Volume 18, numéro 3, september 1991

EN :

The only long-term record of climatic change is the geologic record, which suggests that the surface of the planet has had a remarkably stable thermal history. This stability is remarkable because of an inferred 30% increase in solar luminosity since Early Archean time. The glacial record provides some of the best evidence of thermal perturbation.The major cause of glaciation may be the periodic reduction of atmospheric C02, which is linked, via plate tectonics, to the weathering cycle. Different glacial epochsmay, however, have had different controls.Early Proterozoic glaciation may have occurred because of the combination of enhanced weathering of newly emergent cratons, and the faint early sun. Associated highly weathered rocks may reflect the high C02 content of the atmosphere. Late Proterozoic glaciation is explained as being due tolowering of atmospheric C02 levels by extreme weathering of a supercontinent at low latitudes. Most Phanerozoic glaciation was caused by the combined effects of weathering of an elevated supercontinent (Pangea) and polar positioning. The Cenozoic glaciation may be related to high latitudes of some continental masses and reduced CO2 levels due to enhanced weathering of the continents which become emergent as the Atlantic Ocean floor ages, cools and sinks. What of the future? In the short term, the "Little Ice Age" climatic cycle suggests warming for about the next 1,000 years. Global cooling should follow as the Earth descends into the next severe glaciation predicted by Milankovitch theory. Anthropogenic contribution to the greenhouse effect should enhance the short-term warming trend. There peated cycle of Cenozoic glaciations will end with the initiation of subduction along the Atlantic margins.

FR :

Le climat terrestre est régi par des interac-tions complexes entre l'Océan, l'Atmosphèreet la Cryosphère (neige, glace, glaciers). Lephénomène ENSO (El Nino-Southern Oscil-lation), qui ne représente qu'un type connud'interaction Océan-Atmosphère, est briève-ment décrit. Le rôle des océans en tant quetampon thermique capable de ralentir le ré-chauffement (ou refroidissement) de l'atmo-sphère est également discuté. Finalement,quelques-uns des points faibles des modè-les climatiques présents et futurs sontmentionnés.

EN :

The Earth's climate is determined by complex interactions between the Ocean, the Atmosphere and the Cryosphere (snow, seaice, glaciers). The ENSO phenomenon (El Nino-Southern Oscillation), which represents but one known type of Ocean-Atmosphere interaction, is briefly described. The role of the oceans as a heat buffer capable of slowing down the warming (or cooling) of the atmosphere is also discussed. Finally, some of the weaknesses of current and future climate models are also mentioned.

EN :

The possibility of global warming resulting from the anthropogenic addition of carbon dioxide and other industrial gases into the atmosphere is a topic of much recent concern. Global climate models are used to make predictions for possible future climatic changes, but their ability to simulate climatic states other than the present day is not well constrained. The sedimentary record contains a wealth of information on past global climatic change. From this, we can get a good idea of what past climatic extremes were, and some idea of their underlying causes. If we attempt to simulate the climate of a past time period, we can evaluate the ability of a climate model to represent a climatic state substantially different from the present day.

At present, models are not well utilized to reproduce a global climatology for a particular time period. Rather, the models are used for sensitivity experiments. One geologic factor is varied {e.g., geography or C02 level) while all others are held constant. In this way, the effect on the climate of changing that one factor can be tested, and insights into the mechanisms of global change are gained.The results are then compared with the geologic record. Two case studies are given asexamples.