Wofsy Steven C.

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Steven C.

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  • Preprint
    The terrestrial biosphere as a net source of greenhouse gases to the atmosphere
    ( 2015-12-21) Tian, Hanqin ; Lu, Chaoqun ; Ciais, Philippe ; Michalak, Anna M. ; Canadell, Josep G. ; Saikawa, Eri ; Huntzinger, Deborah N. ; Gurney, Kevin R. ; Sitch, Stephen ; Zhang, Bowen ; Yang, Jia ; Bousquet, Philippe ; Bruhwiler, Lori ; Chen, Guangsheng ; Dlugokencky, Edward J. ; Friedlingstein, Pierre ; Melillo, Jerry M. ; Pan, Shufen ; Poulter, Benjamin ; Prinn, Ronald G. ; Saunois, Marielle ; Schwalm, Christopher R. ; Wofsy, Steven C.
    The terrestrial biosphere can release or absorb the greenhouse gases, carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) and therefore plays an important role in regulating atmospheric composition and climate1. Anthropogenic activities such as land use change, agricultural and waste management have altered terrestrial biogenic greenhouse gas fluxes and the resulting increases in methane and nitrous oxide emissions in particular can contribute to climate warming2,3. The terrestrial biogenic fluxes of individual greenhouse gases have been studied extensively4-6, but the net biogenic greenhouse gas balance as a result of anthropogenic activities and its effect on the climate system remains uncertain. Here we use bottom-up (BU: e.g., inventory, statistical extrapolation of local flux measurements, process-based modeling) and top-down (TD: atmospheric inversions) approaches to quantify the global net biogenic greenhouse gas balance between 1981-2010 as a result of anthropogenic activities and its effect on the climate system. We find that the cumulative warming capacity of concurrent biogenic CH4 and N2O emissions is about a factor of 2 larger than the cooling effect resulting from the global land CO2 uptake in the 2000s. This results in a net positive cumulative impact of the three GHGs on the planetary energy budget, with a best estimate of 3.9±3.8 Pg CO2 eq/yr (TD) and 5.4±4.8 Pg CO2 eq/yr (BU) based on the GWP 100 metric (global warming potential on a 100-year time horizon). Our findings suggest that a reduction in agricultural CH4 and N2O emissions in particular in Southern Asia may help mitigate climate change.
  • Article
    Ecosystem fluxes of hydrogen : a comparison of flux-gradient methods
    (Copernicus Publications on behalf of the European Geosciences Union, 2014-09-03) Meredith, Laura K. ; Commane, R. ; Munger, J. William ; Dunn, A. ; Tang, Jianwu ; Wofsy, Steven C. ; Prinn, Ronald G.
    Our understanding of biosphere–atmosphere exchange has been considerably enhanced by eddy covariance measurements. However, there remain many trace gases, such as molecular hydrogen (H2), that lack suitable analytical methods to measure their fluxes by eddy covariance. In such cases, flux-gradient methods can be used to calculate ecosystem-scale fluxes from vertical concentration gradients. The budget of atmospheric H2 is poorly constrained by the limited available observations, and thus the ability to quantify and characterize the sources and sinks of H2 by flux-gradient methods in various ecosystems is important. We developed an approach to make nonintrusive, automated measurements of ecosystem-scale H2 fluxes both above and below the forest canopy at the Harvard Forest in Petersham, Massachusetts, for over a year. We used three flux-gradient methods to calculate the fluxes: two similarity methods that do not rely on a micrometeorological determination of the eddy diffusivity, K, based on (1) trace gases or (2) sensible heat, and one flux-gradient method that (3) parameterizes K. We quantitatively assessed the flux-gradient methods using CO2 and H2O by comparison to their simultaneous independent flux measurements via eddy covariance and soil chambers. All three flux-gradient methods performed well in certain locations, seasons, and times of day, and the best methods were trace gas similarity for above the canopy and K parameterization below it. Sensible heat similarity required several independent measurements, and the results were more variable, in part because those data were only available in the winter, when heat fluxes and temperature gradients were small and difficult to measure. Biases were often observed between flux-gradient methods and the independent flux measurements, and there was at least a 26% difference in nocturnal eddy-derived net ecosystem exchange (NEE) and chamber measurements. H2 fluxes calculated in a summer period agreed within their uncertainty and pointed to soil uptake as the main driver of H2 exchange at Harvard Forest, with H2 deposition velocities ranging from 0.04 to 0.10 cm s−1.
  • Article
    Precision requirements for space-based XCO2 data
    (American Geophysical Union, 2007-05-26) Miller, C. E. ; Crisp, D. ; DeCola, P. L. ; Olsen, S. C. ; Randerson, James T. ; Michalak, Anna M. ; Alkhaled, A. ; Rayner, Peter ; Jacob, Daniel J. ; Suntharalingam, Parvadha ; Jones, D. B. A. ; Denning, A. S. ; Nicholls, M. E. ; Doney, Scott C. ; Pawson, S. ; Boesch, H. ; Connor, B. J. ; Fung, Inez Y. ; O'Brien, D. ; Salawitch, R. J. ; Sander, S. P. ; Sen, B. ; Tans, Pieter P. ; Toon, G. C. ; Wennberg, Paul O. ; Wofsy, Steven C. ; Yung, Y. L. ; Law, R. M.
    Precision requirements are determined for space-based column-averaged CO2 dry air mole fraction (XCO2) data. These requirements result from an assessment of spatial and temporal gradients in XCO2, the relationship between XCO2 precision and surface CO2 flux uncertainties inferred from inversions of the XCO2 data, and the effects of XCO2 biases on the fidelity of CO2 flux inversions. Observational system simulation experiments and synthesis inversion modeling demonstrate that the Orbiting Carbon Observatory mission design and sampling strategy provide the means to achieve these XCO2 data precision requirements.
  • Article
    Atmospheric carbon dioxide variability in the Community Earth System Model : evaluation and transient dynamics during the twentieth and twenty-first centuries
    (American Meteorological Society, 2013-07-01) Keppel-Aleks, Gretchen ; Randerson, James T. ; Lindsay, Keith ; Stephens, Britton B. ; Moore, J. Keith ; Doney, Scott C. ; Thornton, Peter E. ; Mahowald, Natalie M. ; Hoffman, Forrest M. ; Sweeney, Colm ; Tans, Pieter P. ; Wennberg, Paul O. ; Wofsy, Steven C.
    Changes in atmospheric CO2 variability during the twenty-first century may provide insight about ecosystem responses to climate change and have implications for the design of carbon monitoring programs. This paper describes changes in the three-dimensional structure of atmospheric CO2 for several representative concentration pathways (RCPs 4.5 and 8.5) using the Community Earth System Model–Biogeochemistry (CESM1-BGC). CO2 simulated for the historical period was first compared to surface, aircraft, and column observations. In a second step, the evolution of spatial and temporal gradients during the twenty-first century was examined. The mean annual cycle in atmospheric CO2 was underestimated for the historical period throughout the Northern Hemisphere, suggesting that the growing season net flux in the Community Land Model (the land component of CESM) was too weak. Consistent with weak summer drawdown in Northern Hemisphere high latitudes, simulated CO2 showed correspondingly weak north–south and vertical gradients during the summer. In the simulations of the twenty-first century, CESM predicted increases in the mean annual cycle of atmospheric CO2 and larger horizontal gradients. Not only did the mean north–south gradient increase due to fossil fuel emissions, but east–west contrasts in CO2 also strengthened because of changing patterns in fossil fuel emissions and terrestrial carbon exchange. In the RCP8.5 simulation, where CO2 increased to 1150 ppm by 2100, the CESM predicted increases in interannual variability in the Northern Hemisphere midlatitudes of up to 60% relative to present variability for time series filtered with a 2–10-yr bandpass. Such an increase in variability may impact detection of changing surface fluxes from atmospheric observations.