3.4.1.2 Status quo

Tipping points and the Intergovernmental Panel on Climate Change

The IPCC first addressed climate tipping points in its 3rd Assessment Report (McCarthy et al., 2001), using the terminology of ‘large-scale discontinuities’ in the report of Working Group II (Impacts, Adaptation and Vulnerability). Tipping points were included in a set of ‘reasons for concern’, visualised in the ‘burning embers’ diagram that later motivated the selection of the 2°C temperature goal (Leemans and Vellinga, 2017). At this time, the IPCC concluded that tipping points would only become likely if global average temperatures exceeded 4°C. 

The burning embers were not included in AR4 (IPC, 2007), but scientists independently published an updated figure in 2009 (Smith et al., 2009). It returned in the AR5 WGII Summary for Policymakers (Field et al., 2014), which referred to ‘large-scale singular events’ as a reason for concern, or RFC. AR5 Working Group I defined tipping points as Earth system components that are ‘susceptible to abrupt or irreversible change’, focusing on irreversibility and the likelihood of the occurrence of tipping points in the 21st Century (Collins et al., 2013). More than a decade after AR3, the IPCC updated its risk assessment for the transgression of tipping points, stating that they “become moderate between 0-1°C additional warming [above 1984-2005 average], […]. Risks increase disproportionately as temperature increases between 1-2°C additional warming and become high above 3°C, due to the potential for a large and irreversible sea level rise from ice sheet loss” (Field et al., 2014; Assessment Box SPM.1). AR5 also contained a table listing nine Earth system components that are “susceptible to abrupt or irreversible change” (IPCC AR5 WG, 2014 I, Table 12.4, p.1115). These included AMOC, ice sheets, and tropical and boreal forest dieback. Lenton et al., (2019) showed how the IPCC’s risk assessment of tipping points had changed over time (Figure 3.4.1).

IPCC, 2023) updated the burning embers diagram (IPCC 2023), maintaining the language of ‘large-scale singular events’ (IPCC 2023). The IPCC’s WG I defined a tipping point as “a critical threshold beyond which a system reorganises, often abruptly and/or irreversibly”. It also used the related term abrupt climate change, defined as “a large-scale abrupt change in the climate system that takes place over a few decades or less, persists (or is anticipated to persist) for at least a few decades and causes substantial impacts in human and/or natural systems”. (IPCC 2023). AR6 WGI sometimes uses ‘tipping point’ to refer to a class of abrupt change in which the subsequent rate of change is independent of the forcing [IPCC 2023). AR6 assessed the risk of tipping point transgression as moderate today (at more than 1ºC warming above pre-industrial levels), becoming high around warming of 2ºC, and very high beyond 2.5^ºC.

The IPCC’s WG I provided an updated table (4.10) of “components in the Earth system that have been proposed as susceptible to tipping points/abrupt change, irreversibility, projected 21st Century change”. This table includes 15 items, including some that do not fall under the definition of a tipping point, e.g. global sea level rise (an outcome of ice sheet melting with no clear threshold for system reorganisation in itself).