AGW Observer

Observations of anthropogenic global warming

New research – carbon cycle (September 12, 2016)

Posted by Ari Jokimäki on September 12, 2016

Some of the latest papers on carbon cycle are shown below. First a few highlighted papers with abstracts and then a list of some other papers. If this subject interests you, be sure to check also the other papers – they are by no means less interesting than the highlighted ones.

Highlights

Methane emissions proportional to permafrost carbon thawed in Arctic lakes since the 1950s (Anthony et al. 2016) http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2795.html

Abstract: Permafrost thaw exposes previously frozen soil organic matter to microbial decomposition. This process generates methane and carbon dioxide, and thereby fuels a positive feedback process that leads to further warming and thaw. Despite widespread permafrost degradation during the past ~40 years, the degree to which permafrost thaw may be contributing to a feedback between warming and thaw in recent decades is not well understood. Radiocarbon evidence of modern emissions of ancient permafrost carbon is also sparse. Here we combine radiocarbon dating of lake bubble trace-gas methane (113 measurements) and soil organic carbon (289 measurements) for lakes in Alaska, Canada, Sweden and Siberia with numerical modelling of thaw and remote sensing of thermokarst shore expansion. Methane emissions from thermokarst areas of lakes that have expanded over the past 60 years were directly proportional to the mass of soil carbon inputs to the lakes from the erosion of thawing permafrost. Radiocarbon dating indicates that methane age from lakes is nearly identical to the age of permafrost soil carbon thawing around them. Based on this evidence of landscape-scale permafrost carbon feedback, we estimate that 0.2 to 2.5 Pg permafrost carbon was released as methane and carbon dioxide in thermokarst expansion zones of pan-Arctic lakes during the past 60 years.

Rising Plant-mediated Methane Emissions from Arctic Wetlands (Andresen et al. 2016) http://onlinelibrary.wiley.com/doi/10.1111/gcb.13469/abstract

Abstract: Plant-mediated CH4 flux is an important pathway for land-atmosphere CH4 emissions but the magnitude, timing, and environmental controls, spanning scales of space and time, remain poorly understood in arctic tundra wetlands, particularly under the long term effects of climate change. CH4 fluxes were measured in situ during peak growing season for the dominant aquatic emergent plants in the Alaskan arctic coastal plain, Carex aquatilis and Arctophila fulva, to assess the magnitude and species-specific controls on CH4 flux. Plant biomass was a strong predictor of A. fulva CH4 flux while water depth and thaw depth were co-predictors for C. aquatilis CH4 flux. We used plant and environmental data from 1971-72 from the historic International Biological Program (IBP) research site near Barrow, Alaska, which we resampled in 2010-13, to quantify changes in plant biomass and thaw depth, and used these to estimate species-specific decadal-scale changes in CH4 fluxes. A ~60% increase in CH4 flux was estimated from the observed plant biomass and thaw depth increases in tundra ponds over the past 40 years. Despite covering only ~5% of the landscape, we estimate that aquatic C. aquatilis and A. fulva account for two-thirds of the total regional CH4 flux of the Barrow Peninsula. The regionally observed increases in plant biomass and active layer thickening over the past 40 years not only have major implications for energy and water balance, but have significantly altered land-atmosphere CH4 emissions for this region, potentially acting as a positive feedback to climate warming.

Enhanced carbon export to the abyssal depths driven by atmosphere dynamics (Pedrosa-Pàmies et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016GL069781/abstract

Abstract: Long-term biogeochemical observations are critical to understand the natural ability of the oceans to fix CO2 into organic carbon and export it to the deep as sinking particles. Here we present results from a 3 year (2010–2013) sediment trap deployment that allowed detecting interannual variations of carbon fluxes beyond 4000 m depth in the Eastern Mediterranean Sea. Anomalous atmospheric conditions triggering strong heat losses in winter–spring 2012 resulted in convective mixing, nutrient uplifting, and a diatom-dominated bloom southeast of Crete. Phytoplankton growth, reinforced by the arrival of nutrients from airborne Etna volcano ash, was the highest in the last decade (satellite-derived Chl a concentrations up to 1.9 mg m−3). This situation caused carbon export to increase by 2 orders of magnitude (12.2 mg m−2 d−1) with respect to typical values, which demonstrates how pulses of sinking fresh phytodetritus linked to rare atmospheric processes can episodically impact one of the most oligotrophic environments in the world ocean.

Partitioning uncertainty in ocean carbon uptake projections: Internal variability, emission scenario, and model structure (Lovenduski et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016GB005426/abstract

Abstract: We quantify and isolate the sources of projection uncertainty in annual-mean sea-air CO2 flux over the period 2006–2080 on global and regional scales using output from two sets of ensembles with the Community Earth System Model (CESM) and models participating in the 5th Coupled Model Intercomparison Project (CMIP5). For annual-mean, globally-integrated sea-air CO2 flux, uncertainty grows with prediction lead time and is primarily attributed to uncertainty in emission scenario. At the regional scale of the California Current System, we observe relatively high uncertainty that is nearly constant for all prediction lead times, and is dominated by internal climate variability and model structure, respectively in the CESM and CMIP5 model suites. Analysis of CO2 flux projections over 17 biogeographical biomes reveals a spatially heterogenous pattern of projection uncertainty. On the biome scale, uncertainty is driven by a combination of internal climate variability and model structure, with emission scenario emerging as the dominant source for long projection lead times in both modeling suites.

The sensitivity of soil respiration to soil temperature, moisture, and carbon supply at the global scale (Hursh et al. 2016) http://onlinelibrary.wiley.com/doi/10.1111/gcb.13489/abstract

Abstract: Soil respiration (Rs) is a major pathway by which fixed carbon in the biosphere is returned to the atmosphere, yet there are limits to our ability to predict respiration rates using environmental drivers at the global scale. While temperature, moisture, carbon supply and other site characteristics are known to regulate soil respiration rates at plot scales within certain biomes, quantitative frameworks for evaluating the relative importance of these factors across different biomes and at the global scale require tests of the relationships between field estimates and global climatic data. This study evaluates the factors driving Rs at the global scale by linking global datasets of soil moisture, soil temperature, primary productivity and soil carbon estimates with observations of annual Rs from the Global Soil Respiration Database (SRDB). We find that calibrating models with parabolic soil moisture functions can improve predictive power over similar models with asymptotic functions of mean annual precipitation. Soil temperature is comparable with previously-reported air temperature observations used in predicting Rs, and is the dominant driver of Rs in global models; however, within certain biomes soil moisture or soil carbon emerge as dominant predictors of Rs. We identify regions where typical temperature-driven responses are further mediated by soil moisture, precipitation, and carbon supply and regions in which environmental controls on high Rs values are difficult to ascertain due to limited field data. Because soil moisture integrates temperature and precipitation dynamics, it can more directly constrain the heterotrophic component of Rs, but global-scale models tend to smooth its spatial heterogeneity by aggregating factors that increase moisture variability within and across biomes. We compare statistical and mechanistic models that provide independent estimates of global Rs ranging from 83 to 108 Pg/yr, but also highlight regions of uncertainty where more observations are required or environmental controls are hard to constrain.

Other papers

Methane and carbon dioxide fluxes of a temperate mire in Central Europe (Fortuniak et al. 2016) http://www.sciencedirect.com/science/article/pii/S0168192316303781

Airborne methane remote measurements reveal heavy-tail flux distribution in Four Corners region (Frankenberg et al. 2016) http://www.pnas.org/content/113/35/9734.short

Greenhouse gas emissions from natural ecosystems and agricultural lands in sub-Saharan Africa: synthesis of available data and suggestions for further research (Kim et al. 2016) http://www.biogeosciences.net/13/4789/2016/

Peak season carbon exchange shifts from a sink to a source following 50+ years of herbivore exclusion in an Arctic tundra ecosystem (Lara et al. 2016) http://onlinelibrary.wiley.com/doi/10.1111/1365-2745.12654/abstract

Vegetation carbon sequestration in Chinese forests from 2010 to 2050 (He et al. 2016) http://onlinelibrary.wiley.com/doi/10.1111/gcb.13479/abstract

CH4 concentrations over the Amazon from GOSAT consistent with in situ vertical profile data (Webb et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016JD025263/abstract

CH4 exchanges of the natural ecosystems in China during the past three decades: the role of wetland extent and its dynamics (Wei & Wang, 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016JG003418/abstract

Mesoscale modulation of air-sea CO2 flux in Drake Passage (Song et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016JC011714/abstract

Biomass turnover time in terrestrial ecosystems halved by land use (Erb et al. 2016) http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2782.html

Permafrost carbon as a missing link to explain CO 2 changes during the last deglaciation (Crichton et al. 2016) http://www.nature.com/ngeo/journal/vaop/ncurrent/full/ngeo2793.html

High export via small particles before the onset of the North Atlantic spring bloom (Giering et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016JC012048/abstract

Inorganic carbon cycling and biogeochemical processes in an Arctic inland sea (Hudson Bay) (Burt et al. 2016) http://www.biogeosciences.net/13/4659/2016/

Constrained partitioning of autotrophic and heterotrophic respiration reduces model uncertainties of forest ecosystem carbon fluxes but not stocks (Carbone et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016JG003386/abstract

Century-long increasing trend and variability of dissolved organic carbon export from the Mississippi River basin driven by natural and anthropogenic forcing (Ren et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016GB005395/abstract

Apparent winter CO2 uptake by a boreal forest due to decoupling (Jocher et al. 2016) http://www.sciencedirect.com/science/article/pii/S0168192316303495

Over-estimating climate warming-induced methane gas escape from the seafloor by neglecting multi-phase flow dynamics (Stranne et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016GL070049/abstract

Strong regional atmospheric 14C signature of respired CO2 observed from a tall tower over the mid-western United States (LaFranchi et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2015JG003271/abstract

Underestimation of boreal soil carbon stocks by mathematical soil carbon models linked to soil nutrient status (Ťupek et al. 2016) http://www.biogeosciences.net/13/4439/2016/

Methane Emissions from global rice fields: Magnitude, spatio-temporal patterns and environmental controls (Zhang et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016GB005381/abstract

Modeling pCO2 variability in the Gulf of Mexico (Xue et al. 2016) http://www.biogeosciences.net/13/4359/2016/

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