The interlinked operation of the water and carbon cycles in the Arctic tundra, the diagnostic role of permafrost and frozen stores, and the impact of human activity and climate change (especially permafrost thaw) on its water and carbon balances.

What this dot point is asking

OCR wants you to explain how the water and carbon cycles operate in the Arctic tundra, explain the diagnostic role of permafrost and frozen stores, and assess how human activity and climate change, above all permafrost thaw, disrupt its water and carbon balances. The Arctic tundra is OCR's cold-environment contrasting case study, paired with the tropical rainforest.

The answer

Why permafrost defines the tundra system

The master control is cold, and its product is frozen ground. Permafrost acts as an impermeable, frozen lid that locks up both water (as ground ice) and carbon (as undecomposed organic matter). Liquid water and life are restricted to the active layer, so the tundra runs both cycles at a fraction of rainforest rates. The system is finely balanced on the freezing point: it is stable only while the ground stays frozen, which is why warming, even by a few degrees, can transform it. This sensitivity is the reason OCR pairs the tundra with the rainforest as a contrasting case study.

The water cycle in the tundra

Despite low precipitation (often under $250$ mm a year, much of it snow), the tundra is often waterlogged in summer. The reason is the permafrost: it prevents downward percolation, so meltwater and rain are trapped in the thin active layer, producing widespread ponds, bogs and thermokarst lakes. Evapotranspiration is low because of the cold and sparse vegetation. Stores are dominated by ground ice and snow (the cryosphere); liquid surface and soil water are small and seasonal. River regimes are sharply nival, with a large early-summer peak from snowmelt and very low winter flow when water is frozen.

The carbon cycle in the tundra

The tundra has low net primary productivity: the cold, short growing season and sparse, low-growing vegetation fix little carbon each year. Yet it is one of the planet's largest terrestrial carbon stores, because decomposition is slower still. Cold, waterlogged, frozen soils suppress microbial activity, so dead organic matter accumulates as peat and frozen organic carbon over thousands of years. Permafrost is estimated to hold roughly twice as much carbon as is currently in the atmosphere. The store is therefore the result of a tiny but persistent input meeting an almost-zero output, and it is stable only while frozen.

Human disruption: warming, thaw and extraction

Climate change is the dominant disruptor, and the Arctic is warming far faster than the global average (Arctic amplification). Warming deepens the active layer and thaws permafrost, which disrupts the water cycle (water drains where it once ponded, or new thermokarst lakes form and later drain; snowmelt comes earlier) and the carbon cycle (previously frozen organic matter decomposes, releasing carbon dioxide and, in waterlogged conditions, methane, a potent greenhouse gas). This can switch the tundra from a long-term sink to a source. The effect is amplified by positive feedbacks: released greenhouse gases drive more warming, and retreating snow and ice lower albedo. Resource extraction (oil and gas, pipelines, mining) adds local disruption to drainage, vegetation and permafrost.

Examples in context

Example 1. The Alaskan North Slope and Siberian permafrost. Across the Alaskan North Slope and the vast Siberian tundra, deep permafrost holds an enormous frozen carbon store built over millennia, while summer activity is confined to a thin active layer that waterlogs the surface. Monitoring shows the active layer deepening and widespread thermokarst (ground subsidence as ice melts), with some Arctic sites already measured as net carbon sources in autumn. These are OCR's go-to located examples for the frozen stores and the thaw feedback.

Example 2. Resource extraction at Prudhoe Bay. The Prudhoe Bay oil field and the Trans-Alaska Pipeline illustrate human disruption layered on climate change: infrastructure built on permafrost must be elevated or refrigerated to avoid thawing the ground, roads and gravel pads alter local drainage and vegetation, and warming raises maintenance and spill risks. This shows how extraction modifies the local water and carbon system and adds a human dimension to the climate-driven changes, useful for the management and synoptic strands.

Try this

Q1. Define permafrost and the active layer. [2 marks]

• Cue. Permafrost is ground frozen for at least two consecutive years; the active layer is the surface zone above it that thaws each summer and refreezes in winter.

Q2. Explain why permafrost thaw acts as a positive feedback on climate change. [4 marks]

• Cue. Warming thaws permafrost, allowing decomposition that releases carbon dioxide and methane; these greenhouse gases cause further warming, which thaws more permafrost, amplifying the original change.