Thawing tundra permafrost affects the carbon cycle in which of the following ways? Select all that are correct.

Question

Thawing tundra permafrost affects the carbon cycle in which of the following ways?  Select all that are correct.

BlackTom AI · Accepted Answer

Permafrost stores vast amounts of frozen organic carbon; when it thaws the active layer deepens and microbial decomposition and plant responses both change, producing the outcomes below.

1. Relevant knowledge points and concepts
- Permafrost contains large pools of organic carbon (dead plant/animal matter) locked in frozen soils. The active layer is the seasonally thawed top layer above permafrost.
- Thaw increases active-layer depth, exposing previously frozen organic matter to microbial decomposition, releasing CO2 and CH4.
- Warmer temperatures and higher nutrient availability can stimulate tundra plant growth (a short-term CO2 sink), but plant uptake is limited compared with large permafrost carbon stocks.
- Net effect depends on rates/timescales: short-term increased productivity vs longer-term cumulative carbon release from decomposition.

2. Step-by-step reasoning
- Thaw → active layer deepens → microbes access more organic matter → respiration rates rise → CO2 (and CH4 under anaerobic conditions) emitted to atmosphere.
- Simultaneously, warmer conditions and more available nutrients can enhance plant photosynthesis and biomass, drawing down some CO2.
- Compare magnitudes: enhanced plant uptake often partially offsets respiration for a limited period, but decomposing formerly frozen carbon reservoirs are large and continue to release greenhouse gases for decades to centuries.

3. Calculations
- No numeric calculation required here; reasoning is qualitative and based on relative magnitudes and timescales (short-term plant uptake vs long-term large carbon release).

4. Why the correct answers are correct
- "In the short term, enhanced growth of tundra plants makes it a carbon sink." — True because warming and nutrient release can boost productivity and seasonal carbon uptake.
- "In the long term, enhanced decomposition in the active layers leads it to act as a carbon source." — True because sustained decomposition of large frozen carbon stores eventually releases more CO2/CH4 than plants can sequester.

5. Why the other options are incorrect
- "Enhanced decomposition leads to a negative feedback loop that limits further warming by reducing CO2." — Incorrect: decomposition releases CO2/CH4, a positive feedback that increases warming, not reduces it.
- "In the long term, the balance of photosynthesis and respiration means that tundra is carbon-neutral" — Incorrect: long-term evidence and carbon pool sizes indicate net positive emissions after thaw; not carbon-neutral.

6. Conclusion
The correct selections are the two statements given: short-term enhanced plant growth can make tundra a sink, but in the long term enhanced decomposition turns it into a carbon source, matching the provided answers.

Thawing tundra permafrost affects the carbon cycle in which of the following ways? Select all that are correct. 多项选择题

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