CGD Research: TSS Overview

Terrestrial Sciences Section

CLM/CCSM biogeochemistry

TSS scientists conducted several projects to implement biogeochemistry in CLM and CCSM. This research broadly addresses how biogeochemical coupling of carbon, nitrogen, and iron cycles affects climate, air quality, radiative forcing, and ecosystem function on regional to global scales. It involves two specific research agendas related to the terrestrial carbon cycle and mineral aerosols.

Carbon-nitrogen interactions in CLM

Peter Thornton applied the recently completed CLM3-CN model to evaluate the influence of interactions between terrestrial carbon and nitrogen cycles on the aspects of the global carbon cycle that are most critical to global-scale carbon-climate feedbacks. This involved completion of a series of global simulations in which CLM3-CN is forced by transient signals for historical and potential future CO2 concentrations and rates of mineral nitrogen deposition over the period 1850-2100. The objectives were to determine the sensitivity of the model to increases in CO2, and also the sensitivity to variation in temperature and precipitation, and to compare these sensitivities to a model without C-N interactions.

The study resulted in the following conclusions: (1) The sensitivity to increasing CO2 concentration for the C-N model is about four times smaller than for the C-only model, while this sensitivity for the C-only model was in the same range as found for an analysis of nine other C-only models. The inclusion of C-N coupling has a dramatic influence on one of the major feedback mechanisms in the coupled carbon-climate system, in the direction of a weaker negative feedback of terrestrial ecosystems on CO2 concentration. (2) The magnitude of sensitivities to variation in temperature and precipitation were both reduced by introduction of the C-N coupling, but the signs are the same for C-N and C-only modes: a positive feedback (release of carbon from land) for increasing temperature, and a negative feedback (uptake on land) for increasing precipitation. (3) Under a transient scenario of increasing CO2 concentration, the sensitivity of the land biosphere to temperature and precipitation variation increases for the C-only model, but decreases when the C-N coupling mechanisms are introduced. The spatial patterns of these feedback mechanisms are complex, and the fact that there are likely to be competing positive and negative feedbacks makes it difficult to determine a priori what the influence of introducing the C-N mechanisms would be in a fully-coupled climate-carbon cycle simulation. (Figure 4.)

Aerosols

Over the past year, Natalie Mahowald and collaborators documented the dust cycle in the CAM/CLM models and the response of dust to climate in the CCSM3. With collaborators within CGD, she included the ability of dust to feedback onto the climate, showing that dust could be important for enhancing the drought in the Sahel and also the response of climate to dust under different climates (last glacial maximum, preindustrial and future).

She also examined the biogeochemical implications of dust and other aerosols. This included a published review of atmospheric deposition of dust and the impacts on biogeochemistry, as well as studies of iron availability for ocean biota and combustion sources of iron, which might be more soluble and thus bioavailable than dust sources. She also studied the impact of biomass burning on atmospheric phosphorus deposition.

Additional Information

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CGD Sectional Narratives

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Comparisons.

Figure 4. (High resolution image) Comparison of ?L between C-only and C-N model configurations. a) Trends in ?L over the period 2000 – 2100, for C-only simulations with increasing CO2 (Ca1 and Cb1), C-N simulation with increasing CO2 (CN1, Cn1x), and C-N simulation with increasing CO2 and mineral nitrogen deposition (CN3). Symbols indicate values for eleven C-only models participating in C4MIP (diamonds) and the mean of the C4MIP models (square). C4MIP results from Friedlingstein et al. [2006]. b) Trends in ratio of ?L between C-N and C-only model configurations (CN1:Ca1, CN1x:Ca1, and CN3: Ca1) over the period 2000-2100.