Is the Thawing Tundra a Carbon Source or Sink?
A research station monitors a tundra site underlain by permafrost. Over 15 years, the active layer has deepened from 0.55 m to 0.78 m, and soil temperature at 1 m has risen from −2.1°C to −1.4°C. Summer CO₂ flux from the soil has increased from 85 to 140 g C m⁻² yr⁻¹, while winter CO₂ flux, previously assumed negligible, is now measured at 25 g C m⁻² yr⁻¹. Methane flux has risen from 2 to 6 g CH₄ m⁻² yr⁻¹. Warmer summers have also increased shrub growth, raising plant carbon uptake by an estimated 40 g C m⁻² yr⁻¹ above the 15-year baseline. A model projects cumulative net carbon loss of 8–14 kg C m⁻² by 2100 under a high-emissions pathway, but only 2–4 kg C m⁻² under a low-emissions pathway. The station must advise whether the site should be counted as a carbon source in the regional greenhouse gas inventory.
1. Using the flux data, determine whether the site is currently a net carbon source or sink, showing your reasoning and any necessary assumptions.
Read the worked answer
Step 1 - carbon losses. Soil respiration is the sum of summer and winter fluxes:
- 140 + 25 = 165 g C m⁻² yr⁻¹ released as CO₂.
Methane is a separate loss and must be converted to CO₂-equivalent to compare. Using GWP₁₀₀ = 28 for CH₄: 6 g CH₄ × 28 = 168 g CO₂-eq m⁻² yr⁻¹, i.e. about 6 g CH₄-C corresponds to roughly 8 g CH₄ by mass, so the carbon mass lost as CH₄ is small (about 4.5 g C m⁻² yr⁻¹) but its warming effect is large.
Step 2 - gains. Plant uptake increased by 40 g C m⁻² yr⁻¹ above baseline. The baseline uptake is not given, which is a critical gap: we cannot calculate net ecosystem exchange without it. If baseline uptake were, say, 120 g C m⁻² yr⁻¹, current uptake is 160 g C m⁻² yr⁻¹ and the site is roughly carbon-neutral in CO₂ terms (165 vs 160), but a net warming source once CH₄ is included.
Conclusion: on the measured fluxes the site is at best marginally carbon-neutral and, when CH₄ is included, is a net greenhouse gas source. The correct answer must state the missing baseline uptake rather than assume it.
2. Evaluate the strength of the evidence that this is a positive feedback on warming. Address at least one alternative explanation and one source of uncertainty.
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Supporting a positive feedback: deeper active layer (0.55 → 0.78 m) and warmer soil (−2.1 → −1.4°C) are consistent with thaw exposing previously frozen organic carbon to microbial decomposition. Rising CO₂ and CH₄ fluxes track the thaw, and methane is especially significant because warming also expands anaerobic saturated zones. The model's 8–14 kg C m⁻² loss under high emissions versus 2–4 under low emissions implies the feedback is emissions-dependent, which is the signature of a positive feedback.
Alternative explanation: the flux increase could reflect increased plant productivity and root respiration, not old carbon decomposition. Warmer summers increase shrub growth, and root and mycorrhizal respiration would raise soil CO₂ flux without any permafrost carbon being lost. The 40 g C m⁻² yr⁻¹ uptake increase supports this. The discriminating evidence would be radiocarbon dating of respired CO₂: old carbon indicates permafrost decomposition, young carbon indicates root respiration.
Uncertainty: winter fluxes were previously assumed negligible and are now 25 g C m⁻² yr⁻¹, so past budgets understated losses; measurements are also plot-scale and may not represent the region. The model's range (8–14) shows substantial uncertainty, so the feedback is likely positive but its magnitude is not well constrained.
3. Should the site be counted as a source in the regional inventory? Justify your recommendation, considering the low- versus high-emissions projections.
Read the worked answer
Yes, but as a small and conditional source, with the methane included and the uncertainty stated. The measured fluxes show losses of 165 g C m⁻² yr⁻¹ as CO₂ plus a warming-equivalent CH₄ loss, against a plant uptake whose baseline is not reported. Counting it as zero would be wrong because winter respiration alone (25 g C m⁻² yr⁻¹) was previously omitted and is a real loss.
Conditionality: the model projects 8–14 kg C m⁻² by 2100 under high emissions but only 2–4 under low emissions. This means the site is a source that grows with warming - the inventory should record it as a source now and apply an emissions-dependent projection, rather than a fixed number. Reporting a single mid-range value would hide the fact that mitigation changes the answer.
Practical judgement: the per-area fluxes are small compared with industrial emissions, but tundra covers vast areas, so regional upscaling matters. The honest inventory entry is "net source, magnitude uncertain, likely to increase under high emissions", with the key caveat that the missing baseline plant uptake could shift the site close to neutral in CO₂ terms. Counting it as a large source would overstate the evidence; counting it as a sink would contradict the measurements.