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 (CUP_MR) to actual changes in the CUP of the net ecosystem exchange (CUP_NEE) is missing. In this study, we specifically assess the impact of (i) atmospheric transport (ii) inter-annual variability in CUP_NEE (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 CUP_NEE in the Northern Hemisphere to test the effect of the aforementioned factors in CUP_MR metrics at ten Northern Hemisphere sites. Our analysis indicates a significant damping of changes in the simulated ∆CUP_MR due to the integration of signals with varying CUP_NEE timing across regions. CUP_MR at well-studied sites such as Mauna Loa, Barrow, and Alert showed only 50 % of the applied ∆CUP_NEE 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.

中文翻译：

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大气中的二氧化碳如何让我们了解生物圈碳吸收期的年度和十年变化


摘要。碳吸收期（CUP）是指每年陆地生物圈光合吸收速率超过呼吸速率，导致从大气到陆地净吸收CO ₂ 的时间。由于气候驱动因素影响光合作用和呼吸作用，CUP 为陆地生物圈如何响应气候变化和影响碳预算提供了宝贵的见解。几项研究根据 CO ₂ 摩尔分数测量的季节性指标评估了 CUP 的大规模变化。然而，深入了解源自 CO ₂ 摩尔分数数据 (CUP _MR ) 的 CUP 对净生态系统交换的 CUP 实际变化的敏感性 ( CUP _NEE ）丢失。在本研究中，我们专门评估了 (i) 大气传输 (ii) CUP _NEE 的年际变化 (iii) 对在不同背景地点整合的信号的区域贡献的影响 ₂ 进行干燥空气摩尔分数测量。我们进行了理想化模拟，对北半球的 CUP _NEE 施加已知变化 (Δ)，以测试上述因素对北半球 10 个地点的 CUP _MR 指标的影响。我们的分析表明，由于不同区域的 CUP _NEE 时序不同的信号集成，模拟 ΔCUP _MR 的变化存在显着阻尼。在 Mauna Loa、Barrow 和 Alert 等经过充分研究的地点，CUP _MR 显示，在非年际变化的大气传输条件下，仅应用 ΔCUP _NEE 的 50%。 此外，我们的综合分析得出结论，大气传输的年际变化（IAV）是观测信号变化的重要组成部分。然而，即使在分离了传输 IAV 的贡献之后，之前的研究对 CUP 表面变化的估计也不太可能提供表面发生的实际变化的准确幅度。由于大气平均了来自各个区域的非同步信号，观测到的信号经历了显着的阻尼。