PA G ES/CLIVA

Radiative forcing and the ice core greenhouse gas record Many books and articles deal with the numerous feedbacks and links in the climate-biogeochemical system and their consequences for our society. Here, the focus is on one particular link between the physical climate system and biogeochemical cycles, that is, the radiative forcing imposed on the physical climate system through altered greenhouse gas concentrations and other radiative perturbations. Polar ice cores analysed by different groups around the world leave no doubt that cold polar ice faithfully records past atmospheric concentrations of major greenhouse gases (Fig. 1 over page) (Stauffer et al., 2002). The well-known finding is that the rate of increase in atmospheric carbon dioxide and methane is without precedence at least over the past few tens of millennia, and that CO2 and CH4 concentrations today are larger than at any time over the past 650,000 years, the period spanned by the ice core records. Atmospheric concentration of N2O, another greenhouse gas, is also higher than ever measured, though its ice core record is less complete. Humans, by burning fossil fuels and through land use changes and other activities, are altering the state of the atmosphere. Increasing concentrations of greenhouse gases cause global warming. Greenhouse gases such as CO2 absorb part of the long-wave radiation emitted from the earth surface, thereby altering the temperature on the ground and the temperature distribution in the atmosphere. The greenhouse gas theory was already well established in the 19th century and by the end of the century, Arrhenius had calculated that a doubling of the atmospheric CO2 concentration would cause a global-mean surface warming of about 4oC. In comparison, this climate sensitivity, termed ∆T2x, evaluated with the current set of comprehensive and spatially-resolved climate models falls generally within the range of 1.5 to 4.5oC