CLIMATIC VARIATIONS IN LAST 400.000 YEARS DERIVED FROM POLAR ICE CORES

The study of climate in past times has recently become an important topic in view of the possible social and economic consequences that can be brought on by  climate variations induced by the human activity.

Apart from the   record of  climate parameters obtained by direct measures by man in the few last centuries, one has to resort to analysis of the natural geological and biological record to get information on climate in ancient times. Here polar ices play a very important role. Snow accumulating every year  becomes ice, at temperatures always below melting point. Hence special features of the snow providing information on a given year atmosphere are preserved (including atmospheric dust, hidrates, small volumes of air..). By drilling ice cores, sequences of ice layers can be obtained documenting the history of the atmosphere in the last few hundred thousands of years. Temporal  resolution depends on yearly snow deposit and may sometimes allow seasonal differentiation.  By chemical, isotopic, geochemical and physical measurements one can get information on ancient air temperatures, rain levels, marine and continental aerosols, volcanic activities and so on. One can in particular get information on air composition and hot house effects.

From Antarctic ice cores one can now document the climate variations for the last 400.000 years. Four major climatic cycles can be recognized in this time interval. For about 10\% of this time the climate was similar to the present one (interglacial climate), for the remaining, hence for the most, period, it was colder (glacial climate). Interglacial climates typically last a few thousand years, while glacial climates last tens of thousands, with a series of internal variations, characterized by colder and colder temperatures. Transition to interglacial climates are usually fast.  This cycle appears to be correlated with variations in the Earth orbital parameters.

Ice cores from Greenland allow a finer analysis but are applicable only to the last 100.000 years. The northern climate shows variations with respect to the southern climate. It shows variations of high amplitude and short duration (the Dansgaard-Oescher events), which have been related to variations in the oceanic circulation.  Even with respect to the last transition from glacial to interglacial the northern hemisphere has shown stronger climate variations, some happening in a matter of some dozen years.

The present interglacial, the Olocene, started about 11.500 years ago and has remained essentially stable in the polar regions, while showing important variations in regions that are presently dry or semidry. The relations between the northern and the southern climates are presently being investigated, also in relation with the present increase due to human activity of the gases leading to hot house effects. The effects of human activity show up quite strongly in the polar ice cores. Increase of $CO_2$ and methane have led to values which are respectively 30\% and 140\% higher than the previous maxima in the last 400.000 years; this dramatic increase has been obtained essentially just in the last 200 years!