While some aspects of climate change can be viewed as becoming proportionately larger with increasing forcings, other aspects may feature more complex, nonlinear behaviour. This can include abrupt and/or irreversible change, which may be associated with key thresholds. Such behaviour must be considered differently in assessments of the potential benefits of mitigation: it implies, for example, that certain impacts could be significantly different above certain levels of anthropogenic interference.
This report summarises the key new literature on the main large-scale systems that may feature abrupt and/or irreversible changes (specifically, ice sheets, Arctic sea ice, the Atlantic Meridional Overturning Circulation (AMOC), tropical forests, carbon stored in terrestrial permafrost or below the ocean in permafrost or as methane hydrates, or the response to ocean acidification). This represents an update to the review of Good et al. (2011), and it has been approximately 2 years (18 months) since the submission deadlines for literature for the IPCC AR5 working group I (working group II); the corresponding deadlines for acceptance of literature were 18 months (1 year) ago.