Carbon sequestration has been proposed as an important means for mitigating climate change, particularly in the medium and longer term. Three approaches have been proposed: biological sequestration in trees and soils; capture from the flue gases of large fossil fuel-using plants, particularly for electric power generation, followed by transport and burial; capture directly from the air, followed by burial. Although biological sequestering in soils and biomass is a natural process, and is seen as the cheapest option, the annual potential is small, and storage may reverse later in the century. Carbon capture and sequestration is probably limited to about one third of annual fossil-fuel related emissions, and will have high energy and money costs if plants are not =capture-ready'. There are no limits on air capture, but it may need more energy than the energy content of the input fuels, and in any case is a high cost approach. Sequestration can be in geological reservoirs such as disused oil and gas fields, or saline aquifers. (Many researchers rule out ocean burial, both because of its adverse environmental effects and because of the temporary nature of storage.) There is however, great uncertainty in the total safe capacity of geological reservoirs. Perhaps even more importantly, the time for absorbtion mechanisms to act varies from immediate to hundreds or even thousands of years, so that annual limits on safe disposal may be the limiting factor.