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1. Solar-Induced Chlorophyll Fluorescence (SIF): Towards a Better Understanding of Vegetation Dynamics and Carbon Uptake in Arctic-Boreal Ecosystems

   https://doi.org/10.1007/s40641-024-00194-8  

   Cheng, Rui (June 2024, Current Climate Change Reports)

   Abstract Purpose of ReviewTerrestrial ecosystems in the Arctic-Boreal region play a crucial role in the global carbon cycle as a carbon sink. However, rapid warming in this region induces uncertainties regarding the future net carbon exchange between land and the atmosphere, highlighting the need for better monitoring of the carbon fluxes. Solar-Induced chlorophyll Fluorescence (SIF), a good proxy for vegetation CO$$^{2}$$ ${}^2$uptake, has been broadly utilized to assess vegetation dynamics and carbon uptake at the global scale. However, the full potential and limitations of SIF in the Arctic-Boreal region have not been explored. Therefore, this review aims to provide a comprehensive summary of the latest insights into Arctic-Boreal carbon uptake through SIF analyses, underscoring the advances and challenges of SIF in solving emergent unknowns in this region. Additionally, this review proposes applications of SIF across scales in support of other observational and modeling platforms for better understanding Arctic-Boreal vegetation dynamics and carbon fluxes. Recent FindingsCross-scale SIF measurements complement each other, offering valuable perspectives on Arctic-Boreal ecosystems, such as vegetation phenology, carbon uptake, carbon-water coupling, and ecosystem responses to disturbances. By incorporating SIF into land surface modeling, the understanding of Arctic-Boreal changes and their climate drivers can be mechanistically enhanced, providing critical insights into the changes of Arctic-Boreal ecosystems under global warming. SummaryWhile SIF measurements are more abundant and with finer spatiotemporal resolutions, it is important to note that the coverage of these measurements is still limited and uneven in the Arctic-Boreal region. To address this limitation and further advance our understanding of the Arctic-Boreal carbon cycle, this review advocates for fostering a SIF network providing long-term and continuous measurements across spatial scales. Simultaneously measuring SIF and other environmental variables in the context of a multi-modal sensing system can help us comprehensively characterize Arctic-Boreal ecosystems with spatial details in land surface models, ultimately contributing to more robust climate projections. 

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2. Gridded surface O ₃ , NO _x , and CO abundances for model metrics from the South Korean ground station network

   https://doi.org/10.5194/egusphere-2024-1173  

   Wilson, Calum Patrick; Prather, Michael John (August 2024, EGU)

   Abstract. We present gridded surface air quality datasets over South Korea for three key species – ozone (O3), carbon monoxide (CO), and nitrogen oxides (NOx) during the timeframe of the Korea–US Air Quality (KORUS–AQ) mission (May–June 2016). The tenth degree hourly averaged abundances are constructed from the 300+ air quality network sites using inverse distance weighting with simple declustering. Cross–comparing the interpolated fields against the site data that was used to create them reveals high prediction skill for O3 (80 %) throughout South Korea, and moderate skill (60 %) for CO and NOx on average in densely observed regions after individual mean bias corrections. The gridded O3 and CO interpolations predict the NASA DC–8 observations in the planetary boundary layer (PBL) with high skill (80 %) in the Seoul Metropolitan Area (SMA) after subtracting the mean bias. DC–8 NOx observations were much less predictable on account of consistently negative vertical gradients within the PBL. Our gridded products capture the mean and variability of O3 throughout South Korea, and of CO and surface NOx in most site–dense urban centres (SMA, Cheongju, Gwangju, Daegu, Changwon, and Busan). 

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3. Improved atmospheric constraints on Southern Ocean CO ₂ exchange

   https://doi.org/10.1073/pnas.2309333121  

   Jin, Yuming; Keeling, Ralph F; Stephens, Britton B; Long, Matthew C; Patra, Prabir K; Rödenbeck, Christian; Morgan, Eric J; Kort, Eric A; Sweeney, Colm (February 2024, Proceedings of the National Academy of Sciences)

   We present improved estimates of air–sea CO₂exchange over three latitude bands of the Southern Ocean using atmospheric CO₂measurements from global airborne campaigns and an atmospheric 4-box inverse model based on a mass-indexed isentropic coordinate (M_θe). These flux estimates show two features not clearly resolved in previous estimates based on inverting surface CO₂measurements: a weak winter-time outgassing in the polar region and a sharp phase transition of the seasonal flux cycles between polar/subpolar and subtropical regions. The estimates suggest much stronger summer-time uptake in the polar/subpolar regions than estimates derived through neural-network interpolation of pCO₂data obtained with profiling floats but somewhat weaker uptake than a recent study by Long et al. [Science374, 1275–1280 (2021)], who used the same airborne data and multiple atmospheric transport models (ATMs) to constrain surface fluxes. Our study also uses moist static energy (MSE) budgets from reanalyses to show that most ATMs tend to have excessive diabatic mixing (transport across moist isentrope, θ_e, or M_θesurfaces) at high southern latitudes in the austral summer, which leads to biases in estimates of air–sea CO₂exchange. Furthermore, we show that the MSE-based constraint is consistent with an independent constraint on atmospheric mixing based on combining airborne and surface CO₂observations. 

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4. Magnitude, Trends, and Variability of the Global Ocean Carbon Sink From 1985 to 2018

   https://doi.org/10.1029/2023GB007780  

   DeVries, Tim; Yamamoto, Kana; Wanninkhof, Rik; Gruber, Nicolas; Hauck, Judith; Müller, Jens Daniel; Bopp, Laurent; Carroll, Dustin; Carter, Brendan; Chau, Thi‐Tuyet‐Trang; et al (October 2023, Global Biogeochemical Cycles)

   Abstract This contribution to the RECCAP2 (REgional Carbon Cycle Assessment and Processes) assessment analyzes the processes that determine the global ocean carbon sink, and its trends and variability over the period 1985–2018, using a combination of models and observation‐based products. The mean sea‐air CO₂flux from 1985 to 2018 is −1.6 ± 0.2 PgC yr⁻¹based on an ensemble of reconstructions of the history of sea surface pCO₂(pCO₂products). Models indicate that the dominant component of this flux is the net oceanic uptake of anthropogenic CO₂, which is estimated at −2.1 ± 0.3 PgC yr⁻¹by an ensemble of ocean biogeochemical models, and −2.4 ± 0.1 PgC yr⁻¹by two ocean circulation inverse models. The ocean also degasses about 0.65 ± 0.3 PgC yr⁻¹of terrestrially derived CO₂, but this process is not fully resolved by any of the models used here. From 2001 to 2018, the pCO₂products reconstruct a trend in the ocean carbon sink of −0.61 ± 0.12 PgC yr⁻¹ decade⁻¹, while biogeochemical models and inverse models diagnose an anthropogenic CO₂‐driven trend of −0.34 ± 0.06 and −0.41 ± 0.03 PgC yr⁻¹ decade⁻¹, respectively. This implies a climate‐forced acceleration of the ocean carbon sink in recent decades, but there are still large uncertainties on the magnitude and cause of this trend. The interannual to decadal variability of the global carbon sink is mainly driven by climate variability, with the climate‐driven variability exceeding the CO₂‐forced variability by 2–3 times. These results suggest that anthropogenic CO₂dominates the ocean CO₂sink, while climate‐driven variability is potentially large but highly uncertain and not consistently captured across different methods. 

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5. Regional Inversion Shows Promise in Capturing Extreme‐Event‐Driven CO ₂ Flux Anomalies but Is Limited by Atmospheric CO ₂ Observational Coverage

   https://doi.org/10.1029/2023JD040006  

   Byrne, B; Liu, J; Bowman, K W; Yin, Y; Yun, J; Ferreira, G D; Ogle, S M; Baskaran, L; He, L; Li, X; et al (March 2024, Journal of Geophysical Research: Atmospheres)

   Abstract Extreme climate events are becoming more frequent, with poorly understood implications for carbon sequestration by terrestrial ecosystems. A better understanding will critically depend on accurate and precise quantification of ecosystems responses to these events. Taking the 2019 US Midwest floods as a case study, we investigate current capabilities for tracking regional flux anomalies with “top‐down” inversion analyses that assimilate atmospheric CO₂observations. For this analysis, we develop a regionally nested version of the NASA Carbon Monitoring System‐Flux system for North America (CMS‐Flux‐NA) that allows high resolution atmospheric transport (0.5° × 0.625°). Relative to a 2018 baseline, we find the 2019 US Midwest growing season net carbon uptake is reduced by 11–57 TgC (3%–16%, range across assimilated CO₂data sets). These estimates are found to be consistent with independent “bottom‐up” estimates of carbon uptake based on vegetation remote sensing (15–78 TgC). We then investigate current limitations in tracking regional carbon budgets using “top‐down” methods. In a set of observing system simulation experiments, we show that the ability of atmospheric CO₂inversions to capture regional carbon flux anomalies is still limited by observational coverage gaps for both in situ and satellite observations. Future space‐based missions that allow for daily observational coverage across North America would largely mitigate these observational gaps, allowing for improved top‐down estimates of ecosystem responses to extreme climate events. 

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