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Module 6

Tundra

Beyond the treeline, the tundra stretches to polar ice. Underlain by permafrost, scoured by wind, open to continuous daylight in summer and continuous darkness in winter, this biome is the most sensitive indicator of planetary warming — and among the most threatened. Arctic amplification runs 2–4× the global mean rate.

1. Permafrost — The Defining Feature

Permafrost is ground that remains frozen for at least two consecutive years. It underlies ~23% of the Northern Hemisphere land surface. A thin active layer (20–200 cm) thaws seasonally, allowing roots to grow, but the frozen substrate below blocks drainage and creates the characteristic waterlogged polygon landscapes. Permafrost contains not only frozen water but preserved Pleistocene organic matter — the Yedoma deposits of Siberia and Alaska, estimated to contain ~400 Gt C.

2. Arctic Amplification

Arctic warming is proceeding at 2–4× the global mean rate, primarily due to ice-albedo feedback:

\[ \Delta T_{Arctic} \;\approx\; (2\text{-}4)\cdot \Delta T_{global};\qquad \alpha_{ice} \approx 0.8 \gg \alpha_{ocean} \approx 0.06 \]

Loss of reflective sea ice exposes dark ocean, absorbing more heat, melting more ice — a runaway positive feedback. Secondary amplifiers: water vapour (warm air holds more moisture, a greenhouse gas), snow-albedo feedback on land, and reduced heat-loss through decreased sea-ice insulation.

Simulation: Arctic Amplification & Permafrost Carbon

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3. Plant Strategies

Tundra flora contains no tree species (by definition — the treeline is the ecotone). Life forms emphasise wind-pruned shapes and cold tolerance:

  • Prostrate shrubs: arctic willow, dwarf birch grow 10–30 cm tall, horizontal.
  • Sedges and grasses: Carex, Eriophorum (cotton-grass) dominate wet polygon-trough mosaics; major peat-formers.
  • Mosses & lichens: Sphagnum, reindeer lichen (Cladonia) grow slowly (mm/yr) but are the dominant primary producers in High-Arctic zones; many fix N via cyanobacterial symbionts.
  • Cushion plants: Silene, Saxifraga, Dryas form compact hemispheres that trap solar-warmed air, creating internal micro-greenhouse temperatures up to 15 °C above ambient.

4. Shrubification

Under 2–4°C warming, dwarf-shrub cover has expanded across the Low-Arctic tundra over the past four decades (Myers-Smith 2011). Landsat records show detectable “greening” of tundra. Shrub expansion alters snow depth, ground insulation, and albedo, with complex feedbacks on both carbon storage and permafrost stability.

5. Fauna

Characteristic tundra vertebrates: caribou/reindeer, muskox, Arctic fox, Arctic hare, lemmings, snowy owl, rough-legged hawk. Polar bears (ice-dependent) use coastal tundra as summer retreat. Lemming population cycles drive owl and fox reproductive success with 3–4 year regularity. Migratory birds (shorebirds, waterfowl) nest on the tundra during the brief Arctic summer, then retreat to wintering grounds as far as the Antarctic (Arctic terns).

Key References

• Chapin, F. S. et al. (2005). “Role of land-surface changes in arctic summer warming.” Science, 310, 657–660.

• Schuur, E. A. G. et al. (2015). “Climate change and the permafrost carbon feedback.” Nature, 520, 171–179.

• Myers-Smith, I. H. et al. (2011). “Shrub expansion in tundra ecosystems: dynamics, impacts and research priorities.” Environ. Res. Lett., 6, 045509.

• Serreze, M. C. & Barry, R. G. (2011). “Processes and impacts of Arctic amplification: a research synthesis.” Glob. Planet. Change, 77, 85–96.

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