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1. Resolving the Carbon‐Climate Feedback Potential of Wetland CO ₂ and CH ₄ Fluxes in Alaska

   https://doi.org/10.1029/2022GB007524  

   Ma, Shuang; Bloom, A. Anthony; Watts, Jennifer D.; Quetin, Gregory R.; Donatella, Zona; Euskirchen, Eugénie S.; Norton, Alexander J.; Yin, Yi; Levine, Paul A.; Braghiere, Renato K.; et al (September 2023, Global Biogeochemical Cycles)

   Abstract Boreal‐Arctic regions are key stores of organic carbon (C) and play a major role in the greenhouse gas balance of high‐latitude ecosystems. The carbon‐climate (C‐climate) feedback potential of northern high‐latitude ecosystems remains poorly understood due to uncertainty in temperature and precipitation controls on carbon dioxide (CO₂) uptake and the decomposition of soil C into CO₂and methane (CH₄) fluxes. While CH₄fluxes account for a smaller component of the C balance, the climatic impact of CH₄outweighs CO₂(28–34 times larger global warming potential on a 100‐year scale), highlighting the need to jointly resolve the climatic sensitivities of both CO₂and CH₄. Here, we jointly constrain a terrestrial biosphere model with in situ CO₂and CH₄flux observations at seven eddy covariance sites using a data‐model integration approach to resolve the integrated environmental controls on land‐atmosphere CO₂and CH₄exchanges in Alaska. Based on the combined CO₂and CH₄flux responses to climate variables, we find that 1970‐present climate trends will induce positive C‐climate feedback at all tundra sites, and negative C‐climate feedback at the boreal and shrub fen sites. The positive C‐climate feedback at the tundra sites is predominantly driven by increased CH₄emissions while the negative C‐climate feedback at the boreal site is predominantly driven by increased CO₂uptake (80% from decreased heterotrophic respiration, and 20% from increased photosynthesis). Our study demonstrates the need for joint observational constraints on CO₂and CH₄biogeochemical processes—and their associated climatic sensitivities—for resolving the sign and magnitude of high‐latitude ecosystem C‐climate feedback in the coming decades. 

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2. Saturation of Global Terrestrial Carbon Sink Under a High Warming Scenario

   https://doi.org/10.1029/2020GB006800  

   Shi, Hao; Tian, Hanqin; Pan, Naiqing; Reyer, Christopher P. O.; Ciais, Philippe; Chang, Jinfeng; Forrest, Matthew; Frieler, Katja; Fu, Bojie; Gädeke, Anne; et al (October 2021, Global Biogeochemical Cycles)

   Abstract The terrestrial carbon sink provides a critical negative feedback to climate warming, yet large uncertainty exists on its long‐term dynamics. Here we combined terrestrial biosphere models (TBMs) and climate projections, together with climate‐specific land use change, to investigate both the trend and interannual variability (IAV) of the terrestrial carbon sink from 1986 to 2099 under two representative concentration pathways RCP2.6 and RCP6.0. The results reveal a saturation of the terrestrial carbon sink by the end of this century under RCP6.0 due to warming and declined CO₂effects. Compared to 1986–2005 (0.96 ± 0.44 Pg C yr⁻¹), during 2080–2099 the terrestrial carbon sink would decrease to 0.60 ± 0.71 Pg C yr⁻¹but increase to 3.36 ± 0.77 Pg C yr⁻¹, respectively, under RCP2.6 and RCP6.0. The carbon sink caused by CO₂, land use change and climate change during 2080–2099 is −0.08 ± 0.11 Pg C yr⁻¹, 0.44 ± 0.05 Pg C yr⁻¹, and 0.24 ± 0.70 Pg C yr⁻¹under RCP2.6, and 4.61 ± 0.17 Pg C yr⁻¹, 0.22 ± 0.07 Pg C yr⁻¹, and ‐1.47 ± 0.72 Pg C yr⁻¹under RCP6.0. In addition, the carbon sink IAV shows stronger variance under RCP6.0 than RCP2.6. Under RCP2.6, temperature shows higher correlation with the carbon sink IAV than precipitation in most time, which however is the opposite under RCP6.0. These results suggest that the role of terrestrial carbon sink in curbing climate warming would be weakened in a no‐mitigation world in future, and active mitigation efforts are required as assumed under RCP2.6. 

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3. Stronger Arctic amplification produced by decreasing, not increasing, CO ₂ concentrations

   https://doi.org/10.1088/2752-5295/aceea2  

   Zhou, Shih-Ni; Liang, Yu-Chiao; Mitevski, Ivan; Polvani, Lorenzo M (August 2023, Environmental Research: Climate)

   Abstract Arctic amplification (AA), referring to the phenomenon of amplified warming in the Arctic compared to the warming in the rest of the globe, is generally attributed to the increasing concentrations of carbon dioxide (CO₂) in the atmosphere. However, little attention has been paid to the mechanisms and quantitative variations of AA under decreasing levels of CO₂, when cooling where the Arctic region is considerably larger than over the rest of the planet. Analyzing climate model experiments forced with a wide range of CO₂concentrations (from 1/8× to 8×CO₂, with respect to preindustrial levels), we show that AA indeed occurs under decreasing CO₂concentrations, and it is stronger than AA under increasing CO₂concentrations. Feedback analysis reveals that the Planck, lapse-rate, and albedo feedbacks are the main contributors to producing AAs forced by CO₂increase and decrease, but the stronger lapse-rate feedback associated with decreasing CO₂level gives rise to stronger AA. We further find that the increasing CO₂concentrations delay the peak month of AA from November to December or January, depending on the forcing strength. In contrast, decreasing CO₂levels cannot shift the peak of AA earlier than October, as a consequence of the maximum sea-ice increase in September which is independent of forcing strength. Such seasonality changes are also presented in the lapse-rate feedback, but do not appear in other feedbacks nor in the atmospheric and oceanic heat transport processeses. Our results highlight the strongly asymmetric responses of AA, as evidenced by the different changes in its intensity and seasonality, to the increasing and decreasing CO₂concentrations. These findings have significant implications for understanding how carbon removal could impact the Arctic climate, ecosystems, and socio-economic activities. 

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4. Early Cenozoic Decoupling of Climate and Carbonate Compensation Depth Trends

   https://doi.org/10.1029/2019PA003601  

   Greene, S. E.; Ridgwell, A.; Kirtland Turner, S.; Schmidt, D. N.; Pälike, H.; Thomas, E.; Greene, L. K.; Hoogakker, B. A. A. (June 2019, Paleoceanography and Paleoclimatology)

   Abstract Our understanding of the long‐term evolution of the Earth system is based on the assumption that terrestrial weathering rates should respond to, and hence help regulate, atmospheric CO₂and climate. Increased terrestrial weathering requires increased carbonate accumulation in marine sediments, which in turn is expected to result in a long‐term deepening of the carbonate compensation depth (CCD). Here, we critically assess this long‐term relationship between climate and carbon cycling. We generate a record of marine deep‐sea carbonate abundance from selected late Paleocene through early Eocene time slices to reconstruct the position of the CCD. Although our data set allows for a modest CCD deepening, we find no statistically significant change in the CCD despite >3 °C global warming, highlighting the need for additional deep‐sea constraints on carbonate accumulation. Using an Earth system model, we show that the impact of warming and increased weathering on the CCD can be obscured by the opposing influences of ocean circulation patterns and sedimentary respiration of organic matter. From our data synthesis and modeling, we suggest that observations of warming, declining δ¹³C and a relatively stable CCD can be broadly reproduced by mid‐Paleogene increases in volcanic CO₂outgassing and weathering. However, remaining data‐model discrepancies hint at missing processes in our model, most likely involving the preservation and burial of organic carbon. Our finding of a decoupling between the CCD and global marine carbonate burial rates means that considerable care is needed in attempting to use the CCD to directly gauge global carbonate burial rates and hence weathering rates. 

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5. Climate drivers alter nitrogen availability in surface peat and decouple N₂ fixation from CH₄ oxidation in the Sphagnum moss microbiome

   https://doi.org/10.1111/gcb.16651  

   Petro, Caitlin; Carrell, Alyssa A.; Wilson, Rachel M.; Duchesneau, Katherine; Noble‐Kuchera, Sekou; Song, Tianze; Iversen, Colleen M.; Childs, Joanne; Schwaner, Geoff; Chanton, Jeffrey P.; et al (March 2023, Global Change Biology)

   Abstract Peat mosses (Sphagnumspp.) are keystone species in boreal peatlands, where they dominate net primary productivity and facilitate the accumulation of carbon in thick peat deposits.Sphagnummosses harbor a diverse assemblage of microbial partners, including N₂‐fixing (diazotrophic) and CH₄‐oxidizing (methanotrophic) taxa that support ecosystem function by regulating transformations of carbon and nitrogen. Here, we investigate the response of theSphagnumphytobiome (plant + constituent microbiome + environment) to a gradient of experimental warming (+0°C to +9°C) and elevated CO₂(+500 ppm) in an ombrotrophic peatland in northern Minnesota (USA). By tracking changes in carbon (CH₄, CO₂) and nitrogen (NH₄‐N) cycling from the belowground environment up toSphagnumand its associated microbiome, we identified a series of cascading impacts to theSphagnumphytobiome triggered by warming and elevated CO₂. Under ambient CO₂, warming increased plant‐available NH₄‐N in surface peat, excess N accumulated inSphagnumtissue, and N₂fixation activity decreased. Elevated CO₂offset the effects of warming, disrupting the accumulation of N in peat andSphagnumtissue. Methane concentrations in porewater increased with warming irrespective of CO₂treatment, resulting in a ~10× rise in methanotrophic activity withinSphagnumfrom the +9°C enclosures. Warming's divergent impacts on diazotrophy and methanotrophy caused these processes to become decoupled at warmer temperatures, as evidenced by declining rates of methane‐induced N₂fixation and significant losses of keystone microbial taxa. In addition to changes in theSphagnummicrobiome, we observed ~94% mortality ofSphagnumbetween the +0°C and +9°C treatments, possibly due to the interactive effects of warming on N‐availability and competition from vascular plant species. Collectively, these results highlight the vulnerability of theSphagnumphytobiome to rising temperatures and atmospheric CO₂concentrations, with significant implications for carbon and nitrogen cycling in boreal peatlands. 

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