Variation in climate, disturbance regime, and forest management strongly influence terrestrial carbon sources and sinks. Spatially distributed, process-based, carbon cycle simulation models provide a way to integrate information on these various influences to estimate carbon pools and flux over large areas. A team of scientists, including Andrew Gray with the Forest Service’s Pacific Northwest Research Station, did this for a four-state region—Oregon, Washington, Idaho, and western Montana. They used the Biome-BGC carbon cycle process model to determine net ecosystem production and net ecosystem carbon balance for the region from 1986 to 2010. Landsat satellite data were used to characterize disturbances, and forest inventory data were used to parameterize the model. They found that overall disturbance rate on forest land across the four-state region was 0.8 percent per year. Of this disturbance, 49 percent was from timber harvests, 28 percent was from fire, and 23 percent was from pests or pathogens. Most forestland in the study area for the 2006–2010 interval accumulated carbon (positive net ecosystem production), with maximum values in the Coast Range, intermediate values in the Cascade Range, and relatively low values in the Inland Rocky Mountain ecoregions. Areas with localized negative net ecosystem production were mostly associated with recent disturbances. Regional net ecosystem production varied from year to year, with notably low values across the region in 2003, which was also the warmest year in the interval. The region's net ecosystem carbon balance was positive (14.4 teragrams of carbon, or TgC, per year) from 2006–2010. Public forestland contributed a larger proportion to the total net ecosystem carbon balance than did other ownerships because of its larger area.