Quick questions on Water and carbon in the Arctic tundra: permafrost, frozen stores and thaw - OCR A-Level Geography

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.

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.

Define permafrost and the active layer. [2 marks]

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