3,745 research outputs found
Hypotheses for near-surface exchange of methane on Mars
The Curiosity rover recently detected a background of 0.7 ppb and spikes of 7
ppb of methane on Mars. This in situ measurement reorients our understanding of
the Martian environment and its potential for life, as the current theories do
not entail any geological source or sink of methane that varies sub-annually.
In particular, the 10-fold elevation during the southern winter indicates
episodic sources of methane that are yet to be discovered. Here we suggest a
near-surface reservoir could explain this variability. Using the temperature
and humidity measurements from the rover, we find that perchlorate salts in the
regolith deliquesce to form liquid solutions, and deliquescence progresses to
deeper subsurface in the season of the methane spikes. We therefore formulate
the following three testable hypotheses. The first scenario is that the
regolith in Gale Crater adsorbs methane when dry and releases this methane to
the atmosphere upon deliquescence. The adsorption energy needs to be 36 kJ/mol
to explain the magnitude of the methane spikes, higher than existing laboratory
measurements. The second scenario is that microorganisms convert organic matter
in the soil to methane when they are in liquid solutions. This scenario does
not require regolith adsorption, but entails extant life on Mars. The third
scenario is that deep subsurface aquifers produce the bursts of methane.
Continued in situ measurements of methane and water, as well as laboratory
studies of adsorption and deliquescence, will test these hypotheses and inform
the existence of the near-surface reservoir and its exchange with the
atmosphere.Comment: Accepted for publication on Astrobiolog
Target mass corrections and twist-3 in the nucleon spin structure functions
The Nachtmann moment is employed to study the contribution of twist-3
operator to the nucleon spin structure functions. Target mass corrections to
the Cornwall-Norton moments of the spin structure functions are
discussed. It is found that the corrections play a sizeable role to the
contribution of the twist-3 extracted from the Cornwall-Norton
moments.Comment: 11 pages, 1 figure
Detection of fossil fuel emission trends in the presence of natural carbon cycle variability
Atmospheric COâ observations have the potential to monitor regional fossil fuel emission (FFCOâ) changes to support carbon mitigation efforts such as the Paris Accord, but they must contend with the confounding impacts of the natural carbon cycle. Here, we quantify trend detection time and magnitude in gridded total COâ fluxesâthe sum of FFCOâ and natural carbon fluxesâunder an idealized assumption that monthly total COâ fluxes can be perfectly resolved at a 2°Ă2° resolution. Using Coupled Model Intercomparison Project 5 (CMIP5) 'business-as-usual' emission scenarios to represent FFCOâ and simulated net biome exchange (NBE) to represent natural carbon fluxes, we find that trend detection time for the total COâ fluxes at such a resolution has a median of 10 years across the globe, with significant spatial variability depending on FFCOâ magnitude and NBE variability. Differences between trends in the total COâ fluxes and the underlying FFCOâ component highlight the role of natural carbon cycle variability in modulating regional detection of FFCOâ emission trends using COâ observations alone, particularly in the tropics and subtropics where mega-cities with large populations are developing rapidly. Using COâ estimates alone at such a spatiotemporal resolution can only quantify fossil fuel trends in a few placesâmostly limited to arid regions. For instance, in the Middle East, FFCOâ can explain more than 75% of the total COâ trends in ~70% of the grids, but only ~20% of grids in China can meet such criteria. Only a third of the 25 megacities we analyze here show total COâ trends that are primarily explained (>75%) by FFCOâ. Our analysis provides a theoretical baseline at a global scale for the design of regional FFCOâ monitoring networks and underscores the importance of estimating biospheric interannual variability to improve the accuracy of FFCOâ trend monitoring. We envision that this can be achieved with a fully integrated carbon cycle assimilation system with explicit constraints on FFCOâ and NBE, respectively
Satellite Based Estimation of Global Biogenic Methane Emissions
Atmospheric CH4 is derived from both natural and anthropogenic sources, and the
rapid increase in atmospheric CH4 levels over the past two centuries has predominantly
been a result of increased anthropogenic emissions. Nonetheless, natural sources have
also changed as a result of global change, and quantifying the fluxes of CH4 from these
sources, and their associated climatic feedbacks, is of paramount importance. In this
thesis I have developed a method to upscale the global CH4 emissions from UV irradiation
of foliar pectin (chapter 2). I have quantified the magnitude and distribution
of CH4 emissions from wetlands on a global scale and determined the sensitivity of
wetlands to temporal changes in water volume and temperature (chapters 3 and 4). Finally
I determine that tropical wetland organic matter decomposition on a global scale
behaves non-linearly over seasonal timescales. This implies a substantially different
seasonality in CH4 emissions from wetlands (chapter 5). I show that (i) satellites such
as MODIS and GRACE can be used to improve the understanding of individual CH4
sources and sinks, and (ii) the newly available satellite observations of CH4 can be effectively
used for more than constraining atmospheric chemistry and transport model
inversions. Moreover, the work shown in this thesis has contributed new biogenic CH4
source estimates, but has also posed new questions which will ultimately help guide
new projects in the atmospheric CH4 research area
Large-Scale Controls of Methanogenesis Inferred from Methane and Gravity Spaceborne Data
Wetlands are the largest individual source of methane (CH_4), but the magnitude and distribution of this source are poorly understood on continental scales. We isolated the wetland and rice paddy contributions to spaceborne CH_4 measurements over 2003â2005 using satellite observations of gravity anomalies, a proxy for water-table depth Î, and surface temperature analyses T_S. We find that tropical and higher-latitude CH_4 variations are largely described by Î and T_S variations, respectively. Our work suggests that tropical wetlands contribute 52 to 58% of global emissions, with the remainder coming from the extra-tropics, 2% of which is from Arctic latitudes. We estimate a 7% rise in wetland CH_4 emissions over 2003â2007, due to warming of mid-latitude and Arctic wetland regions, which we find is consistent with recent changes in atmospheric CH_4
Remote-sensing constraints on South America fire traits by Bayesian fusion of atmospheric and surface data
Satellite observations reveal substantial burning during the 2007 and 2010 tropical South America fire season, with both years exhibiting similar total burned area. However, 2010 CO fire emissions, based on satellite CO concentration measurements, were substantially lower (â28%), despite the onceâinâaâcentury drought in 2010. We use Bayesian inference with satellite measurements of CH_4 and CO concentrations and burned area to quantify shifts in combustion characteristics in 2010 relative to 2007. We find an 88% probability in reduced combusted biomass density associated with the 2010 fires and an 82% probability of lower fire carbon losses in 2010 relative to 2007. Higher combustion efficiency was a smaller contributing factor to the reduced 2010 CO emissions. The reduction in combusted biomass density is consistent with a reduction (4â6%) in Global Ozone Monitoring Experiment 2 solarâinduced fluorescence (a proxy for gross primary production) during the preceding months and a potential reduction in biomass (â€8.3%) due to repeat fires
Understanding the land carbon cycle with space data: current status and prospects
Our understanding of the terrestrial carbon cycle has been greatly enhanced since satellite observations of the land surface started. The advantage of remote sensing is that it provides wall-to-wall observations including in regions where in situ monitoring is challenging. This paper reviews how satellite observations of the biosphere have helped improve our understanding of the terrestrial carbon cycle. First, it details how remotely sensed information of the land surface has provided new means to monitor vegetation dynamics and estimate carbon fluxes and stocks. Second, we present examples of studies which have used satellite products to evaluate and improve simulations from global vegetation models. Third, we focus on model data integration approaches ranging from bottom-up extrapolation of single variables to carbon cycle data assimilation system able to ingest multiple types of observations. Finally, we present an overview of upcoming satellite missions which are likely to further improve our understanding of the terrestrial carbon cycle and its response to climate change and extremes
Detection of fossil fuel emission trends in the presence of natural carbon cycle variability
Atmospheric COâ observations have the potential to monitor regional fossil fuel emission (FFCOâ) changes to support carbon mitigation efforts such as the Paris Accord, but they must contend with the confounding impacts of the natural carbon cycle. Here, we quantify trend detection time and magnitude in gridded total COâ fluxesâthe sum of FFCOâ and natural carbon fluxesâunder an idealized assumption that monthly total COâ fluxes can be perfectly resolved at a 2°Ă2° resolution. Using Coupled Model Intercomparison Project 5 (CMIP5) 'business-as-usual' emission scenarios to represent FFCOâ and simulated net biome exchange (NBE) to represent natural carbon fluxes, we find that trend detection time for the total COâ fluxes at such a resolution has a median of 10 years across the globe, with significant spatial variability depending on FFCOâ magnitude and NBE variability. Differences between trends in the total COâ fluxes and the underlying FFCOâ component highlight the role of natural carbon cycle variability in modulating regional detection of FFCOâ emission trends using COâ observations alone, particularly in the tropics and subtropics where mega-cities with large populations are developing rapidly. Using COâ estimates alone at such a spatiotemporal resolution can only quantify fossil fuel trends in a few placesâmostly limited to arid regions. For instance, in the Middle East, FFCOâ can explain more than 75% of the total COâ trends in ~70% of the grids, but only ~20% of grids in China can meet such criteria. Only a third of the 25 megacities we analyze here show total COâ trends that are primarily explained (>75%) by FFCOâ. Our analysis provides a theoretical baseline at a global scale for the design of regional FFCOâ monitoring networks and underscores the importance of estimating biospheric interannual variability to improve the accuracy of FFCOâ trend monitoring. We envision that this can be achieved with a fully integrated carbon cycle assimilation system with explicit constraints on FFCOâ and NBE, respectively
Delayed Resolution of Acute Inflammation in Ulcerative Colitis Is Associated with Elevated Cytokine Release Downstream of TLR4
Background: Ulcerative colitis (UC) is widely viewed as a leukocyte-mediated disorder. Although strong evidence implicates an exuberant response to microbial components in its pathogenesis, no intrinsic immune defect has been identified and the underlying pathogenic mechanisms remain obscure.Methodology/Principal Findings: The acute immune response to bacterial injection was determined in UC patients with quiescent disease and directly compared to healthy control subjects. Monocyte-derived macrophages were used to investigate bacterial recognition mechanisms in vitro. An exuberant and protracted acute inflammatory response to bacteria was evident in patients with UC, which coincides with increased systemic levels of CXCL10. Macrophages stimulated with bacteria and Toll-like receptor (TLR) ligands revealed a specific defect in the TLR4 response in UC. The defect resulted in the over-expression of a number of pro-inflammatory molecules under transcriptional control of the adaptor TIR-domain containing adaptor inducing interferon-b (TRIF).Conclusion: These findings highlight a dysregulated innate immune response with over-expression of molecules associated with leukocyte recruitment and activation that may eventuate in the hallmark chronic immune-mediated inflammation of UC
Global net biome CO2 exchange predicted comparably well using parameterâenvironment relationships and plant functional types
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