Editing IPCC:AR6/WGII/Chapter-16 (section)

===== Reasons for Concern across scales =====

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The five major Reasons for Concern (RFCs), describing risks associated with (1) unique and threatened systems, (2) extreme weather events, (3) distribution of impacts, (4) global aggregate impacts, and (5) large-scale singular events, were updated using expert elicitation. RFC risk levels were assessed with no or low adaptation, but limits to adaptation are a factor in the identification of very high risk levels.

'''Compared with AR5 and SR15, risks increase to high and very high levels at lower global warming levels for all five RFCs (''' '''''high confidence''''' '''), and transition ranges are assigned with greater confidence. Transitions from high to very high risk emerge in all five RFCs, compared with just two RFCs in AR5 (''' '''''high confidence''''' '''). {16.6.3, Figure 16.15}'''

* For unique and threatened systems (RFC1), as before, levels of risk at a given level of warming are higher than for the other RFCs. Risks are already (at current warming of 1.1°C) in the transition from moderate to high ( ''very high confidence'' ), compared with moderate in AR5 and SR15, based on observed and modelled impacts. The transition to very high risk occurs between 1.2°C and 2.0°C warming ( ''high confidence'' ) ''.'' {16.6.3.1}
* For risks from extremes (RFC2), the transition to high risk is between 1.0°C and 1.5°C ( ''high confidence'' ) and to very high risk (new in AR6) between 1.8°C and 2.5°C ( ''medium confidence'' ). {16.6.3.2}
* For risks disproportionately affecting particularly vulnerable societies and socio-ecological systems, including disadvantaged people and communities in countries at all levels of development (RFC3), current risk is moderate ( ''high confidence'' ) and the transition to high risk is between 1.5°C and 2.0°C warming ( ''medium confidence'' ). The transition to very high risk occurs at between 2.0°C and 3.5°C warming ( ''medium confidence'' ). {16.6.3.3}
* The risk of global aggregate impacts, including monetary damages, lives affected, species lost or ecosystem degradation at a global scale (RFC4), has begun to transition to moderate risk ( ''medium confidence'' ), with a transition to high risk between 1.5°C and 2.5°C ( ''medium confidence'' ) and to very high risk (new in AR6) at between 2.5°C and 4.5°C ( ''medium confidence'' ). {16.6.3.4}
* Present-day risks associated with large-scale singular events (sometimes called tipping points or critical thresholds) (RFC5) are already moderate ( ''high confidence'' ), with a transition to high risk between 1.5°C and 2.5°C ( ''medium confidence'' ) and to very high risk (new in AR6) between 2.5°C and 4°C ( ''low confidence'' ). {16.6.3.5}

'''Limiting global warming to 1.5°C would ensure risk levels remain moderate for RFC3, RFC4 and RFC5 (''' '''''medium confidence''''' '''), but risk for RFC2 would have transitioned to a high risk at 1.5°C and RFC1 would be well into the transition to very high risk (''' '''''high confidence''''' '''). Remaining below 2°C warming (but above 1.5°C) would imply that risk for RFC3 through 5 would be transitioning to high, and risk for RFC1 and RFC2 would be transitioning to very high (''' '''''high confidence''''' ''').''' By 2.5°C warming, RFC1 will be in very high risk ( ''high confidence'' ) and all other RFCs will have begun their transitions to very high risk ( ''medium confidence'' for RFC2, RFC3 and RFC4, ''low confidence'' for RFC5).

RFC1, RFC2 and RFC5 include risks that are irreversible, such as species extinction, coral reef degradation, loss of cultural heritage, or loss of a small island due to sea level rise. Once such risks materialise, as is expected at very high risk levels, the impacts would persist even if global temperatures would subsequently decline to levels associated with lower levels of risk in an ‘overshooting’ scenario ( ''high confidence'' ). {16.6.3}

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