Table 1.
Policy-relevant potential future tipping elements in the climate system and (below the empty line) candidates that we considered but failed to make the short list*
| Tipping element | Feature of system, F (direction of change) | Control parameter(s), ρ | Critical value(s),† ρcrit | Global warming†‡ | Transition timescale,†T | Key impacts |
|---|---|---|---|---|---|---|
| Arctic summer sea-ice | Areal extent (−) | Local ΔTair, ocean heat transport | Unidentified§ | +0.5–2°C | ≈10 yr (rapid) | Amplified warming, ecosystem change |
| Greenland ice sheet (GIS) | Ice volume (−) | Local ΔTair | +≈3°C | +1–2°C | >300 yr (slow) | Sea level +2–7 m |
| West Antarctic ice sheet (WAIS) | Ice volume (−) | Local ΔTair, or less ΔTocean | +≈5–8°C | +3–5°C | >300 yr (slow) | Sea level +5 m |
| Atlantic thermohaline circulation (THC) | Overturning (−) | Freshwater input to N Atlantic | +0.1–0.5 Sv | +3–5°C | ≈100 yr (gradual) | Regional cooling, sea level, ITCZ shift |
| El Niño–Southern Oscillation (ENSO) | Amplitude (+) | Thermocline depth, sharpness in EEP | Unidentified§ | +3–6°C | ≈100 yr (gradual) | Drought in SE Asia and elsewhere |
| Indian summer monsoon (ISM) | Rainfall (−) | Planetary albedo over India | 0.5 | N/A | ≈1 yr (rapid) | Drought, decreased carrying capacity |
| Sahara/Sahel and West African monsoon (WAM) | Vegetation fraction (+) | Precipitation | 100 mm/yr | +3–5°C | ≈10 yr (rapid) | Increased carrying capacity |
| Amazon rainforest | Tree fraction (−) | Precipitation, dry season length | 1,100 mm/yr | +3–4°C | ≈50 yr (gradual) | Biodiversity loss, decreased rainfall |
| Boreal forest | Tree fraction (−) | Local ΔTair | +≈7°C | +3–5°C | ≈50 yr (gradual) | Biome switch |
| Antarctic Bottom Water (AABW)* | Formation (−) | Precipitation–Evaporation | +100 mm/yr | Unclear¶ | ≈100 yr (gradual) | Ocean circulation, carbon storage |
| Tundra* | Tree fraction (+) | Growing degree days above zero | Missing‖ | — | ≈100 yr (gradual) | Amplified warming, biome switch |
| Permafrost* | Volume (−) | ΔTpermafrost | Missing‖ | — | <100 yr (gradual) | CH4 and CO2 release |
| Marine methane hydrates* | Hydrate volume (−) | ΔTsediment | Unidentified§ | Unclear¶ | 103 to 105 yr (>TE) | Amplified global warming |
| Ocean anoxia* | Ocean anoxia (+) | Phosphorus input to ocean | +≈20% | Unclear¶ | ≈104 yr (>TE) | Marine mass extinction |
| Arctic ozone* | Column depth (−) | Polar stratospheric cloud formation | 195 K | Unclear¶ | <1 yr (rapid) | Increased UV at surface |
N, North; ITCZ, Inter-tropical Convergence Zone; EEP, East Equatorial Pacific; SE, Southeast.
See SI Appendix 2 for more details about the tipping elements that failed to make the short list.
Numbers given are preliminary and derive from assessments by the experts at the workshop, aggregation of their opinions at the workshop, and review of the literature.
Global mean temperature change above present (1980–1999) that corresponds to critical value of control, where this can be meaningfully related to global temperature.
Meaning theory, model results, or paleo-data suggest the existence of a critical threshold but a numerical value is lacking in the literature.
Meaning either a corresponding global warming range is not established or global warming is not the only or the dominant forcing.
Meaning no subcontinental scale critical threshold could be identified, even though a local geographical threshold may exist.