Limitations in the use of atmospheric CO ₂ observations to directly infer changes in the length of the biospheric carbon uptake period

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) interannual variability in CUPNEE, and (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 (Δ) to the CUPNEE in the Northern Hemisphere to test the effect of the aforementioned factors in CUPMR metrics at 10 Northern Hemisphere sites. Our analysis indicates a significant damping of changes in the simulated ΔCUPMR due to the integration of signals with varying CUPNEE timing across regions. CUPMR at well-studied sites such as Mauna Loa, Utqiaġvik (formerly Barrow), and Alert showed only 50 % of the applied ΔCUPNEE 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.