Abrupt Change/ Tipping PointKey Region(s)Probability to Occur in the 21st CenturyMaximum CO 2 or CH 4 Release in the 21st CenturyPrincipal Development Time ScaleMaximum CO 2 or CH 4 Rate of Change Over the 21st Century(Ir)reversibility Tropical forests dieback (Section 5.4.9.1.1)Amazon watershed Low<200 PgC as CO 2 (Section 5.4.9.1.1; medium confidence)Multi-decadal CO 2: <0.5 ppm yr–1Irreversible at multi-decadal scale (medium confidence) Boreal forests dieback (Sections 5.4.9.1.1, 5.4.3.2)Boreal Eurasia and North AmericaLow<27 Pg (Section 5.4.9.1.2; medium confidence)Multi-decadal Small (low confidence)Irreversible at multi-decadal scale (medium confidence) Biogenic emissions from permafrost thaw (Section 5.4.9.1.2)Pan-Arctic Highup to 240 PgC of CO 2 and up to 5300 Tg of CH 4 (Section 5.4.8.1.2; low confidence)Multi-decadalCO 2: ≤1 ppm yr–1 CH 4: ≤10 ppb yr–1Irreversible at centennial time scales