Over ten times more CO2 is fixed by plants into biomass, and annually released by decomposers and food chains, than is emitted to the atmosphere due to the burning of fossil fuels. Human activity is already directly and indirectly affecting almost half of the terrestrial biological C cycle. Management of even a small fraction of the biological C cycle would make a major contribution to mitigation of this greenhouse gas. Electric power generation is responsible for roughly one third of fossil CO2 emissions. Direct CO2 mitigation processes are those that reduce fossil CO2 emissions from specific power plants. Direct biological CO2 mitigation processes include the cultivation of microalgae on flue-gas or captured CO2, and the cofiring of wood with fossil fuels. Indirect biological processes, such as growing trees for C storage or for fueling dedicated biomass power plants, recapture CO2 that already has entered the atmosphere. Indirect and direct CO2 mitigation processes have the same overall effect in reducing global warming potential. Reducing global CO2 emissions from forest destruction and unsustainable agricultural and land use practices is one of the most cost-effective, and environmentally beneficial actions that can be taken now to arrest global climate change. Another near-term option is to enhance the substitution of fossil fuels with biofuels. Biofuels are a major source of fuel for the poorer half of mankind. Globally, biofuels could replace a substantial fraction of current fossil fuel usage. Cofiring biomass wastes and residues with coal is one of the lowest-cost, nearest-term options for reducing fossil CO2 emissions at existing power plants. Long-term demonstrations of biomass cofiring are required at full-scale coal-fired power plants to document efficiencies, ash characteristics, biomass preparation and feeding, and other technical issues. Biomass fuel resources for cofiring can be expanded in the near-term through greater recovery of wastes and residues in forestry and agriculture, and in the mid-term through systems that produce biomass specifically for use as fuels (energy crops).