Biological processes driven by photosynthesis cycle through the atmosphere well over an order of magnitude more CO2 than is currently emitted from the combustion of fossil fuels. Already human activities control and appropriate almost half the primary photosynthetic productivity of the planet. Better management of natural and man-made ecosystems affords many opportunities for mitigation of greenhouse gases, through sink enhancements, source reductions and substitution of fossil fuels with biofuels. Biofuels can be recovered from most organic wastes, from agricultural and forestry residues, and from biomass produced solely for energy use. However, the currently low costs of fossil fuels limits the markets for biofuels. Accounting for the greenhouse gas mitigation value of biofuels would significantly increase their contribution to world fuel supplies, estimated to be currently equivalent to about 15% of fossil fuel usage. Another limiting factor in expanding the use of biofuels is the relatively low solar energy conversion efficiencies of photosynthesis. Currently well below 1% of solar energy is converted into biomass energy even by intensive agricultural or forestry systems, with peak conversion efficiencies about 2 to 3 % for sugar cane or microalgae cultures. One approach to increase photosynthetic efficiencies, being developed at the University of California Berkeley, is to reduce the amount of light-gathering chlorophyll in microalgae and higher plants. This would reduce mutual shading and also increase photosynthetic efficiencies under full sunlight intensities. Estimates of the potential of photosynthetic greenhouse mitigation processes vary widely. However, even conservative estimates for biofuels substituting for fossil fuels project the potential to reduce a large fraction of current increases in atmospheric CO2 levels. Biofuels production will require integration with existing agronomic, forestry and animal husbandry systems, and improved utilization-conversion processes. The diffuse nature of biomass resources requires relatively small-scale processes for their utilization as solid fuels or conversion to liquid and gaseous fuels. Earlier proposals for enormous energy plantations feeding large power plants, or for establishing huge ocean kelp farms, were impractical. As are some recent "geoengineering" proposals, such as ocean fertilization. In biomass utilization, combustion is generally preferable to more complex processes, such as thermal or biochemical conversions to oils and alcohols. The co-firing of biomass in fossil power plants avoids many of the scale, procurement, and efficiency limitations of stand-alone systems and provides significant near-term opportunities for CO2 mitigation. Landfill gas recovery, due to the large greenhouse gas forcing of methane gas, is another currently available technology that can significantly reduce greenhouse gas emissions. Wastes and residues provide many opportunities for biofuels production and CO2 mitigation. Mitigating global warming with biological processes requires overcoming many scientific, technological, financial, institutional, regulatory and, perhaps most important, environmental barriers. This necessitates a major, world-wide and long-term, sustained research, development and implementation effort.
