목차

• Ⅰ. The Global Timber Model (Forest only)
  Ⅱ. The GTM Incorporating Agriculture and Livestock Sector (CET Approach)
  Ⅲ. Converting CET results to Physical Units of Area

요약문

Forests play traditionally important roles in economic life as a source of wood and other products. Recently, their environmental usefulness is enjoying the spotlight, as forests can provide a carbon sink via forest carbon sequestration as a part of reduction in greenhouse gas. This, in turn, should reduce global warming. In particular, carbon sequestration using forests increased in policy significance because it presents a method for both a relatively low-cost means of removing carbon dioxide from the atmosphere (US Environmental Protection Agency, 1995) and long-term effectiveness for carbon abatement with other policies (Sohngen & Mendelsohn, 2007). In this context, forest economic models grew out of traditional supply-demand ones to link with various environmental problems.
Given that greenhouse gas emission from land use change and deforestation accounted for 17.4% of the total (IPCC 2007), land-use models for understanding relations between agriculture and forest sectors are increasingly important in the analysis of environmental problems. Therefore, there have been various attempts in modeling approaches: Scopes of models of forest economy expanded from wood products in the past to soil, land-use changes, and integrated existing economic models. Such forest models can be roughly divided into computable general equilibrium models (CGE) and partial equilibrium models (PEM).
Computable general equilibrium models include general sectors of production and consumption in economy, and interactions and feedback effects across economic sectors can be considered in these models in cases of various economic environments and changes in structures. On the other hand, partial equilibrium models are for analyzing specific economic sectors of concern without considering relations with other economic sectors. These allow for analysis of concerned sectors in the detailed sectoral model. Examples of computable general equilibrium models are GTAPEM (OECD, 2003) and GTAPE-L (Burniaux & Lee, 2003). Both models are modified versions of the standard Global Trade Analysis Project (Hertel, 1997) and often are used to analyze the impacts of environmental problems on the entire economy. The Global Timber Model (Sohngen et al., 1999) and the Forest and Agricultural Sector Optimization Model (Adams et al., 2005) are examples of partial equilibrium models. Meanwhile, despite the same theoretical backgrounds (or modeling approaches), there are considerably various models based on scale of model, period of analysis, and static or dynamic analysis. For instance, optimization problems solve for each period in static models, while dynamic models are for maximizing the net present value of welfare defined over the whole period by optimal distribution of resources over time through intertemporal optimizing with perfect foresight. The Timber Assessment Market Model (Adams & Haynes, 1980) is an example of a static model; GTM and FASOM are dynamic models. Also, there are regional models and global modes divided by scale. Forest Carbon Budget Model (Smith & Health, 2001), which is the model to estimate carbon in U.S. forests, and the forest carbon flux model FORUG (Verbeeck et al., 2006), which was used to analyze NEE (net ecosystem exchange) for the Hesse Forest in France, are examples of regional models, whereas a GTM is a global model. Recently, model developers often combine or couple existing economic models. Usually, a detailed bottom-up model is linked to a comprehensive top-down model. For example, Tavoni et al. (2007) linked the World Induced Technical Change Hybrid Model (WITCH) to a Global Timber Model built by Sohngen et al. (1999) to investigate the potential contribution of forest management to climate stabilization.
Among these various forest models, a GTM has several appropriate features that are differentiated from others. First, a GTM is a dynamic model. Because forests take time to grow and participate in a dynamic process affected by long-term decision making, a time path should be explicitly considered in a model. For instance, accumulation of carbon in the atmosphere results in rising carbon prices over time and consequently increases incentives, inducing carbon sequestration using forests. Static models have a demerit in that they treat forests as a steady-state stock rather than as a dynamic stock and then fail to explain intertemporal adjustment in response to changing incentives. However, in a GTM, dynamic optimization does account for important adjustments in timber inventories, which are composed of trees at various ages, and the dynamics of forest carbon sequestration are associated with that adjustment.
Second, a GTM is a global model. Regional studies may be useful in that relatively elaborate modeling is possible by using detailed data of a specific region. However, problems such as global warming and changes in land use are not limited to a national but worldwide phenomenon, and environment policies therefore should be conducted on a global basis. In this context, global models can show significant policy implications for environmental problems. For instance, when a nation imposes carbon tax for carbon sequestration, it may induce forest owners to release carbon in other nations. A GTM can present useful information for policy makers both by considering such leakage and by showing different reactions of nations on an environment policy. Or, when international prices of grain increase due to an international shortage of food, forests or pastures of a nation can be converted to compensate for the shortage. Global models are needed to analyze such land conversion and contention. Finally, a GTM includes forest products that are not handled by other models, indicating that it can present a more accurate analysis of environmental problems.
The purposes of this study are to present and illustrate the current status of GTM researches. In this study, three different versions of global timber models are introduced. The paper is organized as follows: A GTM including forests only is introduced, followed by a GTM including agriculture and livestock. Models in which the results of the GTM including agriculture and livestock are converted to physical units are explained.

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