The lateral fluxes of carbon are poorly constrained by measurements and are frequently excluded from models. The river fluxes outlined in the previous section are only beginning to be quantified. The lateral transport of carbon through trade has not been constrained or quantified in global carbon budgets based on inventories of terrestrial carbon stocks and fluxes. Approximately one-third of the 4 PgC fixed annually through agriculture represents harvested products that are directed toward some form of human or domestic animal consumption (Tschirley and Servin, Chapter 21, this volume). In 2001, US$547 million in agricultural and forest-related products entered into international trade, amounting to 9 percent of total trade (excluding services). Imports and exports of cereals, wood, and paper products accounted for about 0.72 PgC of "embodied" carbon traded in 2000.
As we move from the basic carbon budget presented in Table 2.1 to a more detailed review of the fluxes shown in Colorplate 1 and to regional assessments of carbon budgets, it becomes increasingly important to recognize and quantify the lateral transports of carbon. The transport of carbon through trade and commerce represents a flux that is nearly as large as the net terrestrial and oceanic sinks. This carbon is assumed to be respired back to the atmosphere, but if the CO2 is taken out of the atmosphere on one continent, then transported to another continent before being respired, it can have a significant impact on regional budget assessments.
Embodied carbon represents the carbon that resides directly within the product itself (for cereals, wood, and paper, about half of the final weight is carbon). In addition to embodied carbon, at least two other components are usually excluded from estimates of carbon content in international trade. One is the "production" carbon, which comprises the inputs required to produce a final product, primarily energy-related consumption to provide inputs (e.g., pesticides, fertilizers) and processing (e.g., machinery for harvesting, milling, sawing). A second element is the "transport" carbon, which represents the energy costs of transporting the processed products to a final destination. Countries that use intensive production systems and engage in significant exports (pri marily in the developed countries) would have higher ratios of carbon content to product mass exported, possibly up to 20 percent, and would therefore have higher carbon emissions to the atmosphere than currently stated, under a system of full carbon accounting. Other countries would have lower carbon release to the atmosphere under full carbon accounting. Trade and commerce carbon accounting has a political background and differs from budget studies by atmospheric inversion and inventory checks. For example, CO2 emissions during production and transport of products across regional boundaries would fully and partly, respectively, be accounted for by the country of origin using inventory studies. Inverse models based on atmospheric measurements, however, would reflect the emissions as they were actually distributed over the regions. As we begin to move from the global carbon budget to regional carbon budgets, fluxes like those in trade and commerce will need to be better quantified.
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