Abrupt Shifts in the Earth System at Moderate Warming

The gradual rise in greenhouse gas concentrations is projected to drive a mostly smooth increase in global temperature (Stocker et al., 2013). However, our planet Earth is suspected to have a range of ?tipping elements? with the characteristic that their gradual change will be punctuated by critical transitions on regional scales (Lenton et al., 2008; Levermann et al., 2012). That is, for relatively small changes in atmospheric concentrations of greenhouse gases, parts of the Earth System exhibit major changes. Well-known examples of such ?tipping elements? are: the Greenland and Antarctic Ice Sheets, the Amazon rainforest, Arctic sea-ice, the Warm Gulf Stream in the North Atlantic, in particular its northward branch along the British Isles and the Norwegian continental shelf. One of the most concerning issues of human-induced increases in atmospheric greenhouse gas concentrations is the potential for rapid regional transitions in the climate system, associated with crossing a ?tipping point? where small changes have large impacts. Such nonlinear responses to relatively gradual atmospheric composition changes may have major implications for society and how it adapts?