Bhutan Forest Study
Steven C. Wofsy
Rotch Professor of Atmospheric and Environmental Science
Human beings have enormous impact on the chemical composition of the atmosphere, both regionally and globally. Combustion of fossil fuel, clearing of forests, agriculture, and chemical industry have caused concentrations of key atmospheric trace gases (CO2, CH4, N2O) to increase over the past 200 years, with accelerating change in the last 50 years. Emissions of industrial halocarbons (CFCs, such as CF2Cl2, "CFC-12") have caused stratospheric ozone to decrease. These changes may have serious impacts, such as climatic warming, regional and urban air pollution, and increased exposure to solar ultraviolet light.
Professor Wofsy's work is motivated by the need for scientific information and analysis to make wise decisions on the future development of the world's resources. We need quantitative information defining the sources and sinks for important atmospheric gases, and deeper understanding of underlying processes and of the rates for chemical transformations and transport in the atmosphere. We need to understand the interactions between the gases in the atmosphere and the vegetation, soils, and oceans at the earth's surface. We must learn how ecosystems respond to climate change, and learn how to manage biotic resources better.
Prof. Wofsy and colleagues study CO2, CH4, CO, and other important atmospheric gases at long-term measurement stations, located from the subarctic to the equator, and in atmospheric measurement campaigns using aircraft such as the University of North Dakota Citation II, University of Wyoming King Air, and NASA's ER-2 and WB-57F. Recent work has extended downscale to urban domains (McKain et al., PNAS vol. 109, 2012) and upscale to the globe Phil. Trans Roy. Soc A, 28 May 2011 vol. 369 no. 1943 2073-2086, and the focus has broadened to industrial emissions and underlying processes at various scales (e.g. "Anthropogenic emissions of methane in the United States", PNAS vol. 110, 20018¿20022, 2013, doi: 10.1073/pnas.1314392110).
The group projects include developing new airborne sensors to make accurate measurements of CO2, CH4, CO, and N2O, and devising new analysis and modelling procedures to extract quantitiative information about sources, sinks, transformations, and transport of atmospheric trace gases. The long-term goal of these efforts is to understand the factors that regulate atmospheric composition and to help design programs to mitigate undesirable change. Particular effort has focused on the adjoint model-data framework using the Weather Research and Forecasting Model combined with the Stochastic Time-Inverted Lagrangian Transport model (WRF-STILT) (e.g. "WRF Simulations of the Urban Circulation in the Salt Lake City Area for CO2 Modeling" J. Appl Met Clim. 52, 2013)
Experimental field studies of the carbon cycle have focused on long-term eddy-covariance measurements of atmosphere-biosphere exchange in tropical, boreal (Thompson, Manitoba), and midlatitude forests (Harvard Forest; see Goulden et al., 1996, Science 279, 214-217, 1998; Barford, Carol C., et al., Factors Controlling Long- and Short-Term Sequestration of Atmospheric CO2 in a Mid-latitude Forest, Science, 294, 1688-1691, 2001; Factors controlling CO2 exchange on timescales from hourly to decadal at Harvard Forest: Urbanski S, Barford C, Wofsy S, et al.,J. Geophys Res-Biogeosciences 112, G2, G02020, 2007).
We also undertake large-scale aircraft measurement programs to determine the regional and continental scale sources and sinks of major Greenhouse Gases as in the CARVEi experiment in Alaska or the HIPPO program that covered the globe in 2009-2011.
At Harvard Forest we are measuring fluxes of CO2, O3, and NOy, along with ambient concentrations of these gases and many of others (CFCs, H2, CH4, PAN, CO, NMHCs). A good deal of effort is invested in developing advanced instrumentation to make these measurements continuously because we wish to address problems that require long, continuous time series -- to understand quantitatively the sources of variance and the influence of disturbance events, and to determine the factors that regulate concentrations and deposition fluxes of greenhouse gases and pollutants.
An important subset of measurements focuses on the net carbon balance of forest ecosystems. We measure net fluxes of CO2 on an hourly basis, and carefully aggregate to seasons, years, and (at Harvard Forest) decade(s). Often, phenomena are observed at the longer time scales that were not apparent at shorter time scales, representing the primary forcing for biological carbon sequestration. We have carefully compared the carbon budget derived from eddy flux data to more traditional biometric observations the determine periodically how much carbon is actually stored as organic matter at the site. These ecological measurements provide confirmation of the carbon uptake rate inferred from the eddy flux data and help determine the influence of long-term factors, such a prior land use, on net carbon uptake.
The long term flux measurements at Harvard Forest stimulated global interest in establishing similar sites across the USA and in Europe, and there are now two international networks of tower flux sites, AmeriFlux in USA, Canada, Brasil and Costa Rica, and EUROFLUX in western Europe.
Measurements in the stratosphere and the upper troposphere have observed the propagation of the seasonal variations and the long-term increase of CO2 from the troposphere, providing a tracer to help understand pollution and chemical change in the stratosphere and climate change in the stratosphere. The measurements define the mean age of stratospheric air, upwelling velocity, and rates of dispersion for air entering the stratosphere in the tropics, and for air in the climatically sensitive region near the tropical tropopause. (See Boering et al., 1996; Andrews et al., Mean ages of stratospheric air derived from in situ observations of CO2, CH4, and N2O, J. Geophys. Res-Atmos. 106, D23, 32295-32314, 2001; "The CO2 tracer clock for the Tropical Tropopause Layer," Park S, et al., Atmos. Chem. Phys. 7, 14, 3989-4000, 2007 .
Prof. Wofsy's research on:
Atmosphere-biosphere exchange: role of forests in the global carbon cycle, regional and global atmospheric chemistry