Quaternary Climates

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Ice ages were discovered in the mid-19th century. These represented the first clearly documented evidence of large-scale climatic changes, and they have since been at the root of theories of climate and climatic change. From the beginning, two main theories were suggested to explain the geological observations. On the one hand, the astronomical theory claims that changes in the Earth's orbital parameters induce small but persistent imbalances in the ice-sheet mass budget, and consequently drive long-term modification of the Earth glaciated areas. On the other hand, the greenhouse effect largely controls Earth's climate; therefore small but persistent imbalances in the carbon cycle are likely to drive large atmospheric CO2 changes that will affect global climate. Since the mid-19th century, an incredible amount of information on ice ages has been gathered, including geomorphological observations, micropaleontology or geochemistry of sediments from marine cores, ice core records, terrestrial pollen and lacustrine data. In parallel, tremendous progress has been achieved in our understanding of the physics and chemistry of the climate system. Still, the archetypical question of the dynamics of Quaternary climates remains largely open. We know that ice ages are paced by orbital forcing, but Milankovitch's theory predicts 41,000-yearcycles while the geological record shows major transitions occurring every 100,000-years during the last million years. We also know that atmospheric CO2 was significantly lower during ice ages, but there is currently no widely accepted explanation for that fact. Obviously, there is a need for a synthesis between the astronomical and the geochemical theories, in order to account both for the observations and for our current understanding of climate dynamics.