In this section we assess available scientific literature relating to tipping points in the Biosphere, as summarised in Figure 1.3.1 and Table 1.3.1. We focus on the following biomes: forests, savannas, drylands, lakes, coastal ecosystems and marine environments.
Table 1.3.1: Summary of evidence for tipping dynamics, key drivers and biophysical impacts in each system considered in this chapter.
Key: +++ Yes (high confidence), ++ Yes (medium confidence), + Yes (low confidence), – – – No (high confidence), – – No (medium confidence), – No (low confidence), ? Uncertain Primary drivers are bolded, DC: Direct Climate driver (via direct impact of emissions on radiative forcing); CA: Climate-Associated driver (including second-order & related effects of climate change); NC: Non-Climate driver, PF: positive (amplifying) feedback, NF: negative (damping) feedback. Drivers can enhance (↗) the tipping process or counter it (↘)
System (and potential tipping point) | Key drivers | Key biophysical impacts (see S2 for societal impacts) | Key feedbacks | Evidence base | Abrupt / large rate change? | Critical threshold(s)? | Irreversible? (decadal / centennial) | Tipping system? |
---|---|---|---|---|---|---|---|---|
Forests | ||||||||
Amazon rainforest (dieback) | DC: atmospheric warming (↗) NC: deforestation / degradation (↗) DC: drying (↗) CA: fire frequency/intensity increase (↗) DC: heatwaves (↗) CA: ENSO intensification (e.g. Amazon, SE Asia) (↗) CA: AMOC / SPG weakening / collapse (e.g. Amazon) (↗) CA: terrestrial greening (↘, declining) | • Biodiversity loss • Regional rainfall reduction (e.g. from Amazon dieback across Amazon Basin & Southern American Cone) • Carbon emissions (amplifying global warming) • Remote impacts on rainfall patterns all over the planet | Moisture recycling, fire, albedo | • Models • Observations (local scale) | ++ | 1000-1250mm annual rainfall -400 to -450mm max. accumulated water deficit 7-8m dry season length ~20-40% deforestation ~3.5oC (2-6oC) global warming | ++ | +++ (local) ++ (partial dieback / regional) + (full dieback / continental) |
Congo rainforest (dieback) | + | ~1350mm mean annual rainfall; climate change increasing rainfall | + | + (local) | ||||
SE Asia rainforest (dieback) | – | ~1550mm mean annual rainfall | – | +? (local) – – (regional) | ||||
Boreal forest (southern dieback) | DC: drying (↗) CA: fire frequency/intensity increase (↗) DC: atmospheric warming (↗) CA: permafrost thaw (↗) CA: insect outbreaks (↗) NC: deforestation / degradation (↗) DC: heatwaves (↗) CA: terrestrial greening (↘) CA: vegetation albedo (↗) CA: sea ice albedo decline (↗) DC: precipitation change (↘,↗) | • Biodiversity loss • Carbon emissions from dieback, carbon drawdown from expansion • Complex regional biogeophysical effects on warming – dieback = higher albedo (cooling) but less evaporative cooling (warming) & vice versa for expansion | Fire, albedo, moisture recycling | • Models • Observations • Experiments | ++ | ~4oC (1.4-5oC) | + [~100 yr] | ++ (partial / regional)+ (continental) |
Boreal forest (northern expansion) | Fire, albedo, moisture recycling | • Models • Observations • Experiments | + | ~4oC (1.5-7.2oC) | + [~100 yr] | + (partial / regional) | ||
Temperate forests (dieback) | DC: atmospheric warming (↗) DC: droughts (↗) DC: heatwaves (↗) CA: insect outbreaks (↗) CA: windthrow (↗) NC: deforestation & fragmentation (↗) CA: fire frequency increase (↗) | • Biodiversity loss • Carbon emissions • Regional warming in summer due to less evaporative cooling, less cloud cover • Less atmos. water supply • Less groundwater recharge | Moisture recycling, soil moisture -atmosphere, interacting disturbances, albedo | • Models • Observations • Experiments | ++ | Widespread thresholds uncertain | – [decades] | ? (partial / regional) |
Savannas, Grasslands & Drylands | ||||||||
Savanna & Grasslands (degradation) | NC: fire suppression (↗) NC: overgrazing (↗) DC: increased precipitation intensity (↗) CA: terrestrial greening (↗) NC: afforestation (↗) CA: ocean circulation shift (e.g. Sahe), ↗) | • Biodiversity loss Groundwater depletion (with encroachment) • Nutrient cycle disruption • Reduced fires (with encroachment) | Fire, grazing | • Models • Observations (remote sensing & fieldwork) | + | Regionally variable mean annual rainfall; thresholds highly localised; Fire percolation threshold ~ 60% flammable cover | ++ | ++ (local to landscape) ? (regional) |
Drylands (land degradation) | DC: drying (↗) DC: atmospheric warming (↗) NC: land use intensification (e.g. livestock, agriculture, urbanisation)(↗) DC: extreme events (heatwaves, floods) (↗) DC: increased rainfall variability (↗) CA: terrestrial greening (↘) CA: insect outbreaks (↗) CA: invasive species (↗) | • Biodiversity loss • Aridification / Desertification • Groundwater depletion (with encroachment) • Regional rainfall changes • Shift in species composition (e.g. shrub encroachment) • Vegetation recruitment | Soil fertility, / moisture / microbes, vegetation structure, veg-rainfall, fire, herbivory | • Models • Observations (current & historical) • Field experiment | ++ | Aridity index (0.54,0.7 and 0.8) (limited reliability of aridity measures; lack of temporal evidences for some thresholds) | + (shorter timescales possible, e.g. via active restoration) | ++ (local to landscape)+ (regional) |
Freshwater | ||||||||
Lakes (eutrophication-driven anoxia) | NC: nutrient pollution (↗) DC: atmospheric warming (↗) DC: precipitation changes (↗) | • Biodiversity loss • Water quality decline • Increased GHG emissions | Anoxia-driven P release, trophic cascades | • Observations • Models • Experiments | +++ | 20-30 mg P/l No clear warming/rainfall thresholds | ++ (decadal) | +++ (localised, widespread) |
Lakes (DOM loading – ’browning’) | CA: terrestrial greening (↗) NC: afforestation (↗) DC: atmospheric warming (↗) | • Biodiversity loss • Increased GHG emissions | Anoxia-driven P release | • Observations • Models | + | >10 mg DOC/l | ++ (decadal) | ++ (localised, widespread in boreal) |
Lakes (appearance / disappearance) | CA: permafrost thaw-related thermokarst formation / drainage (↗) CA: glacier lake formation / drainage (↗) | • Biodiversity loss • Increased GHG emissions | (can be driven by thermokarst) | • Observations | +++ | As for permafrost thaw | +++ (centennial) | – (localised, widespread on tundra) |
Lakes (N to P limiting switch) | NC: nutrient pollution (atmos. deposition) (↗) | • Biodiversity loss | N/A | • Observations | ++ | Related to elemental ratio | ++ (decadal) | – (localised, regions with high N-deposition) |
Lakes (salinisation) | DC: atmos. warming (↗) DC: drought (in arid regions) (↗) CA: water use intensification (↗) | • Biodiversity loss • Reduced GHG emissions | Salt release from sediment | • Observations | + | Species-specific salinity threshold | ++ (decadal) | – (localised, arid regions) |
Lakes (invasive species) | CA: warming-driven range expansion (↗) NC: human-mediated introduction (↗) | • Biodiversity loss | N/A | • Observations • Models | + | Cannot be defined | ++ (decadal – centennial) | – (localised, widespread) |
Coastal | ||||||||
Warm-water coral reefs (die-off) | DC: ocean warming (↗) DC: marine heatwaves (↗) CA: disease spread (↗) CA: ocean acidification (↗) NC: pollution (nutrient / sediment) (↗) NC: disruption (ships, over-harvesting) (↗) CA: invasive species (↗) DC: storm intensity (↗) CA: sea level rise (↗) | • Biodiversity loss (ecosystem collapse, ~25% marine species have life stages dependent on coral reefs) • Loss of commercial & artisanal fisheries, and other sectors • Coastal protection loss | Thermal stress leading to symbiont expulsion, decarbonisation, loss of structure (habitat) | • Observations • Models | +++ | Region and reef dependent:~1.2oC (1.0-1.5oC) GW Temporally variable heat stress (8-12 Degree Heating Weeks) Long-term consequences of >350 ppm atmospheric CO2 Acidification threshold uncertain | ++ (decadal) | +++ (localised) +++ (regionally clustered) |
Mangroves (die-off) | CA: sea level rise (↗) DC: increased climate extremes (tropical cyclones, El Niño-related heat, drought, & flooding, drops in sea level )(↗) NC: habitat loss (to agri/aquaculture) (↗) DC: increased regional drought (↗) NC: shoreline change (erosion, sedimentation) (↗) NC: nutrient pollution (↗) | • Biodiversity loss • Loss of coastal protection • Loss of carbon sink / increased GHG emissions • Loss of water quality • Sediment salinisation • Subsidence • Enhanced sediment sulphide and methane releases • Hypoxia (seagrasses) • Reduced nutrient recycling | Failed recovery between increasingly frequent extreme events; coastal subsidence and erosion preventing re- establishment | • Palaeo (mangroves) • Observations • Models | + (sea level induced shifts more gradual than for drought or recurrent extremes) | Region dependent (see text for priority regions): ~1.5-2oC global warming ≥ 4-7 mm.yr-1 relative sea level rise rate Recurrent cyclonic exposure (e.g return period below a decade) Soil pore hyper salinisation (site dependent) | ++(decadal) | ++ (regional; region dependent) |
Seagrass (die-off) | DC: marine heatwaves (↗) NC: nutrient pollution (↗) DC: ocean warming (↗) CA: disease spread (↗) NC: sedimentation (↗) NC: turbidity(↗) NC: invasive species (↗) NC: fishing practices (benthos damage) (↗) CA: sea level rise (↗) | ++ | Region dependent: ~1.5oC global warming Degrees heating weeks with higher sensitivity in temperate (site dependent. e.g. 6 -12 °C) Turbidity ( <10-30% of PAR) Nutrient load (site dependent e.g. N loads >1-5 mg/l; P loads >0.03-0.1 mg/l) | ++ (decadal) | ++ (regional; region dependent) | |||
Marine ecosystems and environment | ||||||||
Fisheries (collapse) | NC: over-exploitation (↗) DC: ocean warming (↗) | • Keystone species collapse • Trophic cascades | Trophic cascades, Allee effect | • Observations • Palaeo/historical records • Models | ++ | Warming & over-fishing; thresholds highly localised | + to +++ (decades) | +++ (cod, regional)+ (large fish, regional)– (small fish, regional) |
Marine communities (regime shift) | NC: over-exploitation (↗) DC: ocean warming (↗) NC: nutrient pollution (↗) CA: sea ice loss (↗) | • Regime shifts • Trophic cascades. | Trophic cascades, eutrophication | • Observations • Models | ++ | Multiple drivers (warming, nutrients, overfishing); thresholds highly localised | + (decades) | + (local) |
Kelp forests (die off) | NC: urchin overgrazing (linked to overfishing) (↗) NC: habitat loss (↗) NC: pollution (nutrient/sediment) (↗) DC: ocean warming (↗) DC: marine heatwaves (↗) | • Regime shift to more barren state | Sea urchin recruitment & grazing | • Observations • Models | ++ | Multiple drivers, thresholds highly localised | +++ (months – decades) | +++ (local) |
Biological pump (collapse) | DC: ocean warming (↗) | • Regime shift • Changes to carbon sink • Impacts on ocean biogeochemistry | Diatom recruitment | • Models • Theory | +? | Ocean warming & stratification; thresholds unknown | + (decades) | ? (lipid, regional) – – (gravitational, regional to global) |
Marine oxygenation (hypoxia) | NC: nutrient pollution (↗) DC: ocean warming (↗) | • Major changes in ocean productivity, biodiversity and biogeochemical cycles | Decomposition, sediment P release | • Models • Observations | + | Nutrient load, warming; thresholds highly localised | ++ (months / years to centuries) | + (local)? (regional to global) |