CO2 mitigation options have been overviewed from an engineering point of view. There have been proposed a number of mitigation options, which can be divided into three categories; 1. reduction of energy intensity; 2. reduction of carbon intensity; 3. carbon sequestration. In this review paper, various mitigation options are reviewed focusing on the carbon sequestration options. A reduction in energy intensity is essentially an energy saving. A reduction in carbon intensity could be achieved by switching to energy resources with lower carbon contents. Based on the 2001 IPCC report, the mitigation potential related to energy intensity is estimated at 1,900-2,600 Mt-C/year in 2010, and 3,600-5,050 Mt-C/year in 2020, including other greenhouse gas equivalents. There are additional benefits in implementing these options; they are economically beneficial, and have no associated harmful effects. The carbon sequestration options can be divided into two categories; the enhancement of the natural sinking rates of CO2, and a direct discharge of anthropogenic CO2. The relevant sequestration options in the first category include terrestrial sequestration by vegetation, ocean sequestration by fertilization, and an enhancement of the rock weathering process. In the direct discharge options, the CO2 produced from large point sources, such as thermal power stations, would be captured and separated, then transported and injected either into the ocean or underground. Although the sequestration options are less beneficial in terms of cost per unit CO, reduction compared to other options, technical developments in sequestration options are necessary for the following reasons; 1. A huge potential capacity for carbon sequestration, 2. carbon sequestration enables a continuous use of fossil fuels, which is unavoidable at the moment, before switching to renewable energy sources. Each sequestration option has advantages and disadvantages in terms of capacity, cost, the time scale of the sequestration, the stability of sequestered CO2, and additional environmental impacts, which depend on the location, time, and amount of sequestration. Thus, reliable evaluations of the mitigation efficiency are essential for each sequestration option upon implementation.
