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.