CO2 and astronomical forcing of the late quaternary

The LLN 2-D Northern Hemisphere 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. Experiments have shown that both isotopic stages 11 and 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 stage 5e. Although the insolation changes alone act as a pacemaker for the glacial-interglacial cycles, COP changes help to better reproduce past climatic changes and, in particular, the air temperature and the southern extent of the ice sheets. However CO2 alone is unable to generate any glacial-interglacial cycle. Using the calculated insolation and a few scenarios for CO2, the climate of the next 130 kyr has also been simulated. It shows that our 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.