BG - Latest Articles

Analyzing precipitationsheds to understand the vulnerability of rainfall dependent regions

2012-02-10T00:00:00+01:00

Analyzing precipitationsheds to understand the vulnerability of rainfall dependent regions

Biogeosciences, 9, 733-746, 2012

Author(s): P. W. Keys, R. J. van der Ent, L. J. Gordon, H. Hoff, R. Nikoli, and H. H. G. Savenije

It is well known that rivers connect upstream and downstream ecosystems within watersheds. Here we describe the concept of precipitationsheds to show how upwind terrestrial evaporation source areas contribute moisture for precipitation to downwind sink regions. We illustrate the importance of upwind land cover in precipitationsheds to sustain precipitation in critically water stressed downwind areas, specifically dryland agricultural areas. We first identify seven regions where rainfed agriculture is particularly vulnerable to reductions in precipitation, and then map their precipitationsheds. We then develop a framework for qualitatively assessing the vulnerability of precipitation for these seven agricultural regions. We illustrate that the sink regions have varying degrees of vulnerability to changes in upwind evaporation rates depending on the extent of the precipitationshed, source region land use intensity and expected land cover changes in the source region.

Latitudinal differences in the amplitude of the OAE-2 carbon isotopic excursion: pCO₂ and paleo productivity

2012-02-09T00:00:00+01:00

Latitudinal differences in the amplitude of the OAE-2 carbon isotopic excursion: pCO₂ and paleo productivity

Biogeosciences, 9, 717-731, 2012

Author(s): E. C. van Bentum, G.-J. Reichart, A. Forster, and J. S. Sinninghe Damsté

A complete, well-preserved record of the Cenomanian/Turonian (C/T) Oceanic Anoxic Event 2 (OAE-2) was recovered from Demerara Rise in the southern North Atlantic Ocean (ODP site 1260). Across this interval, we determined changes in the stable carbon isotopic composition of sulfur-bound phytane (δ¹³C_phytane), a biomarker for photosynthetic algae. The δ¹³C_phytane record shows a positive excursion at the onset of the OAE-2 interval, with an unusually large amplitude (~7‰) compared to existing C/T proto-North Atlantic δ¹³C_phytane records (3–6‰). Overall, the amplitude of the excursion of δ¹³C_phytane decreases with latitude. Using reconstructed sea surface temperature (SST) gradients for the proto-North Atlantic, we investigated environmental factors influencing the latitudinal δ¹³C_phytane gradient. The observed gradient is best explained by high productivity at DSDP Site 367 and Tarfaya basin before OAE-2, which changed in overall high productivity throughout the proto-North Atlantic during OAE-2. During OAE-2, productivity at site 1260 and 603B was thus more comparable to the mid-latitude sites. Using these constraints as well as the SST and δ¹³C_phytane-records from Site 1260, we subsequently reconstructed pCO₂ levels across the OAE-2 interval. Accordingly, pCO₂ decreased from ca. 1750 to 900 ppm during OAE-2, consistent with enhanced organic matter burial resulting in lowering pCO₂. Whereas the onset of OAE-2 coincided with increased pCO₂, in line with a volcanic trigger for this event, the observed cooling within OAE-2 probably resulted from CO₂ sequestration in black shales outcompeting CO₂ input into the atmosphere. Together these results show that the ice-free Cretaceous world was sensitive to changes in pCO₂ related to perturbations of the global carbon cycle.

Corrigendum to "Effects of climate variability and functional changes on the interannual variation of the carbon balance in a temperate deciduous forest" published in Biogeosciences, 9, 13–28, 2012

2012-02-08T00:00:00+01:00

Corrigendum to "Effects of climate variability and functional changes on the interannual variation of the carbon balance in a temperate deciduous forest" published in Biogeosciences, 9, 13–28, 2012

Biogeosciences, 9, 715-715, 2012

Author(s): J. Wu, L. van der Linden, G. Lasslop, N. Carvalhais, K. Pilegaard, C. Beier, and A. Ibrom

No abstract available.

Detection of open water dynamics with ENVISAT ASAR in support of land surface modelling at high latitudes

2012-02-08T00:00:00+01:00

Detection of open water dynamics with ENVISAT ASAR in support of land surface modelling at high latitudes

Biogeosciences, 9, 703-714, 2012

Author(s): A. Bartsch, A. M. Trofaier, G. Hayman, D. Sabel, S. Schlaffer, D. B. Clark, and E. Blyth

Wetlands are generally accepted as being the largest but least well quantified single source of methane (CH₄). The extent of wetland or inundation is a key factor controlling methane emissions, both in nature and in the parameterisations used in large-scale land surface and climate models. Satellite-derived datasets of wetland extent are available on the global scale, but the resolution is rather coarse (>25 km). The purpose of the present study is to assess the capability of active microwave sensors to derive inundation dynamics for use in land surface and climate models of the boreal and tundra environments. The focus is on synthetic aperture radar (SAR) operating in C-band since, among microwave systems, it has comparably high spatial resolution and data availability, and long-term continuity is expected.

C-band data from ENVISAT ASAR (Advanced SAR) operating in wide swath mode (150 m resolution) were investigated and an automated detection procedure for deriving open water fraction has been developed. More than 4000 samples (single acquisitions tiled onto 0.5° grid cells) have been analysed for July and August in 2007 and 2008 for a study region in Western Siberia. Simple classification algorithms were applied and found to be robust when the water surface was smooth. Modification of input parameters results in differences below 1 % open water fraction. The major issue to address was the frequent occurrence of waves due to wind and precipitation, which reduces the separability of the water class from other land cover classes. Statistical measures of the backscatter distribution were applied in order to retrieve suitable classification data. The Pearson correlation between each sample dataset and a location specific representation of the bimodal distribution was used. On average only 40 % of acquisitions allow a separation of the open water class. Although satellite data are available every 2–3 days over the Western Siberian study region, the irregular acquisition intervals and periods of unsuitable weather suggest that an update interval of 10 days is more realistic for this domain. SAR data availability is currently limited. Future satellite missions, however, which aim for operational services (such as Sentinel-1 with its C-band SAR instrument), may provide the basis for inundation monitoring for land surface and climate modelling applications.

Chemodiversity of a Scots pine stand and implications for terpene air concentrations

2012-02-06T00:00:00+01:00

Chemodiversity of a Scots pine stand and implications for terpene air concentrations

Biogeosciences, 9, 689-702, 2012

Author(s): J. Bäck, J. Aalto, M. Henriksson, H. Hakola, Q. He, and M. Boy

Atmospheric chemistry in background areas is strongly influenced by natural vegetation. Coniferous forests are known to produce large quantities of volatile vapors, especially terpenes. These compounds are reactive in the atmosphere, and contribute to the formation and growth of atmospheric new particles. Our aim was to analyze the variability of mono- and sesquiterpene emissions between Scots pine trees, in order to clarify the potential errors caused by using emission data obtained from only a few trees in atmospheric chemistry models. We also aimed at testing if stand history and seed origin has an influence on the chemotypic diversity. The inherited, chemotypic variability in mono- and sesquiterpene emission was studied in a seemingly homogeneous 48 yr-old stand in Southern Finland, where two areas differing in their stand regeneration history could be distinguished. Sampling was conducted in August 2009. Terpene concentrations in the air had been measured at the same site for seven years prior to branch sampling for chemotypes. Two main compounds, α-pinene and Δ³-carene formed together 40–97% of the monoterpene proportions in both the branch emissions and in the air concentrations. The data showed a bimodal distribution in emission composition, in particular in Δ³-carene emission within the studied population. 10% of the trees emitted mainly α-pinene and no Δ³-carene at all, whereas 20% of the trees where characterized as high Δ³-carene emitters (Δ³-carene forming >80% of total emitted monoterpene spectrum). An intermediate group of trees emitted equal amounts of both α-pinene and Δ³-carene. The emission pattern of trees at the area established using seeding as the artificial regeneration method differed from the naturally regenerated or planted trees, being mainly high Δ³-carene emitters. Some differences were also seen in e.g. camphene and limonene emissions between chemotypes, but sesquiterpene emissions did not differ significantly between trees. The atmospheric concentrations at the site were found to reflect the species and/or chemodiversity rather than the emissions measured from any single tree, and were strongly dominated by α-pinene. We also tested the effect of chemodiversity on modeled monoterpene concentrations at the site and found out that since it significantly influences the distributions and hence the chemical reactions in the atmosphere, it should be taken into account in atmospheric modeling.

A comparison of biogenic iron quotas during a diatom spring bloom using multiple approaches

2012-02-03T00:00:00+01:00

A comparison of biogenic iron quotas during a diatom spring bloom using multiple approaches

Biogeosciences, 9, 667-687, 2012

Author(s): A. L. King, S. A. Sañudo-Wilhelmy, P. W. Boyd, B. S. Twining, S. W. Wilhelm, C. Breene, M. J. Ellwood, and D. A. Hutchins

Biogenic Fe quotas were determined using three distinct techniques on samples collected concurrently in the subtropical Pacific Ocean east of New Zealand. Fe quotas were measured using radioisotope uptake experiments (24 h incubation), bulk filtration and analysis by inductively-coupled plasma mass spectrometer (ICPMS), and single-cell synchrotron x-ray fluorescence (SXRF) analysis over a sixteen-day period (year days 263 to 278 of 2008) during a quasi-Lagrangian drifter experiment that tracked the evolution of the annual spring diatom bloom within a counter-clockwise open-ocean eddy. Overall, radioisotope uptake-determined Fe quotas (washed with oxalate reagent to remove extracellular Fe) were the lowest (0.5–1.0 mmol Fe:mol P; 4–8 μmol Fe:mol C), followed by single-cell Fe quotas (2.3–7.5 mmol Fe:mol P; 17–57 μmol Fe:mol C), and the highest and most variable quotas were from the bulk filtration ICPMS approach that used the oxalate reagent wash, corrected for lithogenic Fe using Al (0.8–21 mmol Fe:mol P; 4–136 μmol Fe:mol C). During the evolution of the spring bloom within the eddy (year days 263 to 272), the surface mixed layer inventories of particulate biogenic elements (C, N, P, Si) and chlorophyll increased while Fe quotas estimated from all three approaches exhibited a general decline. After the onset of the bloom decline, the drogued buoys exited the eddy center (days 273 to 277). Fe quotas returned to pre-bloom values during this part of the study. Our standardized and coordinated sampling protocols reveal the general observed trend in Fe quotas: ICPMS > SXRF > radioisotope uptake. We discuss the inherent differences between the techniques and argue that each technique has its individual merits and uniquely contributes to the characterization of the oceanic particulate Fe pool.

Estimating the near-surface permafrost-carbon feedback on global warming

2012-02-03T00:00:00+01:00

Estimating the near-surface permafrost-carbon feedback on global warming

Biogeosciences, 9, 649-665, 2012

Author(s): T. Schneider von Deimling, M. Meinshausen, A. Levermann, V. Huber, K. Frieler, D. M. Lawrence, and V. Brovkin

Thawing of permafrost and the associated release of carbon constitutes a positive feedback in the climate system, elevating the effect of anthropogenic GHG emissions on global-mean temperatures. Multiple factors have hindered the quantification of this feedback, which was not included in climate carbon-cycle models which participated in recent model intercomparisons (such as the Coupled Carbon Cycle Climate Model Intercomparison Project – C⁴MIP) . There are considerable uncertainties in the rate and extent of permafrost thaw, the hydrological and vegetation response to permafrost thaw, the decomposition timescales of freshly thawed organic material, the proportion of soil carbon that might be emitted as carbon dioxide via aerobic decomposition or as methane via anaerobic decomposition, and in the magnitude of the high latitude amplification of global warming that will drive permafrost degradation. Additionally, there are extensive and poorly characterized regional heterogeneities in soil properties, carbon content, and hydrology. Here, we couple a new permafrost module to a reduced complexity carbon-cycle climate model, which allows us to perform a large ensemble of simulations. The ensemble is designed to span the uncertainties listed above and thereby the results provide an estimate of the potential strength of the feedback from newly thawed permafrost carbon. For the high CO₂ concentration scenario (RCP8.5), 33–114 GtC (giga tons of Carbon) are released by 2100 (68 % uncertainty range). This leads to an additional warming of 0.04–0.23 °C. Though projected 21st century permafrost carbon emissions are relatively modest, ongoing permafrost thaw and slow but steady soil carbon decomposition means that, by 2300, about half of the potentially vulnerable permafrost carbon stock in the upper 3 m of soil layer (600–1000 GtC) could be released as CO₂, with an extra 1–4 % being released as methane. Our results also suggest that mitigation action in line with the lower scenario RCP3-PD could contain Arctic temperature increase sufficiently that thawing of the permafrost area is limited to 9–23 % and the permafrost-carbon induced temperature increase does not exceed 0.04–0.16 °C by 2300.

Sensitivity analysis of the GEMS soil organic carbon model to land cover land use classification uncertainties under different climate scenarios in senegal

2012-02-03T00:00:00+01:00

Sensitivity analysis of the GEMS soil organic carbon model to land cover land use classification uncertainties under different climate scenarios in senegal

Biogeosciences, 9, 631-648, 2012

Author(s): A. M. Dieye, D. P. Roy, N. P. Hanan, S. Liu, M. Hansen, and A. Touré

Spatially explicit land cover land use (LCLU) change information is needed to drive biogeochemical models that simulate soil organic carbon (SOC) dynamics. Such information is increasingly being mapped using remotely sensed satellite data with classification schemes and uncertainties constrained by the sensing system, classification algorithms and land cover schemes. In this study, automated LCLU classification of multi-temporal Landsat satellite data were used to assess the sensitivity of SOC modeled by the Global Ensemble Biogeochemical Modeling System (GEMS). The GEMS was run for an area of 1560 km² in Senegal under three climate change scenarios with LCLU maps generated using different Landsat classification approaches. This research provides a method to estimate the variability of SOC, specifically the SOC uncertainty due to satellite classification errors, which we show is dependent not only on the LCLU classification errors but also on where the LCLU classes occur relative to the other GEMS model inputs.

Carbon dioxide emissions from an Acacia plantation on peatland in Sumatra, Indonesia

2012-02-01T00:00:00+01:00

Carbon dioxide emissions from an Acacia plantation on peatland in Sumatra, Indonesia

Biogeosciences, 9, 617-630, 2012

Author(s): J. Jauhiainen, A. Hooijer, and S. E. Page

Peat surface CO₂ emission, groundwater table depth and peat temperature were monitored for two years along transects in an Acacia plantation on thick tropical peat (>4 m) in Sumatra, Indonesia. A total of 2300 emission measurements were taken at 144 locations, over a 2 year period. The autotrophic root respiration component of CO₂ emission was separated from heterotrophic emission caused by peat oxidation in three ways: (i) by comparing CO₂ emissions within and beyond the tree rooting zone, (ii) by comparing CO₂ emissions with and without peat trenching (i.e. cutting any roots remaining in the peat beyond the tree rooting zone), and (iii) by comparing CO₂ emissions before and after Acacia tree harvesting. On average, the contribution of autotrophic respiration to daytime CO₂ emission was 21% along transects in mature tree stands. At locations 0.5 m from trees this was up to 80% of the total emissions, but it was negligible at locations more than 1.3 m away. This means that CO₂ emission measurements well away from trees were free of any autotrophic respiration contribution and thus represent only heterotrophic emissions. We found daytime mean annual CO₂ emission from peat oxidation alone of 94 t ha⁻¹ y⁻¹ at a mean water table depth of 0.8 m, and a minimum emission value of 80 t ha⁻¹ y⁻¹ after correction for the effect of diurnal temperature fluctuations, which may result in a 14.5% reduction of the daytime emission. There is a positive correlation between mean long-term water table depth and peat oxidation CO₂ emission. However, no such relation is found for instantaneous emission/water table depth within transects and it is clear that factors other than water table depth also affect peat oxidation and total CO₂ emissions. The increase in the temperature of the surface peat due to plantation establishment may explain over 50% of peat oxidation emissions. Our study sets a standard for greenhouse gas flux studies from tropical peatlands under different forms of agricultural land management. It is the first to purposefully quantify heterotrophic CO₂ emissions resulting from tropical peat decomposition by separating these from autotrophic emissions. It also provides the most scientifically- and statistically-rigorous study to date of CO₂ emissions resulting from anthropogenic modification of this globally significant carbon rich ecosystem. Our findings indicate that past studies have underestimated emissions from peatland plantations, with important implications for the scale of greenhouse gas emissions arising from land use change, particularly in the light of current, rapid agricultural conversion of peatlands in the Southeast Asian region.

Localising the nitrogen imprint of the Paris food supply: the potential of organic farming and changes in human diet

2012-01-31T00:00:00+01:00

Localising the nitrogen imprint of the Paris food supply: the potential of organic farming and changes in human diet

Biogeosciences, 9, 607-616, 2012

Author(s): G. Billen, J. Garnier, V. Thieu, M. Silvestre, S. Barles, and P. Chatzimpiros

The Seine watershed has long been the food-supplying hinterland of Paris, providing most of the animal and vegetal protein consumed in the city. Nowadays, the shift from manure-based to synthetic nitrogen fertilisation, has made possible a strong land specialisation of agriculture in the Seine watershed: it still provides most of the cereal consumed by the Paris agglomeration, but exports 80% of its huge cereal production. On the other hand the meat and milk supply originates mainly from regions in the North and West of France, specialised in animal farming and importing about 30% of their feed from South America. As it works today, this system is responsible for a severe nitrate contamination of surface and groundwater resources. Herein two scenarios of re-localising Paris's food supply are explored, based on organic farming and local provision of animal feed. We show that for the Seine watershed it is technically possible to design an agricultural system able to provide all the plant- and animal-based food required by the population, to deliver sub-root water meeting the drinking water standards and still to export a significant proportion of its production to areas less suitable for cereal cultivation. Decreasing the share of animal products in the human diet has a strong impact on the nitrogen imprint of urban food supply.

Effect of mosaic representation of vegetation in land surface schemes on simulated energy and carbon balances

2012-01-31T00:00:00+01:00

Effect of mosaic representation of vegetation in land surface schemes on simulated energy and carbon balances

Biogeosciences, 9, 593-605, 2012

Author(s): R. Li and V. K. Arora

Energy and carbon balance implications of representing vegetation using a composite or mosaic approach in a land surface scheme are investigated. In the composite approach the attributes of different plant functional types (PFTs) present in a grid cell are aggregated in some fashion for energy and water balance calculations. The resulting physical environmental conditions (including net radiation, soil moisture and soil temperature) are common to all PFTs and affect their ecosystem processes. In the mosaic approach energy and water balance calculations are performed separately for each PFT tile using its own vegetation attributes, so each PFT "sees" different physical environmental conditions and its carbon balance evolves somewhat differently from that in the composite approach. Simulations are performed at selected boreal, temperate and tropical locations to illustrate the differences caused by using the composite versus mosaic approaches of representing vegetation. These idealized simulations use 50% fractional coverage for each of the two dominant PFTs in a grid cell. Differences in simulated grid averaged primary energy fluxes at selected sites are generally less than 5% between the two approaches. Simulated grid-averaged carbon fluxes and pool sizes at these sites can, however, differ by as much as 46%. Simulation results suggest that differences in carbon balance between the two approaches arise primarily through differences in net radiation which directly affects net primary productivity, and thus leaf area index and vegetation biomass.

Water-table height and microtopography control biogeochemical cycling in an Arctic coastal tundra ecosystem

2012-01-31T00:00:00+01:00

Water-table height and microtopography control biogeochemical cycling in an Arctic coastal tundra ecosystem

Biogeosciences, 9, 577-591, 2012

Author(s): D. A. Lipson, D. Zona, T. K. Raab, F. Bozzolo, M. Mauritz, and W. C. Oechel

Drained thaw lake basins (DTLB's) are the dominant land form of the Arctic Coastal Plain in northern Alaska. The presence of continuous permafrost prevents drainage and so water tables generally remain close to the soil surface, creating saturated, suboxic soil conditions. However, ice wedge polygons produce microtopographic variation in these landscapes, with raised areas such as polygon rims creating more oxic microenvironments. The peat soils in this ecosystem store large amounts of organic carbon which is vulnerable to loss as arctic regions continue to rapidly warm, and so there is great motivation to understand the controls over microbial activity in these complex landscapes. Here we report the effects of experimental flooding, along with seasonal and spatial variation in soil chemistry and microbial activity in a DTLB. The flooding treatment generally mirrored the effects of natural landscape variation in water-table height due to microtopography. The flooded portion of the basin had lower dissolved oxygen, lower oxidation-reduction potential (ORP) and higher pH, as did lower elevation areas throughout the entire basin. Similarly, soil pore water concentrations of organic carbon and aromatic compounds were higher in flooded and low elevation areas. Dissolved ferric iron (Fe(III)) concentrations were higher in low elevation areas and responded to the flooding treatment in low areas, only. The high concentrations of soluble Fe(III) in soil pore water were explained by the presence of siderophores, which were much more concentrated in low elevation areas. All the aforementioned variables were correlated, showing that Fe(III) is solubilized in response to anoxic conditions. Dissolved carbon dioxide (CO₂) and methane (CH₄) concentrations were higher in low elevation areas, but showed only subtle and/or seasonally dependent effects of flooding. In anaerobic laboratory incubations, more CH₄ was produced by soils from low and flooded areas, whereas anaerobic CO₂ production only responded to flooding in high elevation areas. Seasonal changes in the oxidation state of solid phase Fe minerals showed that net Fe reduction occurred, especially in topographically low areas. The effects of Fe reduction were also seen in the topographic patterns of pH, as protons were consumed where this process was prevalent. This suite of results can all be attributed to the effect of water table on oxygen availability: flooded conditions promote anoxia, stimulating dissolution and reduction of Fe(III), and to some extent, methanogenesis. However, two lines of evidence indicated the inhibition of methanogenesis by alternative e- acceptors such as Fe(III) and humic substances: (1) ratios of CO₂:CH₄ evolved from anaerobic soil incubations and dissolved in soil pore water were high; (2) CH₄ concentrations were negatively correlated with the oxidation state of the soluble Fe pool in both topographically high and low areas. A second set of results could be explained by increased soil temperature in the flooding treatment, which presumably arose from the increased thermal conductivity of the soil surface: higher N mineralization rates and dissolved P concentrations were observed in flooded areas. Overall, these results could have implications for C and nutrient cycling in high Arctic areas where warming and flooding are likely consequences of climate change.

Plant-driven variation in decomposition rates improves projections of global litter stock distribution

2012-01-30T00:00:00+01:00

Plant-driven variation in decomposition rates improves projections of global litter stock distribution

Biogeosciences, 9, 565-576, 2012

Author(s): V. Brovkin, P. M. van Bodegom, T. Kleinen, C. Wirth, W. K. Cornwell, J. H. C. Cornelissen, and J. Kattge

Plant litter stocks are critical, regionally for their role in fueling fire regimes and controlling soil fertility, and globally through their feedback to atmospheric CO₂ and climate. Here we employ two global databases linking plant functional types to decomposition rates of wood and leaf litter (Cornwell et al., 2008; Weedon et al., 2009) to improve future projections of climate and carbon cycle using an intermediate complexity Earth System model. Implementing separate wood and leaf litter decomposabilities and their temperature sensitivities for a range of plant functional types yielded a more realistic distribution of litter stocks in all present biomes with the exception of boreal forests and projects a strong increase in global litter stocks by 35 Gt C and a concomitant small decrease in atmospheric CO₂ by 3 ppm by the end of this century. Despite a relatively strong increase in litter stocks, the modified parameterization results in less elevated wildfire emissions because of a litter redistribution towards more humid regions.

Diagenetic alterations of amino acids and organic matter in the upper Pearl River Estuary surface sediments

2012-01-27T00:00:00+01:00

Diagenetic alterations of amino acids and organic matter in the upper Pearl River Estuary surface sediments

Biogeosciences, 9, 555-564, 2012

Author(s): J. Zhang, R. Zhang, Q. Wu, and N. Xu

The objective of this study was to investigate the diagenetic alteration of sediment organic matter (OM) in the upper Pearl River Estuary. Sediment analyses were conducted for three size fractions of OM, including coarse particulate OM (CPOM), fine particulate OM (FPOM), and ultrafiltered dissolved OM (UDOM). Results showed that the highest and lowest carbon (C): nitrogen (N) ratios were in CPOM and UDOM, respectively, indicating that CPOM was relatively enriched in organic C. The highest average total N content in the FPOM fraction showed that FPOM was enriched in N-containing molecules. Our study showed that the "size-reactivity continuum" model was applicable to sediment particulate and dissolved OM. Distributions of amino acids and their D-isomers among the sediment fractions indicated that the amino acid-based diagenetic index, C:N ratio, and percentage of total N represented by total hydrolysable amino acids could be used as diagenetic indicators. Furthermore, the diagenetic state of sediment OM could also be characterized by C- and N-normalized yields of total D-amino acids, and C- and N-normalized yields of D-alanine, D-glutamic acid, and D-serine.

Biomass burning emissions estimated with a global fire assimilation system based on observed fire radiative power

2012-01-27T00:00:00+01:00

Biomass burning emissions estimated with a global fire assimilation system based on observed fire radiative power

Biogeosciences, 9, 527-554, 2012

Author(s): J. W. Kaiser, A. Heil, M. O. Andreae, A. Benedetti, N. Chubarova, L. Jones, J.-J. Morcrette, M. Razinger, M. G. Schultz, M. Suttie, and G. R. van der Werf

The Global Fire Assimilation System (GFASv1.0) calculates biomass burning emissions by assimilating Fire Radiative Power (FRP) observations from the MODIS instruments onboard the Terra and Aqua satellites. It corrects for gaps in the observations, which are mostly due to cloud cover, and filters spurious FRP observations of volcanoes, gas flares and other industrial activity. The combustion rate is subsequently calculated with land cover-specific conversion factors. Emission factors for 40 gas-phase and aerosol trace species have been compiled from a literature survey. The corresponding daily emissions have been calculated on a global 0.5^° × 0.5^° grid from 2003 to the present. General consistency with the Global Fire Emission Database version 3.1 (GFED3.1) within its accuracy is achieved while maintaining the advantages of an FRP-based approach: GFASv1.0 makes use of the quantitative information on the combustion rate that is contained in the FRP observations, and it detects fires in real time at high spatial and temporal resolution. GFASv1.0 indicates omission errors in GFED3.1 due to undetected small fires. It also exhibits slightly longer fire seasons in South America and North Africa and a slightly shorter fire season in Southeast Asia. GFASv1.0 has already been used for atmospheric reactive gas simulations in an independent study, which found good agreement with atmospheric observations. We have performed simulations of the atmospheric aerosol distribution with and without the assimilation of MODIS aerosol optical depth (AOD). They indicate that the emissions of particulate matter need to be boosted by a factor of 2–4 to reproduce the global distribution of organic matter and black carbon. This discrepancy is also evident in the comparison of previously published top-down and bottom-up estimates. For the time being, a global enhancement of the particulate matter emissions by 3.4 is recommended. Validation with independent AOD and PM₁₀ observations recorded during the Russian fires in summer 2010 show that the global Monitoring Atmospheric Composition and Change (MACC) aerosol model with GFASv1.0 aerosol emissions captures the smoke plume evolution well when organic matter and black carbon are enhanced by the recommended factor. In conjunction with the assimilation of MODIS AOD, the use of GFASv1.0 with enhanced emission factors quantitatively improves the forecast of the aerosol load near the surface sufficiently to allow air quality warnings with a lead time of up to four days.

The impacts of climate, land use, and demography on fires during the 21st century simulated by CLM-CN

2012-01-26T00:00:00+01:00

The impacts of climate, land use, and demography on fires during the 21st century simulated by CLM-CN

Biogeosciences, 9, 509-525, 2012

Author(s): S. Kloster, N. M. Mahowald, J. T. Randerson, and P. J. Lawrence

Landscape fires during the 21st century are expected to change in response to multiple agents of global change. Important controlling factors include climate controls on the length and intensity of the fire season, fuel availability, and fire management, which are already anthropogenically perturbed today and are predicted to change further in the future. An improved understanding of future fires will contribute to an improved ability to project future anthropogenic climate change, as changes in fire activity will in turn impact climate.

In the present study we used a coupled-carbon-fire model to investigate how changes in climate, demography, and land use may alter fire emissions. We used climate projections following the SRES A1B scenario from two different climate models (ECHAM5/MPI-OM and CCSM) and changes in population. Land use and harvest rates were prescribed according to the RCP 45 scenario. In response to the combined effect of all these drivers, our model estimated, depending on our choice of climate projection, an increase in future (2075–2099) fire carbon emissions by 17 and 62% compared to present day (1985–2009). The largest increase in fire emissions was predicted for Southern Hemisphere South America for both climate projections. For Northern Hemisphere Africa, a region that contributed significantly to the global total fire carbon emissions, the response varied between a decrease and an increase depending on the climate projection.

We disentangled the contribution of the single forcing factors to the overall response by conducting an additional set of simulations in which each factor was individually held constant at pre-industrial levels. The two different projections of future climate change evaluated in this study led to increases in global fire carbon emissions by 22% (CCSM) and 66% (ECHAM5/MPI-OM). The RCP 45 projection of harvest and land use led to a decrease in fire carbon emissions by −5%. The RCP 26 and RCP 60 harvest and landuse projections caused decreases around −20%. Changes in human ignition led to an increase of 20%. When we also included changes in fire management efforts to suppress fires in densely populated areas, global fire carbon emission decreased by −6% in response to changes in population density.

We concluded from this study that changes in fire emissions in the future are controlled by multiple interacting factors. Although changes in climate led to an increase in future fire emissions this could be globally counterbalanced by coupled changes in land use, harvest, and demography.

Biogeochemical factors affecting mercury methylation rate in two contaminated floodplain soils

2012-01-26T00:00:00+01:00

Biogeochemical factors affecting mercury methylation rate in two contaminated floodplain soils

Biogeosciences, 9, 493-507, 2012

Author(s): T. Frohne, J. Rinklebe, U. Langer, G. Du Laing, S. Mothes, and R. Wennrich

An automated biogeochemical microcosm system allowing controlled variation of redox potential (E_H) in soil suspensions was used to assess the effect of various factors on the mobility of mercury (Hg) as well as on the methylation of Hg in two contaminated floodplain soils with different Hg concentrations (approximately 5 mg Hg kg⁻¹ and >30 mg Hg kg^–1). The experiment was conducted under stepwise variation from reducing (approximately −350 mV at pH 5) to oxidizing conditions (approximately 600 mV at pH 5). Results of phospholipid fatty acids (PLFA) analysis indicate the occurrence of sulfate reducing bacteria (SRB) such as Desulfobacter species (10Me16:0, cy17:0, 10Me18:0, cy19:0) or Desulfovibrio species (18:2ω6,9), which are considered to promote Hg methylation. The products of the methylation process are lipophilic, highly toxic methyl mercury species such as the monomethyl mercury ion [MeHg⁺], which is named as MeHg here. The ln(MeHg/Hg_t) ratio is assumed to reflect the net production of monomethyl mercury normalized to total dissolved Hg (Hg_t) concentration. This ratio increases with rising dissolved organic carbon (DOC) to Hg_t ratio (ln(DOC/Hg_t) ratio) (R² = 0.39, p<0.0001, n= 63) whereas the relation between ln(MeHg/Hg_t ratio and lnDOC is weaker (R² = 0.09; p<0.05; n = 63). In conclusion, the DOC/Hg_t ratio might be a more important factor for the Hg net methylation than DOC alone in the current study. Redox variations seem to affect the biogeochemical behavior of dissolved inorganic Hg species and MeHg indirectly through related changes in DOC, sulfur cycle, and microbial community structure whereas E_H and pH values, as well as concentration of dissolved Fe³⁺/Fe²⁺ and Cl^– seem to play subordinate roles in Hg mobilization and methylation under our experimental conditions.

Climate impacts on the structures of the North Pacific air-sea CO₂ flux variability

2012-01-25T00:00:00+01:00

Climate impacts on the structures of the North Pacific air-sea CO₂ flux variability

Biogeosciences, 9, 477-492, 2012

Author(s): V. Valsala, S. Maksyutov, M. Telszewski, S. Nakaoka, Y. Nojiri, M. Ikeda, and R. Murtugudde

Some dominant spatial and temporal structures of the North Pacific air-sea CO₂ fluxes in response to the Pacific Decadal Oscillation (PDO) are identified in three data products from three independent sources: an assimilated CO₂ flux product and two forward model solutions. The interannual variability of CO₂ flux is found to be an order of magnitude weaker compared to the seasonal cycle of CO₂ flux in the North Pacific. A statistical approach is employed to quantify the signal-to-noise ratio in the reconstructed dataset to delineate the representativity errors. The dominant variability with a signal-to-noise ratio above one is identified and its correlations with PDO are examined. A tentative four-pole pattern in the North Pacific air-sea CO₂ flux variability linked to PDO emerges in which two positively correlated poles are oriented in the northwest and southeast directions and contrarily, the negatively correlated poles are oriented in the northeast and southwest directions. This pattern is identified in three products, providing CO₂ and pCO₂. Its relations to the interannual El Niño-Southern Oscillation (ENSO) and lower-frequency PDO are separately identified. A combined EOF analysis between air-sea CO₂ flux and key variables representing ocean-atmosphere interactions is carried out to elicit robust oscillations in the North Pacific CO₂ flux in response to the PDO. The proposed spatial and temporal structures of the North Pacific CO₂ fluxes are insightful since they separate the secular trends of the surface ocean carbon from the interannual variability. The regional characterization of the North Pacific in terms of PDO and CO₂ flux variability is also instructive for determining the homogeneous oceanic domains for the Regional Carbon Cycle and Assessment Processes (RECCAP).

North American CO₂ exchange: inter-comparison of modeled estimates with results from a fine-scale atmospheric inversion

2012-01-25T00:00:00+01:00

North American CO₂ exchange: inter-comparison of modeled estimates with results from a fine-scale atmospheric inversion

Biogeosciences, 9, 457-475, 2012

Author(s): S. M. Gourdji, K. L. Mueller, V. Yadav, D. N. Huntzinger, A. E. Andrews, M. Trudeau, G. Petron, T. Nehrkorn, J. Eluszkiewicz, J. Henderson, D. Wen, J. Lin, M. Fischer, C. Sweeney, and A. M. Michalak

Atmospheric inversion models have the potential to quantify CO₂ fluxes at regional, sub-continental scales by taking advantage of near-surface CO₂ mixing ratio observations collected in areas with high flux variability. This study presents results from a series of regional geostatistical inverse models (GIM) over North America for 2004, and uses them as the basis for an inter-comparison to other inversion studies and estimates from biospheric models collected through the North American Carbon Program Regional and Continental Interim Synthesis. Because the GIM approach does not require explicit prior flux estimates and resolves fluxes at fine spatiotemporal scales (i.e. 1° × 1°, 3-hourly in this study), it avoids temporal and spatial aggregation errors and allows for the recovery of realistic spatial patterns from the atmospheric data relative to previous inversion studies. Results from a GIM inversion using only available atmospheric observations and a fine-scale fossil fuel inventory were used to confirm the quality of the inventory and inversion setup. An inversion additionally including auxiliary variables from the North American Regional Reanalysis found inferred relationships with flux consistent with physiological understanding of the biospheric carbon cycle. Comparison of GIM results with bottom-up biospheric models showed stronger agreement during the growing relative to the dormant season, in part because most of the biospheric models do not fully represent agricultural land-management practices and the fate of both residual biomass and harvested products. Comparison to earlier inversion studies pointed to aggregation errors as a likely source of bias in previous sub-continental scale flux estimates, particularly for inversions that adjust fluxes at the coarsest scales and use atmospheric observations averaged over long periods. Finally, whereas the continental CO₂ boundary conditions used in the GIM inversions have a minor impact on spatial patterns, they have a substantial impact on the continental carbon budget, with a difference of 0.8 PgC yr⁻¹ in the total continental flux resulting from the use of two plausible sets of boundary CO₂ mixing ratios. Overall, this inter-comparison study helps to assess the state of the science in estimating regional-scale CO₂ fluxes, while pointing towards the path forward for improvements in future top-down and bottom-up modeling efforts.

Modelling LAI, surface water and carbon fluxes at high-resolution over France: comparison of ISBA-A-gs and ORCHIDEE

2012-01-25T00:00:00+01:00

Modelling LAI, surface water and carbon fluxes at high-resolution over France: comparison of ISBA-A-gs and ORCHIDEE

Biogeosciences, 9, 439-456, 2012

Author(s): S. Lafont, Y. Zhao, J.-C. Calvet, P. Peylin, P. Ciais, F. Maignan, and M. Weiss

The Leaf Area Index (LAI) is a measure of the amount of photosynthetic leaves and governs the canopy conductance to water vapor and carbon dioxide. Four different estimates of LAI were compared over France: two LAI products derived from satellite remote sensing, and two LAI simulations derived from land surface modelling. The simulated LAI was produced by the ISBA-A-gs model and by the ORCHIDEE model (developed by CNRM-GAME and by IPSL, respectively), for the 1994–2007 period. The two models were driven by the same atmospheric variables and used the same land cover map (SAFRAN and ECOCLIMAP-II, respectively). The MODIS and CYCLOPES satellite LAI products were used. Both products were available from 2000 to 2007 and this relatively long period allowed to investigate the interannual and the seasonal variability of monthly LAI values. In particular the impact of the 2003 and 2005 droughts were analyzed. The two models presented contrasting results, with a difference of one month between the average leaf onset dates simulated by the two models, and a maximum interannual variability of LAI simulated at springtime by ORCHIDEE and at summertime by ISBA-A-gs. The comparison with the satellite LAI products showed that, in general, the seasonality was better represented by ORCHIDEE, while ISBA-A-gs tended to better represent the interannual variability, especially for grasslands. While the two models presented comparable values of net carbon fluxes, ORCHIDEE simulated much higher photosynthesis rates than ISBA-A-gs (+70%), while providing lower transpiration estimates (−8%).

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Biogeochemical factors affecting mercury methylation rate in two contaminated floodplain soils

Climate impacts on the structures of the North Pacific air-sea CO2 flux variability

North American CO2 exchange: inter-comparison of modeled estimates with results from a fine-scale atmospheric inversion

Modelling LAI, surface water and carbon fluxes at high-resolution over France: comparison of ISBA-A-gs and ORCHIDEE

Latitudinal differences in the amplitude of the OAE-2 carbon isotopic excursion: pCO₂ and paleo productivity

Climate impacts on the structures of the North Pacific air-sea CO₂ flux variability

North American CO₂ exchange: inter-comparison of modeled estimates with results from a fine-scale atmospheric inversion