Astronomical theory of climate change

Abstract
The astronomical theory of paleoclimates aims to explain the climatic variations occurring with quasi-periodicities lying between tens and hundreds of thousands of years. Such variations are recorded in deep-sea sediments, in ice sheets and in continental archives. The origin of these quasi-cycles lies in the astronomically driven changes in the latitudinal and seasonal distributions of the energy that the Earth receives from the Sun. These changes are then amplified by the feedback mechanisms which characterize the natural behaviour of the climate system like those involving the albedo-, the water vapor-, and the vegetation- temperature relationships. Climate models of different complexities are used to explain the chain of processes which finally link the long-term variations of three astronomical parameters to the long-term climatic variations at time scale of tens to hundreds of thousands of years. In particular, sensitivity analysis to the astronomically driven insolation changes and to the CO₂ atmospheric concentrations have been performed with the 2-dimension climate model of Louvain-la-Neuve. It could be shown that this model simulates more or less correctly the entrance into glaciation around 2.75 Myr BP, the late Pliocene-early Pleistocene 41-kyr cycle, the emergence of the 100-kyr cycle around 850 kyr BP and the glacial-interglacial cycles of the last 600 kyr. During the 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 CO₂ scenarios, it has been shown that stage 11 and stage 1 request a high CO₂ to reach the interglacial level. Moreover, the insolation pattern at both stages and modeling results lead to conclude that stage 11 is 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 climatic changes and, in particular, the air temperature and the southern extend of the Northern Hemisphere ice sheets. Insolation and CO₂ scenarios for the next 130 kyr lead to an interglacial which will most probably last particularly long (50 kyr). This conclusion is reinforced by the possible intensification of the greenhouse effect which might result from man's activities over the next centuries.