Can we reconcile differences in estimates of carbon fluxes from land-use change and forestry for the 1990s?

摘要

The effect of Land Use Change and Forestry (LUCF) on terrestrial carbonfluxes can be regarded as a carbon credit or debit under the UNFCCC, butscientific uncertainty in the estimates for LUCF remains large. Here, weassess the LUCF estimates by examining a variety of models of differenttypes with different land cover change maps in the 1990s. Annual carbonpools and their changes are separated into different components for separategeographical regions, while annual land cover change areas and carbon fluxesare disaggregated into different LUCF activities and the biospheric responsedue to CO₂ fertilization and climate change. We developed aconsolidated estimate of the terrestrial carbon fluxes that combinesbook-keeping models with process-based biogeochemical models and inventoryestimates and yields an estimate of the global terrestrial carbon flux thatis within the uncertainty range developed in the IPCC 4th Assessment Report.We examined the USA and Brazil as case studies in order to assess the causeof differences from the UNFCCC reported carbon fluxes. Major differences inthe litter and soil organic matter components are found for the USA.Differences in Brazil result from assumptions about the LUC for agriculturalpurposes. The effects of CO₂ fertilization and climate change also varysignificantly in Brazil. Our consolidated estimate shows that the small sinkin Latin America is within the uncertainty range from inverse models, butthat the sink in the USA is significantly smaller than the inverse modelsestimates. Because there are different sources of errors at the countrylevel, there is no easy reconciliation of different estimates of carbonfluxes at the global level. Clearly, further work is required to developdata sets for historical land cover change areas and models ofbiogeochemical changes for an accurate representation of carbon uptake oremissions due to LUC.