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1. Accelerated river avulsion frequency on lowland deltas due to sea-level rise

   https://doi.org/10.1073/pnas.1912351117  

   Chadwick, Austin J. ; Lamb, Michael P. ; Ganti, Vamsi ( July 2020 , Proceedings of the National Academy of Sciences)

   Sea-level rise, subsidence, and reduced fluvial sediment supply are causing river deltas to drown worldwide, affecting ecosystems and billions of people. Abrupt changes in river course, called avulsions, naturally nourish sinking land with sediment; however, they also create catastrophic flood hazards. Existing observations and models conflict on whether the occurrence of avulsions will change due to relative sea-level rise, hampering the ability to forecast delta response to global climate change. Here, we combined theory, numerical modeling, and field observations to develop a mechanistic framework to predict avulsion frequency on deltas with multiple self-formed lobes that scale with backwater hydrodynamics. Results show that avulsion frequency is controlled by the competition between relative sea-level rise and sediment supply that drives lobe progradation. We find that most large deltas are experiencing sufficiently low progradation rates such that relative sea-level rise enhances aggradation rates—accelerating avulsion frequency and associated hazards compared to preindustrial conditions. Some deltas may face even greater risk; if relative sea-level rise significantly outpaces sediment supply, then avulsion frequency is maximized, delta plains drown, and avulsion locations shift inland, posing new hazards to upstream communities. Results indicate that managed deltas can support more frequent engineered avulsions to recover sinking land; however, theremore »is a threshold beyond which coastal land will be lost, and mitigation efforts should shift upstream.

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2. Microbial Growth and Organic Matter Cycling in the Pacific Ocean Along a Latitudinal Transect Between Subarctic and Subantarctic Waters

   https://doi.org/10.3389/fmars.2021.764383  

   Giebel, Helge-Ansgar ; Arnosti, Carol ; Badewien, Thomas H. ; Bakenhus, Insa ; Balmonte, John Paul ; Billerbeck, Sara ; Dlugosch, Leon ; Henkel, Rohan ; Kuerzel, Birgit ; Meyerjürgens, Jens ; et al ( December 2021 , Frontiers in Marine Science)

   The Pacific Ocean constitutes about half of the global oceans and thus microbial processes in this ocean have a large impact on global elemental cycles. Despite several intensely studied regions large areas are still greatly understudied regarding microbial activities, organic matter cycling and biogeography. Refined information about these features is most important to better understand the significance of this ocean for global biogeochemical and elemental cycles. Therefore we investigated a suite of microbial and geochemical variables along a transect from the subantarctic to the subarctic Pacific in the upper 200 m of the water column. The aim was to quantify rates of organic matter processing, identify potential controlling factors and prokaryotic key players. The assessed variables included abundance of heterotrophic prokaryotes and cyanobacteria, heterotrophic prokaryotic production (HPP), turnover rate constants of amino acids, glucose, and acetate, leucine aminopeptidase and β-glucosidase activities, and the composition of the bacterial community by fluorescence in situ hybridization (FISH). The additional quantification of nitrate, dissolved amino acids and carbohydrates, chlorophyll a , particulate organic carbon and nitrogen (POC, PON) provided a rich environmental context. The oligotrophic gyres exhibited the lowest prokaryotic abundances, rates of HPP and substrate turnover. Low nucleic acid prokaryotes dominated in thesemore »gyres, whereas in temperate and subpolar regions further north and south, high nucleic acid prokaryotes dominated. Turnover rate constants of glucose and acetate, as well as leucine aminopeptidase activity, increased from (sub)tropical toward the subpolar regions. In contrast, HPP and bulk growth rates were highest near the equatorial upwelling and lowest in the central gyres and subpolar regions. The SAR11 clade, the Roseobacter group and Flavobacteria constituted the majority of the prokaryotic communities. Vertical profiles of the biogeochemical and microbial variables markedly differed among the different regions and showed close covariations of the microbial variables and chlorophyll a , POC and PON. The results show that hydrographic, microbial, and biogeochemical properties exhibited distinct patterns reflecting the biogeographic provinces along the transect. The microbial variables assessed contribute to a better and refined understanding of the scales of microbial organic matter processing in large areas of the epipelagic Pacific beyond its well-studied regions.« less
3. Pronounced and unavoidable impacts of low-end global warming on northern high-latitude land ecosystems

   https://doi.org/10.1088/1748-9326/ab702b  

   Ito, Akihiko ; Reyer, Christopher P. O. ; Gädeke, Anne ; Ciais, Philippe ; Chang, Jinfeng ; Chen, Min ; François, Louis ; Forrest, Matthew ; Hickler, Thomas ; Ostberg, Sebastian ; et al ( March 2020 , Environmental Research Letters)

   Arctic ecosystems are particularly vulnerable to climate change because of Arctic amplification. Here, we assessed the climatic impacts of low-end, 1.5 °C, and 2.0 °C global temperature increases above pre-industrial levels, on the warming of terrestrial ecosystems in northern high latitudes (NHL, above 60 °N including pan-Arctic tundra and boreal forests) under the framework of the Inter-Sectoral Impact Model Intercomparison Project phase 2b protocol. We analyzed the simulated changes of net primary productivity, vegetation biomass, and soil carbon stocks of eight ecosystem models that were forced by the projections of four global climate models and two atmospheric greenhouse gas pathways (RCP2.6 and RCP6.0). Our results showed that considerable impacts on ecosystem carbon budgets, particularly primary productivity and vegetation biomass, are very likely to occur in the NHL areas. The models agreed on increases in primary productivity and biomass accumulation, despite considerable inter-model and inter-scenario differences in the magnitudes of the responses. The inter-model variability highlighted the inadequacies of the present models, which fail to consider important components such as permafrost and wildfire. The simulated impacts were attributable primarily to the rapid temperature increases in the NHL and the greater sensitivity of northern vegetation to warming, which contrasted with themore »less pronounced responses of soil carbon stocks. The simulated increases of vegetation biomass by 30–60 Pg C in this century have implications for climate policy such as the Paris Agreement. Comparison between the results at two warming levels showed the effectiveness of emission reductions in ameliorating the impacts and revealed unavoidable impacts for which adaptation options are urgently needed in the NHL ecosystems.

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4. Variation in Fine Root Characteristics and Nutrient Dynamics Across Alaskan Ecosystems

   https://doi.org/10.1007/s10021-020-00583-8  

   McCulloch, Lindsay A. ; Kropp, Heather ; Kholodov, Alexander ; Cardelús, Catherine L. ; Natali, Susan M. ; Loranty, Michael M. ( November 2020 , Ecosystems)

   Carbon cycle perturbations in high-latitude ecosystems associated with rapid warming can have implications for the global climate. Belowground biomass is an important component of the carbon cycle in these ecosystems, with, on average, significantly more vegetation biomass belowground than aboveground. Large quantities of dead root biomass are also in these ecosystems owing to slow decomposition rates. Current understanding of how live and dead root biomass carbon pools vary across highlatitude ecosystems and the environmental conditions associated with this variation is limited due to the labor- and time-intensive nature of data collection. To that end, we examined patterns and factors (abiotic and biotic) associated with the variation in live and dead fine root biomass (FRB) and FRB carbon (C), nitrogen (N) and phosphorus concentrations for 23 sites across a latitudinal gradient in Alaska, spanning both boreal forest and tundra biomes. We found no difference in the live or dead FRB variables between these biomes, despite large differences in predominant vegetation types, except for significantly higher live FRB C:N ratios in boreal sites. Soil C:N ratio, moisture, and temperature, along with moss cover, explained a substantial portion of the dead:live FRB ratio variability across sites. We find all these factors have negativemore »relationships with dead FRB, while having positive or no relationship with live FRB. This work demonstrates that FRB does not necessarily correlate with aboveground vegetation characteristics, and it highlights the need for finer-scale measurements of abiotic and biotic factors to understand FRB landscape variability now and into the future.« less
5. Calibration of temperature-dependent ocean microbial processes in the cGENIE.muffin (v0.9.13) Earth system model

   https://doi.org/10.5194/gmd-14-125-2021  

   Crichton, Katherine A. ; Wilson, Jamie D. ; Ridgwell, Andy ; Pearson, Paul N. ( January 2021 , Geoscientific Model Development)

   Abstract. Temperature is a master parameter in the marine carbon cycle, exerting a critical control on the rate of biological transformation of a variety of solid and dissolved reactants and substrates. Although in the construction of numerical models of marine carbon cycling, temperature has been long recognised as a key parameter in the production and export of organic matter at the ocean surface, its role in the ocean interior is much less frequently accounted for. There, bacteria (primarily) transform sinking particulate organic matter (POM) into its dissolved constituents and consume dissolved oxygen (and/or other electron acceptors such as sulfate). The nutrients and carbon thereby released then become available for transport back to the surface, influencing biological productivity and atmospheric pCO2, respectively. Given the substantial changes in ocean temperature occurring in the past, as well as in light of current anthropogenic warming, appropriately accounting for the role of temperature in marine carbon cycling may be critical to correctly projecting changes in ocean deoxygenation and the strength of feedbacks on atmosphericpCO2. Here we extend and calibrate a temperature-dependent representation ofmarine carbon cycling in the cGENIE.muffin Earth system model, intended forboth past and future climate applications. In this, we combine atemperature-dependent remineralisation schememore »for sinking organic matterwith a biological export production scheme that also includes a dependenceon ambient seawater temperature. Via a parameter ensemble, we jointlycalibrate the two parameterisations by statistically contrasting model-projected fields of nutrients, oxygen, and the stable carbon isotopicsignature (δ13C) of dissolved inorganic carbon in the oceanwith modern observations. We additionally explore the role of temperature inthe creation and recycling of dissolved organic matter (DOM) and hence itsimpact on global carbon cycle dynamics. We find that for the present day, the temperature-dependent version showsa fit to the data that is as good as or better than the existing tuned non-temperature-dependent version of the cGENIE.muffin. The main impact ofaccounting for temperature-dependent remineralisation of POM is in drivinghigher rates of remineralisation in warmer waters, in turn driving a morerapid return of nutrients to the surface and thereby stimulating organicmatter production. As a result, more POM is exported below 80 m but onaverage reaches shallower depths in middle- and low-latitude warmer waterscompared to the standard model. Conversely, at higher latitudes, colderwater temperature reduces the rate of nutrient resupply to the surface andPOM reaches greater depth on average as a result of slower subsurface ratesof remineralisation. Further adding temperature-dependent DOM processeschanges this overall picture only a little, with a slight weakening ofexport production at higher latitudes. As an illustrative application of the new model configuration andcalibration, we take the example of historical warming and briefly assessthe implications for global carbon cycling of accounting for a more completeset of temperature-dependent processes in the ocean. We find that betweenthe pre-industrial era (ca. 1700) and the present (year 2010), in response to asimulated air temperature increase of 0.9 ∘C and an associatedprojected mean ocean warming of 0.12 ∘C (0.6 ∘C insurface waters and 0.02 ∘C in deep waters), a reduction inparticulate organic carbon (POC) export at 80 m of just 0.3 % occurs (or 0.7 % including a temperature-dependent DOM response). However, due to this increased recycling nearer the surface, the efficiency of the transfer of carbon away from the surface (at 80 m) to the deep ocean (at 1040 m) is reduced by 5 %. In contrast, with no assumed temperature-dependent processes impacting production or remineralisation of either POM or DOM, global POC export at 80 m falls by 2.9 % between the pre-industrial era and the present day as a consequence of ocean stratification and reduced nutrient resupply to the surface. Our analysis suggests that increased temperature-dependent nutrient recycling in the upper ocean has offset much of the stratification-induced restriction in its physical transport.« less