4 February, 2020 (Jung, Nature, Compensatory)

Jung, M., Reichstein, M., Schwalm, C. et al. Compensatory water effects link yearly global land CO₂ sink changes to temperature. Nature 541, 516-520 (2017). https://doi.org/10.1038/nature20780

• Question:

  1. How do changes in temperature and water availability effect on gross primary productivity (GPP), terrestrial ecosystem respiration (TER), and net ecosystem exchange (NEE) at local and global scales?

• Context:

  • Despite temperature is known to contribute to the inter annual variation (IAV) of the terrestrial carbon cycle, several studies propose that water availability play an important role in the carbon cycle relates such as the carbon balance of semi-tropics and the sensitivity to IAV.
  • An understanding of the process behind the climatic control of terrestrial carbon cycle on interannual timescales and across spatial scales is still lacking.

• Answer

  1. At local scale (grid cell, ~ 50 km), water availability is the major driver of the IAV in GPP, TER, and NEE.
     • Temperature also have amplifying effect on NEE, but the magnitude is negligible.
  2. At global scale, temperature becomes the dominant driver of the IAV in GPP, TER, and NEE.
     • There seems a local compensation mechanism: Local variability in GPP and TER compensate locally, dampening water-driven NEE variability
     • There are spatial compensations: Spatial variability in water availability also compensates.

• Implication:

  • This study reconciles existing seemingly contradictory conclusions many studies (i.e. temperature vs water availability)
  • The apparent temperature-dominated IAV of the residual land sink contains little information on local carbon-cycle processes
  • The spatial covariation of climate variables drives the integrated global carbon-cycle response, as well as local one, and perhaps even the strength of climate-carbon cycle feedbacks.

• Unanswered:

  • Why temperature contributes little to the local-scale GPP and TER?
  • How does the local compensation mechanism occur?
    • The authors guessed that the concomitant positive relationship of soil moisture with productivity and with respiration was it (i.e. Both GPP and TER increase with SM, remaining NEE unchanged).
    • Then, what are the differences between high GPP and TER under pleasing SM condition and low GPP and TER under harsh SM condition?
  • Is not there any mechanism that offset the temperature effect? Does it make sense that the temperature effect at global scale remains without compensation?

• Comment:

  • Can we be sure that the water availability hardly affect the global scale productivity? Are there any other ways?
  • It is interesting and encouraging that the TRENDY results generally match those from FLUXCOM data although TRENDY showed under-/over-estimation to a certain degree.
  • It is also interesting how the authors took steps to solve their question and how they deal with the large set of locally gridded and global-scale data.
  • They suggested some explanations for their results, but it seems that this study created many questions unsolved, which values this study.