The Carbon Budget of California

The carbon budget of a region can be defined as the sum of annual fluxes of carbon dioxide and methane greenhouse gases (GHGs) into and out of the regional surface coverage area. According to the state government’s recent inventory, California's carbon budget is presently dominated by fossil fuel emissions of CO2 (at >85% of total annual GHG emissions) to meet energy and transportation requirements. Other notable (non-ecosystem) sources of carbon GHG emissions in 2004 were from cement- and lime-making industries, livestock-based agriculture, and waste treatment activities. The NASA-CASA (Carnegie Ames Stanford Approach) simulation model based on satellite observations of monthly vegetation cover (including those from the Moderate Resolution Imaging Spectroradiometer - MODIS) has been used to estimate net ecosystem fluxes and vegetation biomass production over the period 1990-2004. California's annual NPP for all ecosystems in the early 2000s, estimated by CASA at 120 million metric tons of carbon equivalent (MMTCE) per year, was roughly equal to its annual fossil fuel emission rates for carbon. However, since natural ecosystems can accumulate only a small fraction of this annual NPP total in long-term storage pools, the net ecosystem sink flux for atmospheric carbon across the state was estimated at a maximum rate of between 15-24 MMTCE per year under favorable precipitation conditions. Under less favorable precipitation conditions, such as those experienced during the early 1990s, ecosystems statewide were estimated to have lost nearly 15 MMTCE per year to the atmosphere. Considering the large amounts of carbon stored in standing biomass of forests, shrublands, and rangelands across the state, the implications of changing climate and land use practices on ecosystems must be factored into the state’s planning to reduce overall GHG emissions.