From the astronomical theory of paleo-climates to global warming

Résumé En

The future of our climate is a primary concern not only at the human time scale but also at the geological time scale. At both time scales, modeling the climate system, estimating the climatic sensitivity from past climates or researching best analogs in the past are the most usual tools to predict what might be the future climate. Most of the time, paleoclimatologists feel that the last interglacial is a good candidate for our future warm climate. They study its stability and length that they compare to our pre¬ sent interglacial. Limitations to such procedures have been stressed. In particular, a special attention must be care to the forcings of past climates (both insolation and C02, in particu¬ lar) because they are important factors which shape the complex latitudinal distribution and seasonal cycle of the world climates. At least from the astronomical point of view the Eemian is not a good choice for the future of the Holocene. Better candidates would be around 400,000 YBP (isotopic stage 11) and even better around 2,000,000 YBP where interglacials lasted longer than during the middle Pleistocene. But first of all, it must be stressed that the orbital forcing for the present and next tens of thousands of years is almost unique, that the predicted C02 con- centration for the next centuries (and already the present ones) are unprecedented and finally that, according to our modeling results, the present Holocene interglacial will, most probably, last exceptionally long, with no counterpart over the last million years. Such a long duration is a very robust feature in our numerical experiments. It seems to be related to the pretty high value of CO 2 which lasts already since 10 kyr BP compensating for the declining astronomical forcing. In addition, our results show not only that the Holocene might last particularly long, but also that the sensitivity of our climate system to the green¬ house gas forcing might be exacerbated. As a consequence, there is a threshold in the greenhouse gas concentration of about 700 ppmv beyond which the Greenland ice sheet melts in about 5000 years and does not recover before a few tens of thousands of years. Such results reinforce the need of limiting the greenhouse gas emissions from human activities as soon as possible. In Bel¬ gium, in particular, one way of doing it efficiently is without any doubt by increasing the share of our electricity production from the nuclear sector. Scenarios for the next decades show clearly that this is unavoidable if Belgium is willing to meet its Kyoto target.