Peter Thornton

Scientist II
TIIMES - CGD
BEACHON - BGS

Contact Information:
PO Box 3000, Boulder, CO 80307-3000
Office: ML - 202B
Telephone: 303-497-1727
Email: thornton@ucar.edu
Home Page | FY07 Abstracts

Project Summary:

Research Summary FY2007

My research over the past year has continued to focus on the interactions between carbon and nitrogen cycles in terrestrial ecosystems, and how this coupling affects feedbacks within the climate system.  Major activities included:

1. Publication of a new canopy integration model that improves the representation of sunlit and shaded canopy fractions, and makes explicit the connection between canopy structure and canopy function mediated by carbon-nitrogen cycle coupling;

2. Completion of a study of the impact of carbon-nitrogen cycle coupling on the response of terrestrial ecosystems to increasing CO2 concentration and interannual climate variability; and

3. A series of coupled model experiments to evaluate the influence of carbon-nitrogen cycle coupling on climate-carbon cycle feedbacks. 

These three topics are detailed individually below.

1. Canopy Integration

Click on picture to view the entire figure.

Measured specific leaf area plotted against overlying leaf area for two deciduous species (a,b) and two evergreen species (c,d), with species-specific regression lines. Each plot shows measurements taken at multiple sites and multiple canopy levels.

In the early stages of development for CLM-CN, we identified the need to introduce a more detailed treatment of radiation interception and carbon and water fluxes within the vegetation canopy, compared to what existed already in CLM.  Based on a combination of new theory and new observations, we developed and implemented a canopy integration scheme that deals explicitly with the observed vertical gradients in leaf morphological characteristics and the related variation in photosynthetic behavior (Thornton and Zimmerman, J. Climate, 2007).  We showed that this new model solved a problem with low productivity under the influence of nitrogen limitation and prognostic leaf area calculation experienced with the original formulation in CLM, and the new treatment has now been adopted for all CLM configurations (Dickinson et al., J. Climate, 2006, and Lawrence et al, J. Hydrometeorology, 2007).

Title, abstract, and figure for relevant publication:

Thornton, P. E., and N. E. Zimmermann, 2007. An improved canopy integration scheme for a land surface model with prognostic canopy structure, Journal of Climate, 20, 3902-3923.

A new logical framework relating the structural and functional characteristics of a vegetation canopy is presented, based on the hypothesis that the ratio of leaf area to leaf mass (specific leaf area) varies linearly with overlying leaf area index within the canopy.  Measurements of vertical gradients in specific leaf area and leaf carbon:nitrogen ratio for five species (two deciduous and three evergreen) in a temperate climate support this hypothesis.  This new logic is combined with a two-leaf (sunlit and shaded) canopy model to arrive at a new canopy integration scheme for use in the land surface component of a climate system model.  An inconsistency in the released model radiation code is identified and corrected.  We also introduce here a prognostic canopy model with coupled carbon and nitrogen cycle dynamics.  The new scheme is implemented within the Community Land Model and tested in both diagnostic and prognostic canopy modes.  The new scheme increases global gross primary production by 66% (from 65 to 108 Pg carbon y-1) for diagnostic model simulations driven with reanalysis surface weather, with similar results (117 PgC y-1) for the new prognostic model.  Comparison of model predictions to global syntheses of observations shows generally good agreement for net primary productivity (NPP) across a range of vegetation types, with likely underestimation of NPP in tundra and larch communities  Vegetation carbon stocks are higher than observed in forest systems, but the ranking of stocks by vegetation type is captured accurately.

2. Offline evaluation of CLM-CN

Click on picture to view the entire figure.

Trends in land sensitivity to atmospheric CO2, showing carbon-only (C) and coupled carbon-nitrogen (CN) responses.

A preliminary test of the newly completed CLM-CN was to drive the model with reanalysis surface weather, to document its present-day predictions for carbon fluxes and stocks and to evaluate the influence of changes in atmospheric CO2 concentration and changes in the rate of mineral nitrogen deposition on carbon, nitrogen, water, and energy cycles.  This study (Thornton et al., Global Biogeochemical Cycles, in press) found that the introduction of carbon-nitrogen coupling significantly altered the model response to increasing CO2, and the sensitivities of net land carbon flux to variation in temperature and precipitation.  That study suggested that C-N coupling would have an important impact on the magnitude and possibly the sign of climate-carbon cycle feedbacks when exercised in the fully coupled Community Climate System Model (CCSM).

Title, abstract, and figure from relevant publication:

Thornton, P.E., J.-F. Lamarque, N.A. Rosenbloom, N. Mahowald, in press. Effects of terrestrial carbon-nitrogen cycle coupling on climate-carbon cycle dynamics. Global Biogeochemical Cycles.

Nutrient cycling affects carbon uptake by the terrestrial biosphere and imposes controls on carbon cycle response to variation in temperature and precipitation, but nutrient cycling is ignored in most global coupled models of the carbon cycle and climate system.  We demonstrate here that the inclusion of nutrient cycle dynamics, specifically the close coupling between carbon and nitrogen cycles, in a terrestrial biogeochemistry component of a global coupled climate system model leads to fundamentally altered behavior for several of the most critical feedback mechanisms operating between the land biosphere and the global climate system.  Carbon-nitrogen cycle coupling reduces the simulated global terrestrial carbon uptake response to increasing atmospheric CO2 concentration by 74%, relative to a carbon-only counterpart model.  Global integrated responses of net land carbon exchange to variation in temperature and precipitation are significantly damped by carbon-nitrogen cycle coupling.  The carbon cycle responses to temperature and precipitation variation are reduced in magnitude as atmospheric CO2 concentration rises for the coupled carbon-nitrogen model, but increase in magnitude for the carbon-only counterpart.  Our results suggest that previous carbon-only treatments of climate-carbon cycle coupling likely over-estimate the terrestrial biosphere’s capacity to ameliorate atmospheric CO2 increases through direct fertilization.  The next generation of coupled climate-biogeochemistry model projections for future atmospheric CO2 concentration and climate change should include explicit, prognostic treatment of terrestrial carbon-nitrogen cycle coupling.

3. Fully-coupled experiments including CLM-CN

We have recently completed a series of fully-coupled simulations, using CLM-CN as well as a new ocean ecosystem model as components of CCSM, testing the sensitivity of global climate-carbon cycle feedbacks to carbon-nitrogen cycle coupling in the land model.  A manuscript describing these results is in preparation, with co-authors from several institutions.

Future Plans

Over the coming year we will continue to document the behavior of the fully-coupled model, with several additional manuscripts in preparation or in planning stages at present.  We will also be exploring several new avenues of model development, including coupling CLM-CN and the existing capability in CLM for prognostic biogeography (Dynamic Global Vegetation Model – DGVM), and the introduction of coupling between the carbon, nitrogen, and phosphorus cycles.  We are participating in an NSF-sponsored project focused on fire at the intersection of carbon and water cycles, and in that context we will be evaluating and hopefully improving the existing simple treatment of prognostic fire fluxes in CLM-CN.  We recently participated in a workshop aimed at coordinating the development of new scenarios within the integrated assessment community and the needs and capabilities of the new generation of coupled climate-biogeochemistry models.  We are exploring the interactions of carbon and nitrogen cycling with landuse and landcover change, and have performed some preliminary experiments in the CCSM framework.  We are also in the early stages of exploring a collaboration with another NSF-sponsored project on hydrologic synthesis, led from the University of Illinois, Champaign-Urbana.

TIIMES External Collaborators:

Kathryn Alexander, Arizona State University
Joseph Alfieri, Purdue University
Ray Anderson, University of California, Irvine
Ryan Anderson, University of Minnesota
Yaxidhi Bamutaze, Makerere University
Valerie Bennington-Benesh, University of Wisconsin
Kathryn Berger, University of New Hampshire
Fredi Birsan, Indiana University
Joseph Blankinship, Northern Arizona University
Bryan Brandel, University of Colorado
Martha Butler, The Pennsylvania State University
Mariah Carbone, University of California, Irvine
Erica Cate, University of New Hampshire
Laura Chasmer, Queens University
K Dhanyalekshmi, Max Planck Institute for Biogeochemistry
Darren Drewry, Duke University
Jordan Golinkoff, University of Montana-Missoula
Sharon Gourdji, University of Michigan
Arun Govind, University of Saskatchewan
Victor Gutierrez Velez, Clark University
Sherri Heck, University of Colorado & NCAR
Jennifer Jensen, University of Idaho
Ranjeet John, University of Toledo
Fuu-Ming Kai, University of California, Irvine
Jenny Kao-Kniffin, University of Wisconsin
Myroslava Khomik, McMaster University
Angela Kross, McGill University
Lynette Laffea, University of Colorado
Kendra Morliengo-Bredlau, University of Colorado
Tomohira Oda, Osaka University of Japan
Nicholas Parazoo, Colorado State University
Alicia Peduzzi, Virginia Polytechnic Institute and State University
Wei Ren, Auburn University
Diego Riveros, Montana State University-Bozeman
Miguel Roman, Boston University
Koichi Sagaguchi, University of Arizona
Cheney Shreve, University of Virginia
Oliver Sonnentag, University of Toronto
Shannon Spencer, Colorado State University
Ann Thijs, University of Texas at Austin
Erico Tomelleri, Max Planck Institute for Biogeochemistry
Rodrigo Vargas, University of California, Riverside
Lixin Wang, University of Virginia
Sonia Wharton, University of California, Davis
Kyle Whittinghill, University of Minnesota
Stephen Wood, Monash University
Xiaojuan Yang, University of Illinois at Urbana-Champaign
Jose Zerpa, North Carolina State University

Publications:

Thornton, P. E., J. F. Lamarque, N. A. Rosenbloom, N. Mahowald, 2007: Inclusion of carbon-nitrogen feedback fundamentally changes response of land carbon model to CO2 fertilization and climate variability. Global Biogeochemical Cycles. (In Press)

Lawrence, D. M., P. E. Thornton, K. W. Oleson, G. B. Bonan, 2007: The partitioning of evapotranspiration into transpiration, soil evaporation, and canopy evaporation in a GCM: Impacts on land-atmosphere interaction. J. Hydrometeorol., 8, 862-880, doi: 10.1175/JHM596.1.

Thornton, P. E., N. E. Zimmermann, 2007: An improved canopy integration scheme for a land surface model with prognostic canopy structure. J. Climate, 20, 3902-3923.

Turner, D. P., W. D. Ritts, J. Styles, Z.-L. Yang, W. B. Cohen, B. E. Law, P. E. Thornton, 2006: A diagnostic carbon flux model to monitor the effects of disturbance and interannual variation in climate on regional NEP. Tellus, 58B, 476-490.