| CLIMATE - The key to the future lies in the past
As concerns over future climate change crystallize, scientists are devoting more of their energies to understanding the climates of the past.
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 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.