Theoretical and observational evidence for climate tipping points

Some components of the Earth system could change their state abruptly in response to a warming atmosphere and associated changes in climate conditions. This possibility has been recognized as one of the greatest potential threats associated with anthropogenic climate change. Examples  include the Atlantic Meridional Overturning Circulation, the polar ice sheets, the Amazon rainforest, and possibly the tropical monsoon systems.  The empirical evidence for abrupt climate transitions comes from paleoclimate proxy records, but also in observational records, signs of stability loss for some of the major tipping elements have been suggested. Here we explain some of the key theoretical concepts suggesting that tipping events may happen under ongoing climate change and summarize the empirical evidence for stability loss in some Earth system components with focus on candidates for future abrupt transitions. We argue that the critical forcing levels and rates are subject to large uncertainties and hence difficult to prdict. Improvements will require combining information from paleoclimate records, simulations with a hierarchy of models, and from observation-based data.