A. Berger; M.F. Loutre
Universite Catholique de Louvain. Institute d'Astronomie et de Geophysique G. Lemaitre B- 1348 Louvain-La Neuve, Belgium
"CO2 and astronomical forcing of the late Quaternary"
The LLN 2-D climate model has been used to reconstruct the long-term climatic variations over the Quaternary Ice Age. Sensitivity analyses to the astronomically-driven insolation changes and to the CO2 atmospheric concentration have been performed. In particular, an atmospheric CO2 concentration decreasing linearly from 320 ppmv at 3 Myr BP (Late Pliocene) to 200 ppmv at the Last Glacial Maximum was used to force the model in addition to the insolation. Under such condition, the model simulates the intensification of glaciation around 2.75 Myr BP, the late Pliocene - early Pleistocene 41 kyr cycle, the emergence of the 100-kyr cycle around 900 kyr BP, and the glacial-interglacial cycles of the last 600 kyr. The hyphotesis was put forward that during Late Pliocene (in an ice-free - warm world) ice sheets can only develop during times of sufficiently low summer insolation. This occurs during large eccentricity times when climatic precession and obliquity combine to obtain such low values, leading to the 41 - kyr period between 3 and 1 Myr BP. On the contrary in a glacial world, ice sheets persist most of the time except when insolation is very high in polar latitudes, requiring large eccentricity again, but leading this time to interglacial and finally to the 100-kyr period of the last 1 Myr. Using a reconstructed CO2 concentration over the last 600 kyr from a regression based upon SPECMAP, it has been shown that stage 11 and stage 1 request a high CO2 to reach the interglacial level. The insolation profile at both stages and modeling results tend to show that stage 11 might be a better analogue for our future climate than the Eem. Although the insolation changes alone act as a pacemaker for the glacial - interglacial cycles, CO2 changes help to better reproduce past climate changes and, in particular, the air temperature and the southern extent od the ice sheets. Using the calculated insolation and a few scenarios for CO2, the climate of the next 130 kyr has been simulated. It shows that an interglacial will most probably last particularly long (50 kyr). This conclusion is reinforced if we take into account the possible intensification of the greenhouse effect which might result from man's activities over the next centuries.