The transitions here are informed by these assessments, along with the assessment in Chapter 2 on species high extinction risk and on ecosystem transitions. It also draws substantially from information in Cross-Chapter Paper 1 and Table SM16.22 on risks to unique and threatened biological systems. Some unique and threatened systems, such as coral reefs and sea-ice-dependent ecosystems, were already showing attributable impacts with high confidence  based on data collected in the mid to latter 20th century, when global warming of 0.5℃ above pre-industrial levels had taken place, as noted already in AR3. In this AR6 assessment, the temperature range for the transition from undetectable to moderate risk is still located at a median value of 0.5℃ above pre-industrial levels, with very high confidence. Since impacts were first detected in coral reef systems in the 1980s when warming of ~0.4℃ of global warming had occurred , this provides the temperature at which the transition begins. The September Arctic sea ice volume has declined by 55-65% between 1979 and 2010  as global warming increased from around 0.36°C in 1979 to around 0.9ºC in 2010. These provide evidence of a start to the transition from undetectable to moderate risk at 0.4℃ above pre-industrial levels. Recent evidence of observed impacts on mountaintop ecosystems and sea-ice-dependent species, and of range shifts in multiple ecosystems during 1990-2000, which AR6 WGI now assesses as corresponding to a global warming of 0.69ºC , provides evidence for an upper limit to this transition of 0.7℃ with very high confidence. Overall, the transition is located at a median of 0.5°C with lower and upper limits of 0.4℃ and 0.7℃, respectively, with very high confidence.
IPCC
Climate Change 2022: Impacts, Adaptation and Vulnerability
IPCC Report