Global Warming – Positive Feedbacks & Tipping Points

Positive vs. Negative Feedback Loops
- Positive: downstream change reinforces the initial change ➜ amplification.
- Negative: downstream change counters the initial change ➜ stabilization.
- “Positive” ≠ “good”; it only means “self-reinforcing.”
- Both loop types regulate Earth-system and human physiology (e.g., body temperature, blood glucose).
- Rising concentrations of greenhouse gases (GHGs) are currently strengthening multiple positive feedbacks, pushing Earth’s climate system away from equilibrium.

Marine snow: dead plankton/organisms sink ➜ become food for benthic decomposers (archaea & bacteria).
Warmer surface waters ➜ more phytoplankton biomass ➜ increased organic matter sinking.
Greater benthic decomposition ➜ more \text{CO}_2 released to deep water & potentially out-gassed to atmosphere.
Loop: warmer water → higher primary production → more sinking carbon → more benthic \text{CO}_2 → stronger greenhouse effect → even warmer water.

Albedo = fraction of incoming sunlight reflected by a surface.
- High-albedo (bright) surfaces: fresh snow, sea ice, white sand.
- Low-albedo (dark) surfaces: open ocean, soil, rock, asphalt.
Melting sea-ice exposes dark ocean ➜ absorbs more solar radiation ➜ warms water ➜ melts more ice (ice-albedo feedback).
Same mechanism in terrestrial cryosphere (glaciers, permafrost surfaces).
Everyday analogy: barefoot on light cement vs. black asphalt under identical sunlight.

Peat bogs = water-logged, acidic, low-oxygen soils that accumulate partially decomposed plant matter.
Under cool conditions → act as carbon sinks; microbes/enzyme activity slowed.
Warming climate ➜ increased microbial metabolism ➜ faster decomposition ➜ release of \text{CO}_2.
Transition from sink → source if emissions exceed accumulation.
Feedback: higher temp → faster peat decay → more \text{CO}_2 → intensifies greenhouse effect → higher temp.

Permafrost: soil at or below 0^\circ\text{C} for ≥2 consecutive years; stores vast amounts of frozen organic matter.
Warming ➜ deeper seasonal thaw layers ➜ activation of microbes.
- Respiring microbes → \text{CO}_2.
- Methanogenic archaea → \text{CH}_4 (methane).
Both \text{CO}2 & \text{CH}4 are potent GHGs; \text{CH}4 has ≈28{-}34 times the warming potential of \text{CO}2 over 100 yr.
Classic positive loop:
\text{higher T} \Rightarrow \text{permafrost melt} \Rightarrow \text{GAS release (CO}2!/!CH4) \Rightarrow \text{stronger greenhouse effect} \Rightarrow \text{higher T}

Climate change alters atmospheric circulation, generating drought conditions (precipitation < ecosystem/agricultural demand).
Dry, dead biomass + heat waves ➜ more frequent & intense wildfires.
Fires combust forest carbon and soil organic matter ➜ pulse of \text{CO}_2.
Smoke aerosols may further darken ice or snow, compounding albedo reduction.

Latitude: high-northern, coldest land biome.
Normal temperature range: -5^\circ\text{C} \text{ to } 5^\circ\text{C}.
Dependent on winter snowfall; snowmelt = primary water source during growing season.

Warmer winters → reduced snow accumulation.
Lower spring/summer snowmelt → soil-moisture deficit.
Water stress suppresses photosynthesis → reduced net primary production (NPP).
Trees lose needles, pigments fade → forest browning.
Extensive browning/drought → heightened vulnerability to pests, disease & wildfire ignition.
Fires combust:
- Recent plant carbon (wood, needles).
- Legacy carbon stored centuries in cold soils.
Massive \text{CO}_2 efflux flips forest from carbon sink → carbon source.

Organic layers built under cold, low-decomposition conditions.
Fire penetrates duff/soil ➜ releases ancient carbon stocks.
Adds to atmospheric GHG burden that modern ecosystems cannot re-sequester quickly.

Threshold in a complex system where incremental change triggers a large, potentially irreversible shift in state/function.
In taiga example: cumulative drought, browning & fire push forest past threshold so net carbon balance switches sign (accumulation → loss).

Positive feedbacks described above are coupled; e.g., wildfire soot on Arctic ice → lowers albedo → accelerates ice melt.
Multiple feedbacks can synchronize, hastening approach to global tipping points (e.g., collapse of Greenland ice sheet, Amazon dieback).
Feedback recognition critical for climate-policy: early mitigation yields non-linear benefits by avoiding loop activation.
Ethical dimension: feedback-driven changes may be irreversible on human timescales, imposing burdens on future generations and biodiversity.

Albedo values (approximate):
- Fresh snow: 0.8{-}0.9.
- Sea ice with melt ponds: 0.3{-}0.5.
- Open ocean: 0.06{-}0.1.
Peatlands store ≈550\text{–}650\,\text{Gt C} globally.
Permafrost carbon pool: ≈1300\text{–}1600\,\text{Gt C}, almost twice atmospheric carbon.
Boreal forests sequester \sim0.5\,\text{Gt C yr}^{-1} under undisturbed conditions; large fire years can emit comparable magnitudes.

Deep-ocean \text{CO}_2, albedo loss, peat/thaw decomposition, methane, drought/fire = five key warming feedbacks.
Boreal forest tipping point shows how hydrological changes drive carbon cycle reversal.
All mechanisms reinforce warming, underscoring urgency of emissions mitigation & ecosystem management.