Regional Budgets

Although atmospheric CO2 concentrations are changing on a global scale, the spatial scales of natural processes, as well as the scales of human interventions and the associated societal mechanisms, have a profound regional character. Industrialized countries, for instance, dominate fossil fuel CO2 emissions by direct release and through trade, whereas developing countries have become the primary CO2 emitters through land use changes (Romero Lankao, Chapter 19, this volume). Insufficient observations limit our ability to conduct regional-scale assessments globally. However, the potential benefits of such approaches in identifying the biogeochemical and human processes responsible for controlling fluxes makes these studies very important. A regional-scale carbon budget assessment also provides a unique opportunity to verify and bridge independent methods and observations made over a range of spatial scales (e.g., top-down atmospheric inversion estimates versus land-based or ocean-based bottom-up observations).

In the land-based bottom-up method, carbon sinks and sources from various ecosystems (forests, croplands, grasslands, and organic soil wetlands) are aggregated over all regions to provide a large-scale perspective. The land-based approach can therefore provide information about which ecosystems and regions are accumulating carbon and which are losing carbon to the atmosphere. The diversity of land mosaics, the complexity of human activities, and the lateral transports of carbon in different components make it difficult, however, to provide a comprehensive carbon budget.

Ocean-based bottom-up methods can be used to assess the complex and often competitive controls of heat flux, mixing, and biology on air-sea gas exchange, as well as to develop proxies for extrapolating limited observations to larger time and space scales. Monitoring changes in ocean interior properties can also provide valuable information on surface processes and fluxes.

Atmosphere-based approaches, in contrast, give no information about which ecosystems or processes are contributing to a sink or source but produce a more consistent large-scale assessment of the net carbon flux to the atmosphere. No one approach holds the key to understanding regional variability in the carbon cycle. A suite of approaches much be used. In addition to providing different, but complementary, information, a combination of the bottom-up and top-down methods puts independent constraints on the integrated carbon balance of regional budgets.

The use of multiple approaches can also be used to identify key fluxes that the inventory approach may have missed. For example, Janssens et al. (2003) used inventories to estimate that the European continent sink is on the order of 0.11 PgC y-1, compared with a mean atmospheric inversion estimate of 0.29 PgC y-1. The discrepancy between these estimates is attributed to the intercontinental displacement of organic matter via trade, emissions of non-CÜ2 gases, and a slight overestimation of fossil-fuel emissions. This example illustrates not only the need for comprehensive assessments of the major vertical and lateral fluxes, but also the power of multiple approaches for highlighting areas where additional studies are needed.

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