Projects

        Arctic and boreal regions are the first areas we expect to see changes due to global climate change.  I developed a technique to detect the timing of soil freeze and thaw using passive microwave satellite data.  The length of time between soil thaw and freeze dictates the growing season length at high northern latitudes, and I found a significant increase in growing season length in all biomes in northern North America since 1988 (Smith et al., 2004).  An earlier spring and later fall will significantly impact the net C balance of northern ecosystems because both plant productivity and soil respiration will change in response to changing temperatures.  This work was cited by the most recent IPCC working group 1 report (section 4.7.3.3).  This record is updated as the SSM/I brightness temperature data are released (typically a 6-8 month lag).  Annual dates of thaw, freeze and growing season length are available by contacting nicole.downey at jsg.utexas.edu

        Using satellite radar backscatter measurements, I am developing a 10-year record of surface water cover in arctic and boreal regions.  Changes in hydrology in the north will have important feedbacks on carbon cycling.  An overall drying will lead to increased soil respiration and CO2 emissions, whereas increases in lake and wetland cover will increase emissions of CH4.  I will use the remote sensing record of water extent and changes to drive a model of soil carbon emissions and quantify changes over time. 

       A transition to hydrogen as a significant energy source may substantially increase emissions of molecular hydrogen (H2) to the atmosphere, which may in turn lead to significant stratospheric ozone (O3) loss.  The largest sink of H2 in the troposphere is uptake by soils, but little was known about the fundamental processes governing the global sink strength.  I designed laboratory experiments to describe the temperature and moisture dependence of soil H2 uptake (Smith-Downey et al., 2006) and spent one year making measurements of soil H2 uptake and profiles across different ecosystems in California (Smith-Downey et al., 2008).  In the field, we discovered that the uptake of H2 is severely diffusion limited, and that variation in the uptake capacity of soil had little effect on the net flux.  I then developed a global mechanistic model of soil H2 fluxes driven by GCM and remote sensing observations to estimate the global distribution and strength of the soil sink (Smith-Downey, 2006).  Because soil uptake is diffusion limited, the flux is proportional to the concentration gradient, so an increase in the tropospheric H2 burden will lead to a corresponding increase in soil H2 consumption.

       Soils are the largest non-lithospheric reservoir of mercury (Hg) in the environment and are a major source of Hg to the atmosphere.  Hg in soils is tightly bound to soil organic carbon, and the cycling of Hg in soils is coupled to the cycling of C in soils.  Taking advantage of this association, I developed a global model of soil mercury storage and fluxes based on the CASA biogeochemical model.  This is the first global mechanistic model of soil Hg storage and emissions, and has been a very powerful tool in evaluating the magnitude of soil fluxes and the impact of anthropogenic Hg emissions on soil Hg.  We find that anthropogenic Hg is concentrated in the most labile carbon pools, and that emissions of previously deposited anthropogenic Hg from soils to the atmosphere is equivalent to 75% of primary anthropogenic emissions (Smith-Downey and Jacob, submitted).  We are currently working on a global estimate of Hg emissions from biomass burning, and a simulation of the Hg cycle through 2050.  This model is one component of the global land-atmosphere-ocean Hg simulation developed in the GEOS-Chem atmospheric chemistry model at Harvard University.  For more information about the GEOS-Chem Hg simulation - go here. 

Trend in growing season length from 1988 to 2002 (Smith et al., 2004).

Model schematic for diffusion of H2 into soils (Smith-Downey, 2006).

Global emissions of Hg from soils to the atmosphere since 1840 (Smith-Downey and Jacob, submitted).