Soil organic matter is the largest global terrestrial C pool and is a source of CO2, CH4 and other greenhouse gases. Changes in soil organic C (SOC) content and fossil fuel C emissions in response to conversion of conventional tillage to conservation tillage in the contiguous USA for field crop production by the year 2020 were projected by developing a model based on published data, and geographic databases of current conservation tillage usage and agricultural SOC. Three scenarios of conservation tillage use, 27% (current usage), and 57% (low), and 76% (high) of field crops planted were considered. Conversion of land to conservation tillage alone is not likely to sequester sufficient C to offset the impact of C released by fossil fuel consumption.