable signs of pollution. These projects have produced a wealth of sedimentary, chemical (pH, presence of pollutants, soot particles, etc.) and biological data and permitted the study of diatomea populations. Like foraminifers, these unicellular algae can tell us a great deal about the climates of the past. As for traces of water levels, these tell us about changing rainfall patterns.
"We are in the process of analysing the links between living creatures and the environmental parameters concerned with climate change,” explains Karel Brabec, a researcher at the University of Masaryk (CZ) and a member of the Eurolimpacs project. “Macroinvertebrates – such as insect larva – and plant groups living on the ocean bed are key groups for studying the reactions of living organisms.”
Bogged down with history Much can also be learned from pollen, especially in the peat bogs that are abundant in Europe. These grains, measuring between 10 and 100µm in diameter, can indicate the flowering plant from which they originated. There are currently comprehensive databases listing these plants and the environmental conditions that favoured them. An abundance of a combination of certain types of pollen can give us quite a precise picture of the climate in the past.
“In some cases, however, this interpretation is limited by a poor knowledge of the age of sediments,” explains Raynaud. “But our dating methods are developing rapidly and have already allowed us to make a great deal of progress over the past 30 years.”
History on ice Finally, there are of course the ice caps, the subject of particular attention over recent years. This is a field in which Europeans can boast a wealth of impressive results, with two projects in particular that made the headlines in Nature.
The first, the European Project for Ice Coring in Antarctica – or Epica as it is more commonly known – achieved the considerable technical feat of extracting, from the Antarctican ice cap, an ice core 3 130 metres long, representing 740 000 years of climatic archives. Ultimately, researchers hope to go back more than 900 000 years by the time they arrive at the extremity of the ice deposit.
The second project, the North Greenland Ice Core Project, or N Grip, extracted an ice core of similar length – 3 085 metres, to be precise – but going back less far: 123 000 years. The difference is because the ice in Greenland is much less compacted than at the South Pole, providing a very good time resolution: at the base of the boring one year is imprinted in more than 1 cm of ice. These lower strata contain valuable information dating back to the end of the last warm period the Earth experienced before our own.
“We want to study how this period ended,” stresses Sigfus Johnsen, a glaciologist at Copenhagen University (DK). “As we are disturbing our climate with CO2 emissions, we need to understand how a warm period ends when there is no interference. That should allow us to better understand how our own period could end.”
Ice is uniquely interesting because it samples the atmosphere directly without passing through the intermediary of biological structures, such as algae or foraminifers. It traps minute bubbles of air that the researchers can then release by melting the ice or crushing it in a vacuum. In this way, 100 grams of ice can produce up to 10 cm3 of air. When analysed, this air can tell us how much oxygen, carbon dioxide, methane or sulphur oxides the atmosphere contained at the time, enabling us to identify how their levels varied in line with temperature changes.
It is partly thanks to Epica researchers that we now know that there is a clear correlation between the Earth’s temperature and the concentration of greenhouse gases (especially CO2), and that, for most of these gases, the present levels are higher than at any time during the past half a million years.
Ice also tells us that the period that was probably closest to our own in climatic terms (Earth’s axis and orbit, CO2 levels, etc.) was about 420 000 years ago. This warm period, separated by two ice ages, is known as MIS 11 (Marine Isotope Stage 11) and lasted 28 000 years. Such a duration suggests that we could at present be in a ‘super interglacial period’ and, therefore, have no grounds for hoping that natural cooling will offset the climatic warming our activities are currently triggering.
During the past 740 000 years the Earth has experienced eight climatic cycles with alternating ice ages and warmer periods known as interglacial periods. “It is the position of the Earth in relation to the Sun that lies at the basis of these different periods. But these variations only give ...
During the past 740 000 years the Earth has experienced eight climatic cycles with alternating ice ages and warmer periods known as interglacial periods. “It is the position of the Earth in relation to the Sun that lies at the basis of these different periods. But these variations only give an initial impetus. The system is not linear and, as it is complex, it produces responses whose logic we are now trying to understand,” explains Dominique Raynaud, a member of the Epica project. “The changes are sometimes sudden and brutal, as there are many threshold effects.”
In its next report, the Intergovernmental Panel on Climate Change will be publishing, for the first time, a complete section devoted to the study of past climates. A conference entitled ‘Paleoclimate: reducing uncertainties’ was organised in July 2004 by the Utrecht Centre of Geosciences (NL) and was supported by the EU. It started the process of identifying certain avenues for research, such as a more in-depth analysis of interglacial periods. Finally, the Sixth Framework programme includes a call for proposals for Integrated Projects that Dominique Raynaud believes "will enable European paleoclimatologists to join forces to propose something really exciting.”