Preprints
https://doi.org/10.5194/egusphere-2024-1382
https://doi.org/10.5194/egusphere-2024-1382
05 Jul 2024
 | 05 Jul 2024
Status: this preprint is open for discussion and under review for Atmospheric Chemistry and Physics (ACP).

How atmospheric CO2 can inform us on annual and decadal shifts in the biospheric carbon uptake period

Theertha Kariyathan, Ana Bastos, Markus Reichstein, Wouter Peters, and Julia Marshall

Abstract. The carbon uptake period (CUP) refers to the time of each year during which the rate of photosynthetic uptake surpasses that of respiration in the terrestrial biosphere, resulting in a net absorption of CO2 from the atmosphere to the land. Since climate drivers influence both photosynthesis and respiration, the CUP offers valuable insights into how the terrestrial biosphere responds to climate variations and affects the carbon budget. Several studies have assessed large-scale changes in CUP based on seasonal metrics from CO2 mole fraction measurements. However, an in-depth understanding of the sensitivity of the CUP as derived from the CO2 mole fraction data (CUPMR) to actual changes in the CUP of the net ecosystem exchange (CUPNEE) is missing. In this study, we specifically assess the impact of (i) atmospheric transport (ii) inter-annual variability in CUPNEE (iii) regional contribution to the signals that integrate at different background sites where CO2 dry air mole fraction measurements are made. We conducted idealized simulations where we imposed known changes (∆) to the CUPNEE in the Northern Hemisphere to test the effect of the aforementioned factors in CUPMR metrics at ten Northern Hemisphere sites. Our analysis indicates a significant damping of changes in the simulated ∆CUPMR due to the integration of signals with varying CUPNEE timing across regions. CUPMR at well-studied sites such as Mauna Loa, Barrow, and Alert showed only 50 % of the applied ∆CUPNEE under non interannually-varying atmospheric transport conditions. Further, our synthetic analyses conclude that interannual variability (IAV) in atmospheric transport accounts for a significant part of the changes in the observed signals. However, even after separating the contribution of transport IAV, the estimates of surface changes in CUP by previous studies are not likely to provide an accurate magnitude of the actual changes occurring over the surface. The observed signal experiences significant damping as the atmosphere averages out non-synchronous signals from various regions.

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Theertha Kariyathan, Ana Bastos, Markus Reichstein, Wouter Peters, and Julia Marshall

Status: open (until 16 Aug 2024)

Comment types: AC – author | RC – referee | CC – community | EC – editor | CEC – chief editor | : Report abuse
  • CC1: 'The meaning of mixing ratio', Andrew Kowalski, 10 Jul 2024 reply
    • AC1: 'Reply on CC1', Theertha Kariyathan, 15 Jul 2024 reply
Theertha Kariyathan, Ana Bastos, Markus Reichstein, Wouter Peters, and Julia Marshall
Theertha Kariyathan, Ana Bastos, Markus Reichstein, Wouter Peters, and Julia Marshall

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Short summary
The carbon uptake period (CUP) refers to the time of the year when there is net absorption of CO2 from the atmosphere to the land. Several studies have assessed changes in CUP based on seasonal metrics from CO2 mole fraction measurements to understand the response of terrestrial biosphere to climate variations. However, we find that the CUP derived from CO2 mole fraction measurements are not likely to provide an accurate magnitude of the actual changes occurring over the surface.