CGD's Dr. Peter Thornton

Abstract

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.

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

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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.

Abstract

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.

Figure caption: 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.

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Lawrence, D.M., P.E. Thornton, K.W. Oleson, G.B. Bonan, in press. The partitioning of evapotranspiration into transpiration, soil evaporation, and canopy evaporation in a GCM. Journal of Hydrometeorology.

Abstract

Although the global partitioning of evapotranspiration (ET) into transpiration, soil evaporation, and canopy evaporation is not well known, most current land surface schemes and the few available observations indicate that transpiration is the dominant component on the global scale, followed by soil evaporation and canopy evaporation. The Community Land Model version 3 (CLM3), however, does not reflect this global view of ET partitioning, with soil evaporation and canopy evaporation far outweighing transpiration. One consequence of this unrealistic ET partitioning in CLM3 is that photosynthesis, which is linked to transpiration through stomatal conductance, is significantly underestimated on a global basis. A number of modifications to CLM3 vegetation and soil hydrology parameterizations are described that improve ET partitioning and reduce an apparent dry soil bias in CLM3. The modifications reduce canopy interception and evaporation, reduce soil moisture stress on transpiration, increase transpiration through a more realistic canopy integration scheme, reduce within-canopy soil evaporation, eliminate lateral drainage of soil water, increase infiltration of water into the soil, and increase the vertical redistribution of soil water. The partitioning of ET is improved, with notable increases seen in transpiration (13%-41% of global ET) and photosynthesis (65-148 Pg C yr_1). Soils are wetter and exhibit a far more distinct soil moisture annual cycle and greater interseasonal soil water storage, which permits plants to sustain transpiration through the dry season. The broader influences of improved ET partitioning on land-atmosphere interaction are diverse. Stronger transpiration and reduced canopy evaporation yield an extended ET response to rain events and a shift in the precipitation distribution toward more frequent small- to medium-size rain events. Soil moisture memory time scales decrease particularly at deeper soil levels. Subsurface soil moisture exerts a slightly greater influence on precipitation. These results indicate that partitioning of ET is an important responsibility for land surface schemes, a responsibility that will gain in relevance as GCMs evolve to incorporate ever more complex treatments of the earth's carbon and hydrologic cycles.

Figure caption: Percent of annual mean ET fro transpiration, soil evaporation, and canopy evaporation, respectively, and total ET for the offline CLM3 control simulation (CONTROL _oπ).

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Dickinson, R.E., K.W. Oleson, G.B. Bonan, F.M. Hoffman, P.E. Thornton, M. Vertenstein, Z.-L. Yang, X. Zeng, 2006. The Community Land Model and its climate statistics as a component of the Community Climate System Model. Journal of Climate, 19: 2302-2324.

Abstract

Several multidecadal simulations have been carried out with the new version of the Community Climate System Model (CCSM). This paper reports an analysis of the land component of these simulations. Global annual averages over land appear to be within the uncertainty of observational datasets, but the seasonal cycle over land of temperature and precipitation appears to be too weak. These departures from observations appear to be primarily a consequence of deficiencies in the simulation of the atmospheric model rather than of the land processes. High latitudes of northern winter are biased sufficiently warm to have a significant impact on the simulated value of global land temperature. The precipitation is approximately doubled from what it should be at some locations, and the snowpack and spring runoff are also excessive. The winter precipitation over Tibet is larger than observed. About two-thirds of this precipitation is sublimated during the winter, but what remains still produces a snowpack that is very large compared to that observed with correspondingly excessive spring runoff. A large cold anomaly over the Sahara Desert and Sahel also appears to be a consequence of a large anomaly in downward longwave radiation; low column water vapor appears to be most responsible. The modeled precipitation over the Amazon basin is low compared to that observed, the soil becomes too dry, and the temperature is too warm during the dry season.

Figure caption: CCSM3.0_T85 annual (a) 2-m air temperature (K), (b) precipitation (mm day^-1), and (c) total runoff (mm day^-1) for northern South America compared to observations. Observations are from Willmont and Matsuura (2000; air temperature and precipitation) and Fekete et al. (2002; runoff).