The Model GORCAM

(Graz / Oak Ridge Carbon Accounting Model)


B. Schlamadinger+, G. Marland*, L. Canella+

+JOANNEUM RESEARCH, Institute of Energy Research, Elisabethstrasse 5, A-8010 Graz, AUSTRIA
* Oak Ridge National Laboratory, Environmental Sciences Division, Oak Ridge, TN 37831-6335, USA


Introduction

Land management and biomass utilization strategies offer opportunities to mitigate the increase of the CO2 concentration in the earth's atmosphere. For example, land can be used to sequester carbon (afforestation, forest protection), to produce bioenergy as a substitute for fossil fuels, or to produce other renewable raw materials like timber.

The Model

GORCAM (Graz / Oak Ridge Carbon Accounting Model) is a spreadsheet model that has been developed to calculate the net fluxes of carbon to and from the atmosphere associated with such strategies.

The model considers

Input parameters of the model describe the management regime (harvest cycle, growth rate etc.), the land use before the project, and the way in which the biomass is used for carbon mitigation. The model output is presented in diagrams with cumulative carbon sequestration over time.

The system of carbon pools and fluxes (Figure 1)

The left side of Figure 1 represents the biosphere part of the model (1 vegetation pool, 5 litter pools, 1 soil pool). The flux from "atmosphere" to "vegetation" corresponds to net primary production. Dead plant material is transferred from the vegetation pool to the 5 litter pools. Decay of organic matter in litter pools produces CO2 that is directly emitted to the atmosphere, and some carbon from litter pools is added to the soil carbon pool which itself also releases CO2 to the atmosphere.

Part of the biomass in the vegetation pool is harvested. Some harvesting residues remain on site as indicated by dotted arrows from the vegetation pool to litter pools.

The right side represents the biomass utilization part of the model. The harvested biomass can be used for bioenergy and for long-, short-, and very-short-lived biomass products. Long- and short-lived products can store significant amounts of C, whereas carbon storage in the very-short-lived pool (e.g. some paper products) is neglected. Waste wood can either be landfilled, recycled for other biomass products, or be used as a biofuel. Whenever harvested biomass or waste wood are burned for energy, CO2 is released to the atmosphere. Decay of wood products is another source of CO2 into the atmosphere.

The five boxes in the lower right corner (Figure 1) represent the impact of using biofuels and wood products on CO2 emissions from fossil fuel burning.

Model results: conventional forestry (Figure 2)

Figure 2 shows the model output for a forest of 160 tC/ha that is harvested at the time = 0 to produce wood products and biofuels and replanted. Due to the initial harvest there is a net loss of on-site carbon, so that the baseline of the plot starts at -160 tC/ha. The rotation period is 60 years. The diagram shows, successively from the bottom, net carbon (C) uptake in soil and litter (net decreases are represented by a drop in the baseline of the plot), net C increase in trees, net C storage in long-lived products, net C storage in short-lived products, net C storage in landfills, C in fossil fuels not burned due to substitution of woodbased materials for more energy-intensive materials like steel, and C in fossil fuels displaced by biofuels.

The initial surplus of soil and litter C is due to logging residues that are left in the forest. The total carbon benefit of this forestry scenario, marked with the thick black line, is only small after 20 years, but increases thereafter (e.g. 65 tC/ha after 50 years).

Model results: fuelwood plantation on agricultural land (Figure 3)

Figure 3 shows the model output for 1 hectare of agricultural land that is afforested to produce a biofuel with a 20 year rotation. The diagram shows, succesively from the bottom, net carbon (C) uptake in soil and litter, net C increase in trees and saved C emissions from fossil fuels because biofuels from the plantation are used instead.

There is an input of fossil fuels required for land management, for processing of biofuels, etc., which is larger than the comparable upstream energy requirements of the displaced fossil fuel. The appropriate amount of C emissions is substracted from the top line and the final net C sequestration is represented with the thick black line.

The total carbon benefit of this forestry scenario is 520 tC/ha after 100 years, which corresponds to a mean annual carbon sequestration of 5.2 tC/ha.


Acknowledgements

This work was arried out within the Environment Research Program of the European Union (contract number EV5V-CT92-0119) with funds from the Austrian Federal Ministry of Science and Research and JOANNEUM RESEARCH. Gregg Marland was supported by the U.S. Department of Energy under Contract DE-AC05-96OR22464 with Lockheed Martin Energy Research Corp.


Publications

1. Marland G. and B. Schlamadinger (1995), Biomass Fuels and Forest Management Strategies: How do we Calculate the Greenhouse-Gas Emissions Benefits?, Energy 20: 1131-1140.

2. Schlamadinger B. and G. Marland (1996), Carbon Implications of Forest Management Strategies, in Apps M. and Price D.T. (eds.): Forest ecosystems, forest management and the global carbon cycle, NATO ASI Series Vol. I 40: 217-232.

3. Schlamadinger B. and G. Marland (1996), Full Fuel Cycle Carbon Balances of Bioenergy and Forestry Options. Energy Conversion and Management 37: 813-818.

4. Schlamadinger B. and G. Marland (1996), The Role of Forest and Bioenergy Strategies in the Global Carbon Cycle, Biomass and Bioenergy 10: 5/6, 275-300

5. Marland G. and B. Schlamadinger (1997), Forests for Carbon Sequestration or Fossil Fuel Substitution? A Sensitivity Analysis