Deglaciation: a matter of CO2

After having analyzed ice from 5 deep drillings in Antarctica, a European research group, that involves Cnr-Institute for the Dynamics of Environmental Processes (Idpa), has discovered that the increase in Antarctic temperature during the last deglaciation (20,000 to 10,000 years ago) occurred at the same time than the increase in atmospheric CO2. This finding, published in 'Science', challenges earlier results showing a lag of CO2 with respect to Antarctic temperature.

Ice sheets form the best archive of past variations of atmosphere and polar climate. They cover 800,000 years of history in Antarctica and allow to better know glaciation-deglaciation cycles which characterize this period. While the main driver of glacial-interglacial variations lies in changes in the Earth's orbit around the Sun, the response of the climate system involves interplay between changes in ice sheets, lands, oceans and the atmosphere, modulating natural variations in greenhouse gas concentration in the atmosphere.

On the whole, Antarctic temperatures were warm during periods of high CO2 concentrations, and cold during periods of low CO2 concentrations.But this correlation does not allow to disentangle the causal link between CO2 and temperature: was it the greenhouse effect due to CO2 which induced a warming or was it the warming which induced the increase in CO2? A supplementary clue comes from the sequence of events between CO2 and temperature: during a common variation, is it CO2 or temperature which varied first? This problem is not trivial because, while Antarctic temperatures are recorded at the surface of ice sheets, atmospheric gases such as CO2 are trapped at about 100 m depth, at the base of the firn where the bubbles close off, this depth being dependent of past climatic conditions. First results, based on a model of this lock-in depth during the past, concluded that CO2 had started to rise 800 years after Antarctic temperature at the end of the last ice age.

European scientists have revised this CO2/Antarctic temperature lead/lag estimate by inferring the air lock-in depth from isotope 15 of nitrogen in air bubbles, which is enriched proportionally to the firn height. They also applied an innovative statistical method to determine the Antarctic temperature and its lead/lag to CO2. They determined that CO2 and Antarctic temperature increased at the same time at the end of the last ice age, within 200 years.
This finding now makes it likely that CO2 was responsible, at least partly, of the Antarctic warming at the end of the last ice age. New data and climate models experiments are necessary to disentangle the different contributors of this past natural global warming.

Fonte: Carlo Barbante, Istituto per la dinamica dei processi ambientali, Venezia, tel. 041/2348942, email carlo.barbante@idpa.cnr.it