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The MIT Integrated Global System Model: Ecosystems Impacts

Ecosystems Components: Terrestrial Ecosystems Model (TEM) | Natural Fluxes
Other IGSM Components: Climate and Chemistry and Anthropogenic Emissions

Changes in terrestrial ecosystems due to changes in climate are an important consideration in policy discussions. But climate-driven changes in the terrestrial biosphere also affect climate dynamics, through feedbacks on both the carbon cycle and the natural emissions of trace gases.

GLS diagram

The terrestrial component of the IGSM includes dynamically linked hydrologic and ecologic models in a Global Land System framework, as depicted at right. Hydrologic processes and surface-heat fluxes are represented by the Community Land Model (CLM), which is based on a multi-institutional collaboration of land models. Within the IGSM, CLM is dynamically linked to the global Terrestrial Ecosystems Model (TEM), developed by The Ecosystems Center at the Marine Biology Laboratory (MBL). Within the IGSM, TEM is used to simulate the carbon dynamics of terrestrial ecosystems. Methane and nitrogen exchange are considered through the Natural Emissions Model (NEM), which is driven by dynamic inputs from both TEM and CLM. The coupled CLM/TEM/NEM model system represents the geographical distribution of global land cover and plant diversity through a mosaic approach, in which all major land cover types and plant functional types are considered over a given domain, and are area-weighted to obtain aggregate fluxes and storages.

The biogeophysical and biogeochemical pathways in the IGSM Global Land System are represented in the schematic above. As shown, coupling exists between the atmospheric model (which also includes linkages to the air chemistry and ocean models) and the land model components of the IGSM2. Also shown are the linkages between the biogeophysical (CLM) and biogeochemical (TEM) subcomponents. All green shaded boxes indicate fluxes/storage that are explicitly calculated/tracked by this Global Land System (GLS). The blue shaded boxes indicate those quantities that are calculated by the atmospheric model of the IGSM2.

TEM has been used with the IGSM to examine the responses of terrestrial ecosystems to climate change, enhanced atmospheric CO2 concentrations, nitrogen availability and land-use change. And changes in natural ecosystems, as predicted by TEM, are used as a measure of terrestrial effects, or as inputs to analysis of the impact of climate change on agriculture. The version of TEM currently used in the IGSM also incorporates the influence of ozone on plant productivity and the influence of soil thermal regime on terrestrial carbon and nitrogen dynamics.

TEM diagram
The Terrestrial Ecosystem Model (TEM) depicted in the schematic is used for predictions of the future state of ecosystems and the fluxes of carbon dioxide between the atmosphere and the land biosphere.

TEM is a process-based ecosystem model that simulates important carbon and nitrogen fluxes and pools for 18 terrestrial ecosystems. It runs at a monthly time step. Driving variables include monthly average climate (precipitation, mean temperature and mean cloudiness), soil texture (sand, clay and silt proportion), elevation, vegetation and water availability. The model incorporates a water balance model to generate hydrological input (e.g., potential evapotranspiration, soil moisture). For global extrapolation, TEM uses spatially-explicit data sets at a resolution of 0.5 degrees. The global data sets include long-term average climate, potential natural vegetation, soil texture and elevation.

Utilizing the IGSM's climate predictions, TEM generates predictions of natural ecosystems states, including land vegetation changes, land CO2 fluxes, and soil composition, which feed back to the coupled chemistry/climate, and natural emissions models. The representations of ecosystem change produced by TEM are a crucial link to the estimation of economic and ecological effects. The change in carbon and nitrogen fluxes, and in carbon storage in vegetation and soils, are themselves useful indicators of climate change impact, and they provide a key step in the estimation of more specific models of effects on agriculture and natural ecosystems. With these linked components, the IGSM is used to study how climate-driven changes in the terrestrial biosphere affect climate dynamics through feedbacks on both the carbon cycle and the natural emissions of trace gases.


The Natural Emissions Model (NEM) is used to simulate the emissions of methane (CH4) and nitrous oxide (N2O) from the terrestrial biosphere to the atmosphere. The natural terrestrial fluxes from soils and wetlands are important contributors to the global budgets for these gases. Because these fluxes are dependent on climate, global models to simulate the relevant biogeochemical processes are incorporated in the IGSM.

The global emission model for N2O, which focuses on soil biogenic N2O emissions, has a 2.5 degree spatial resolution. The model can predict daily emissions for N2O, N2, NH3 and CO2 and daily soil uptake of CH4. It is a process-oriented biogeochemical model including soil C and N dynamic processes for decomposition, nitrification, and denitrification. The model takes into account the spatial and temporal variability of the driving variables, which include vegetation type, total soil organic carbon, soil texture, and climate parameters. Climatic influences, particularly temperature and precipitation, determine dynamic soil temperature and moisture profiles and shifts of aerobic-anaerobic conditions.

The methane emission model is developed specifically for wetlands and has a spatial resolution of 1 degree. For high latitude wetlands, the emission model uses a two-layer hydrological model to predict the water table level and the bog soil temperature, which are then used in an empirical formula to predict methane emissions. For tropical wetlands, a two-factor model (temperature and water availability) is used to model the methane flux by taking into account the temperature and moisture dependence of activity of methanogens. Methane emissions from wet tundra are calculated by assuming a constant small methane flux and an emission season defined by the time period when the surface temperature is above the freezing point. The hydrological model and the two-factor model are driven by surface temperature and precipitation, which links methane.

Further description of the Terrestrial Ecosystem Model, and other components of the Global Land System in the IGSM, and sample applications, are described in the following publications:

Publications Describing Other IGSM Components:

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