AGW Observer

Observations of anthropogenic global warming

New research – carbon cycle (August 9, 2016)

Posted by Ari Jokimäki on August 9, 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

Sources of uncertainty in future projections of the carbon cycle (Hewitt et al. 2016) http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-16-0161.1

Abstract: The inclusion of carbon cycle processes within CMIP5 Earth System Models provides the opportunity to explore the relative importance of differences in scenario and climate model representation to future land and ocean carbon fluxes. A two-way ANOVA approach was used to quantify the variability owing to differences between scenarios and between climate models at different lead times.

For global ocean carbon fluxes, the variance attributed to differences between Representative Concentration Pathway scenarios exceeds the variance attributed to differences between climate models by around 2025, completely dominating by 2100. This contrasts with global land carbon fluxes, where the variance attributed to differences between climate models continues to dominate beyond 2100. This suggests that modelled processes that determine ocean fluxes are currently better constrained than those of land fluxes, thus we can be more confident in linking different future socio-economic pathways to consequences of ocean carbon uptake than for land carbon uptake. The contribution of internal variance is negligible for ocean fluxes and small for land fluxes, indicating that there is little dependence on the initial conditions.

The apparent agreement in atmosphere-ocean carbon fluxes, globally, masks strong climate model differences at a regional level. The North Atlantic and Southern Ocean are key regions, where differences in modelled processes represent an important source of variability in projected regional fluxes.

Rapid carbon loss and slow recovery following permafrost thaw in boreal peatlands (Jones et al. 2016) http://onlinelibrary.wiley.com/doi/10.1111/gcb.13403/abstract

Abstract: Permafrost peatlands store one-third of the total carbon (C) in the atmosphere and are increasingly vulnerable to thaw as high latitude temperatures warm. Large uncertainties remain about C dynamics following permafrost thaw in boreal peatlands. We used a chronosequence approach to measure C stocks in forested permafrost plateaus (forest) and thawed permafrost bogs, ranging in thaw age from young (100 years) in two Interior Alaska chronosequences. Permafrost originally aggraded simultaneously with peat accumulation (syngenetic permafrost) at both sites. We found that upon thaw, C loss of the forest peat C is equivalent to ~30% of the initial forest C stock and is directly proportional to the pre-thaw C stocks. Our model results indicate that permafrost thaw turned these peatlands into net C sources to the atmosphere for a decade following thaw, after which post-thaw bog peat accumulation returned sites to net C sinks. It can take multiple centuries to millennia for a site to recover its pre-thaw C stocks; the amount of time needed for them to regain their pre-thaw C stocks is governed by the amount of C that accumulated prior to thaw. Consequently, these findings show that older peatlands will take longer to recover pre-thaw C stocks, whereas younger peatlands will exceed pre-thaw stocks in a matter of centuries. We conclude that the loss of sporadic and discontinuous permafrost by 2100 could result in a loss of up to 24 Pg of deep C from permafrost peatlands.

Ectomycorrhizal fungi slow soil carbon cycling (Averill & Hawkes, 2016) http://onlinelibrary.wiley.com/doi/10.1111/ele.12631/abstract

Abstract: Respiration of soil organic carbon is one of the largest fluxes of CO2 on earth. Understanding the processes that regulate soil respiration is critical for predicting future climate. Recent work has suggested that soil carbon respiration may be reduced by competition for nitrogen between symbiotic ectomycorrhizal fungi that associate with plant roots and free-living microbial decomposers, which is consistent with increased soil carbon storage in ectomycorrhizal ecosystems globally. However, experimental tests of the mycorrhizal competition hypothesis are lacking. Here we show that ectomycorrhizal roots and hyphae decrease soil carbon respiration rates by up to 67% under field conditions in two separate field exclusion experiments, and this likely occurs via competition for soil nitrogen, an effect larger than 2 °C soil warming. These findings support mycorrhizal competition for nitrogen as an independent driver of soil carbon balance and demonstrate the need to understand microbial community interactions to predict ecosystem feedbacks to global climate.

Brazil’s Amazonian forest carbon: the key to Southern Amazonia’s significance for global climate (Fearnside, 2016) http://link.springer.com/article/10.1007%2Fs10113-016-1007-2

Abstract: Southern Amazonia is the first region of Brazil’s Amazon area to be exposed to intensive conversion to agriculture and ranching. This conversion emits greenhouse gases from the carbon stock in the biomass and soils of the previous vegetation. Quantifying these carbon stocks is the first step in quantifying the impact on global warming from this conversion. This review is limited to information on Brazilian Amazonia’s carbon stocks. It indicates large amounts of carbon at risk of emission in both biomass and soils, as well as considerable uncertainty in estimates. Reducing uncertainty is a priority for research but the existence of uncertainty must not be used as an excuse for delaying measures to contain deforestation. The magnitude of carbon stocks is proportional to greenhouse gas emissions per hectare of deforestation and consequently to impact on global climate.

Long-term drainage reduces CO2 uptake and increases CO2 emission on a Siberian floodplain due to shifts in vegetation community and soil thermal characteristics (Kwon et al. 2016) http://www.biogeosciences.net/13/4219/2016/

Abstract: With increasing air temperatures and changing precipitation patterns forecast for the Arctic over the coming decades, the thawing of ice-rich permafrost is expected to increasingly alter hydrological conditions by creating mosaics of wetter and drier areas. The objective of this study is to investigate how 10 years of lowered water table depths of wet floodplain ecosystems would affect CO2 fluxes measured using a closed chamber system, focusing on the role of long-term changes in soil thermal characteristics and vegetation community structure. Drainage diminishes the heat capacity and thermal conductivity of organic soil, leading to warmer soil temperatures in shallow layers during the daytime and colder soil temperatures in deeper layers, resulting in a reduction in thaw depths. These soil temperature changes can intensify growing-season heterotrophic respiration by up to 95 %. With decreased autotrophic respiration due to reduced gross primary production under these dry conditions, the differences in ecosystem respiration rates in the present study were 25 %. We also found that a decade-long drainage installation significantly increased shrub abundance, while decreasing Eriophorum angustifolium abundance resulted in Carex sp. dominance. These two changes had opposing influences on gross primary production during the growing season: while the increased abundance of shrubs slightly increased gross primary production, the replacement of E. angustifolium by Carex sp. significantly decreased it. With the effects of ecosystem respiration and gross primary production combined, net CO2 uptake rates varied between the two years, which can be attributed to Carex-dominated plots’ sensitivity to climate. However, underlying processes showed consistent patterns: 10 years of drainage increased soil temperatures in shallow layers and replaced E. angustifolium by Carex sp., which increased CO2 emission and reduced CO2 uptake rates. During the non-growing season, drainage resulted in 4 times more CO2 emissions, with high sporadic fluxes; these fluxes were induced by soil temperatures, E. angustifolium abundance, and air pressure.

Other papers

Quantifying Peat Carbon Accumulation in Alaska Using a Process-Based Biogeochemistry Model (Wang et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016JG003452/abstract

Patterns of carbon processing at the seafloor: the role of faunal and microbial communities in moderating carbon flows (Woulds et al. 2016) http://www.biogeosciences.net/13/4343/2016/

Informing climate models with rapid chamber measurements of forest carbon uptake (Metcalfe et al. 2016) http://onlinelibrary.wiley.com/doi/10.1111/gcb.13451/abstract

Direct and indirect effects of climatic variations on the interannual variability in net ecosystem exchange across terrestrial ecosystems (Shao et al. 2016) http://www.tellusb.net/index.php/tellusb/article/view/30575

Decadal and long-term boreal soil carbon and nitrogen sequestration rates across a variety of ecosystems (Manies et al. 2016) http://www.biogeosciences.net/13/4315/2016/

Hotspots of gross emissions from the land use sector: patterns, uncertainties, and leading emission sources for the period 2000–2005 in the tropics (Roman-Cuesta et al. 2016) http://www.biogeosciences.net/13/4253/2016/

Large net CO2 loss from a grass-dominated tropical savanna in south-central Brazil in response to seasonal and interannual drought (De Arruda et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016JG003404/abstract

How much CO2 is taken up by the European terrestrial biosphere? (Reuter et al. 2016) http://journals.ametsoc.org/doi/abs/10.1175/BAMS-D-15-00310.1

Coastal-ocean uptake of anthropogenic carbon (Bourgeois et al. 2016) http://www.biogeosciences.net/13/4167/2016/

Drivers of atmospheric methane uptake by montane forest soils in the southern Peruvian Andes (Jones et al. 2016) http://www.biogeosciences.net/13/4151/2016/

Persistent high temperature and low precipitation reduce peat carbon accumulation (Delarue, 2016) http://onlinelibrary.wiley.com/doi/10.1111/gcb.13433/abstract

Methane oxidation in contrasting soil types: responses to experimental warming with implication for landscape-integrated CH4 budget (D’Imperio et al. 2016) http://onlinelibrary.wiley.com/doi/10.1111/gcb.13400/abstract

Seeing the forest not for the carbon: why concentrating on land-use-induced carbon stock changes of soils in Brazil can be climate-unfriendly (Boy et al. 2016) http://link.springer.com/article/10.1007%2Fs10113-016-1008-1

Stability of grassland soil C and N pools despite 25 years of an extreme climatic and disturbance regime (Wilcox et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016JG003370/abstract

Regional carbon fluxes from land use and land cover change in Asia, 1980–2009 (Calle et al. 2016) http://iopscience.iop.org/article/10.1088/1748-9326/11/7/074011/meta

Carbon cycle responses of semi-arid ecosystems to positive asymmetry in rainfall (Haverd et al. 2016) http://onlinelibrary.wiley.com/doi/10.1111/gcb.13412/abstract

Impact of increasing inflow of warm Atlantic water on the sea-air exchange of carbon dioxide and methane in the Laptev Sea (Wåhlström et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2015JG003307/abstract

Air-sea exchange of carbon dioxide in the Southern Ocean and Antarctic marginal ice zone (Butterworth & Miller, 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016GL069581/abstract

Earlier snowmelt reduces atmospheric carbon uptake in mid-latitude subalpine forests (Winchell et al. 2016) http://onlinelibrary.wiley.com/doi/10.1002/2016GL069769/abstract

Four decades of modeling methane cycling in terrestrial ecosystems (Xu et al. 2016) http://www.biogeosciences.net/13/3735/2016/

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