Climate and global change

The climate of our planet has remained within a range compatible with life since several billion years. Theories of star evolution suggest that the amount of heat emitted by the sun increased by about 10% since 4 109 years, so that several factors of the climatic system, such as the chemical composition of the atmosphere, must have changed in order to maintain the air temperature within a range permitting the occurrence of abundant liquid water at the earth's surface. On the 107 -108 year time scale, climatic variations are important and strongly linked to the plate tectonics and to the atmospheric CO2 concentration changes. In particular warm conditions prevailing during the Cretaceous are best explained by the absence of continental masses in polar position and by higher amount of carbon dioxide in the atmosphere. On the 104 -105 year time scale, seasonal insolation changes which are linked to the variations of the earth's orbit around the sun, exhibit periodicities which are dominant within the spectrum of climatic variations. This so-called Milankovitch mechanism seems to be responsible for most of the Quaternary climatic variations, including the major glaciations. Although climatic variations on the 103 year time scale may be explained as the non-linear response of the climatic system to the Milankovitch forcing, most of the variability on the 10 -103 year range is still poorly understood. The variability of the cryosphere and of the ocean circulation must be taken into account and both are still poorly documented. The last glacial-interglacial cycle (the last 150,000 years) provides the best reference frame to which mechanisms of climatic change and their impact on the atmosphere, ocean, and biosphere may be analyzed. Data can be obtained from the analysis of deep sea and continental sediments as well as from ice cores drilled through the major ice caps. Temperature estimates can be derived from micropaleontological data (pollen, foraminifera, diatoms, coccoliths) and isotopic measurements in ice and fossil marine foraminifera. Precipitation can be estimated from pollen and diatom fossils and sedimentological analysis. The chemical composition of the atmosphere can be reconstructed from the analysis of air bubbles included in ice cores and from geochemical indices, results show that during the peak of the last glaciation, the mean temperature of the Earth was about 4°C lower than today, but that the cooling was much stronger at high latitudes than in the tropics. The water cycle experienced dramatic changes, with a strong reduction of the amount of water available at the surface of the continents. Changes in ocean circulation and chemistry have been important in triggering both important changes in the atmospheric CO2 content and abrupt climatic variations during the glacial to modem interglacial transition.