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1. Diel, seasonal, and inter-annual variation in carbon dioxide effluxes from lakes and reservoirs

   https://doi.org/10.1088/1748-9326/acb834  

   Golub, Malgorzata ; Koupaei-Abyazani, Nikaan ; Vesala, Timo ; Mammarella, Ivan ; Ojala, Anne ; Bohrer, Gil ; Weyhenmeyer, Gesa A. ; Blanken, Peter D. ; Eugster, Werner ; Koebsch, Franziska ; et al ( March 2023 , Environmental Research Letters)

   Accounting for temporal changes in carbon dioxide (CO₂) effluxes from freshwaters remains a challenge for global and regional carbon budgets. Here, we synthesize 171 site-months of flux measurements of CO₂based on the eddy covariance method from 13 lakes and reservoirs in the Northern Hemisphere, and quantify dynamics at multiple temporal scales. We found pronounced sub-annual variability in CO₂flux at all sites. By accounting for diel variation, only 11% of site-months were net daily sinks of CO₂. Annual CO₂emissions had an average of 25% (range 3%–58%) interannual variation. Similar to studies on streams, nighttime emissions regularly exceeded daytime emissions. Biophysical regulations of CO₂flux variability were delineated through mutual information analysis. Sample analysis of CO₂fluxes indicate the importance of continuous measurements. Better characterization of short- and long-term variability is necessary to understand and improve detection of temporal changes of CO₂fluxes in response to natural and anthropogenic drivers. Our results indicate that existing global lake carbon budgets relying primarily on daytime measurements yield underestimates of net emissions.

    

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2. High-latitude fire activity of recent decades derived from microscopic charcoal and black carbon in Greenland ice cores

   https://doi.org/10.1177/09596836221131711  

   Brugger, Sandra O ; Chellman, Nathan J ; McConnell, Callie ; McConnell, Joseph R ( February 2023 , The Holocene)

   Warming temperatures and prolonged drought periods cause rapid changes of fire frequencies and intensities in high-latitude ecosystems. Associated smoke plumes deposit dark particles from incomplete combustion on the Greenland ice sheet that reduce albedo but also provide a detailed record of paleofire history. Here, we apply an emerging microscopic charcoal technique in combination with established black carbon and lead pollution measurements to an array of 10 ice cores from southern to central Greenland that span recent decades. We found that microscopic charcoal deposition is highly variable among sites, with a few records suggesting recently increasing biomass burning possibly in response to growing fire activity in boreal forest ecosystems. This stands in contrast to decreasing trends in black carbon measured in the same ice cores, consistent with contributions from industrial fossil fuel emissions. Decreasing trends of lead pollution and occurrence of microscopic spheroidal carbonaceous particles (SCP), a microfossil tracer of fossil fuel emissions, further support our interpretation that black carbon in this region is influenced by industrial emissions during recent decades. We conclude that microscopic charcoal analyses in ice may help disentangle biomass burning from fossil-fuel emissions during the industrial period and have potential to contribute to better understanding of regional high-latitude fire regimes. 

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3. Fixing a snag in carbon emissions estimates from wildfires

   https://doi.org/10.1111/gcb.14716  

   Stenzel, Jeffrey_E ; Bartowitz, Kristina_J ; Hartman, Melannie_D ; Lutz, James_A ; Kolden, Crystal_A ; Smith, Alistair_M_S ; Law, Beverly_E ; Swanson, Mark_E ; Larson, Andrew_J ; Parton, William_J ; et al ( July 2019 , Global Change Biology)

   Wildfire is an essential earth‐system process, impacting ecosystem processes and the carbon cycle. Forest fires are becoming more frequent and severe, yet gaps exist in the modeling of fire on vegetation and carbon dynamics. Strategies for reducing carbon dioxide (CO₂) emissions from wildfires include increasing tree harvest, largely based on the public assumption that fires burn live forests to the ground, despite observations indicating that less than 5% of mature tree biomass is actually consumed. This misconception is also reflected though excessive combustion of live trees in models. Here, we show that regional emissions estimates using widely implemented combustion coefficients are 59%–83% higher than emissions based on field observations. Using unique field datasets from before and after wildfires and an improved ecosystem model, we provide strong evidence that these large overestimates can be reduced by using realistic biomass combustion factors and by accurately quantifying biomass in standing dead trees that decompose over decades to centuries after fire (“snags”). Most model development focuses on area burned; our results reveal that accurately representing combustion is also essential for quantifying fire impacts on ecosystems. Using our improvements, we find that western US forest fires have emitted 851 ± 228 Tg CO₂(~half of alternative estimates) over the last 17 years, which is minor compared to 16,200 Tg CO₂from fossil fuels across the region.

    

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4. Sources of Uncertainty in Regional and Global Terrestrial CO 2 Exchange Estimates

   https://doi.org/10.1029/2019GB006393  

   Bastos, A. ; O'Sullivan, M. ; Ciais, P. ; Makowski, D. ; Sitch, S. ; Friedlingstein, P. ; Chevallier, F. ; Rödenbeck, C. ; Pongratz, J. ; Luijkx, I. T. ; et al ( February 2020 , Global Biogeochemical Cycles)

   The Global Carbon Budget 2018 (GCB2018) estimated by the atmospheric COgrowth rate, fossil fuel emissions, and modeled (bottom‐up) land and ocean fluxes cannot be fully closed, leading to a “budget imbalance,” highlighting uncertainties in GCB components. However, no systematic analysis has been performed on which regions or processes contribute to this term. To obtain deeper insight on the sources of uncertainty in global and regional carbon budgets, we analyzed differences in Net Biome Productivity (NBP) for all possible combinations of bottom‐up and top‐down data sets in GCB2018: (i) 16 dynamic global vegetation models (DGVMs), and (ii) 5 atmospheric inversions that match the atmospheric COgrowth rate. We find that the global mismatch between the two ensembles matches well the GCB2018 budget imbalance, with Brazil, Southeast Asia, and Oceania as the largest contributors. Differences between DGVMs dominate global mismatches, while at regional scale differences between inversions contribute the most to uncertainty. At both global and regional scales, disagreement on NBP interannual variability between the two approaches explains a large fraction of differences. We attribute this mismatch to distinct responses to El Niño–Southern Oscillation variability between DGVMs and inversions and to uncertainties in land use change emissions, especially in South America and Southeast Asia. We identify key needs to reduce uncertainty in carbon budgets: reducing uncertainty in atmospheric inversions (e.g., through more observations in the tropics) and in land use change fluxes, including more land use processes and evaluating land use transitions (e.g., using high‐resolution remote‐sensing), and, finally, improving tropical hydroecological processes and fire representation within DGVMs.

    

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5. Atmospheric CO 2 and Sea Surface Temperature Variability Cannot Explain Recent Decadal Variability of the Ocean CO 2 Sink

   https://doi.org/10.1029/2021GL096018  

   DeVries, Tim ( April 2022 , Geophysical Research Letters)

   The ocean is one of the most important sinks for anthropogenic CO₂emissions. Here, I use an ocean circulation inverse model (OCIM), ocean biogeochemical models, and pCO₂interpolation products to examine trends and variability in the oceanic CO₂sink. The OCIM quantifies the impacts of rising atmospheric CO₂, changing sea surface temperatures, and gas transfer velocities on the oceanic CO₂sink. Together, these effects account for an oceanic CO₂uptake of 2.2 ± 0.1 PgC yr⁻¹from 1994 to 2007, and a net increase in the oceanic carbon inventory of 185 PgC from 1780 to 2020. However, these effects cannot account for the majority of the decadal variability shown in data‐based reconstructions of the ocean CO₂sink over the past 30 years. This implies that decadal variability of the ocean CO₂sink is predominantly driven by changes in ocean circulation or biology that act to redistribute both natural and anthropogenic carbon in the ocean.

    

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