<P> Geoengineering is seen by Olivier Sterck as an alternative to mitigation and adaptation, but by Gernot Wagner as an entirely separate response to climate change . In a literature assessment, Barker et al. (2007) described geoengineering as a type of mitigation policy . IPCC (2007) concluded that geoengineering options, such as ocean fertilization to remove CO from the atmosphere, remained largely unproven . It was judged that reliable cost estimates for geoengineering had not yet been published . </P> <P> Chapter 28 of the National Academy of Sciences report Policy Implications of Greenhouse Warming: Mitigation, Adaptation, and the Science Base (1992) defined geoengineering as "options that would involve large - scale engineering of our environment in order to combat or counteract the effects of changes in atmospheric chemistry ." They evaluated a range of options to try to give preliminary answers to two questions: can these options work and could they be carried out with a reasonable cost . They also sought to encourage discussion of a third question--what adverse side effects might there be . The following types of option were examined: reforestation, increasing ocean absorption of carbon dioxide (carbon sequestration) and screening out some sunlight . NAS also argued "Engineered countermeasures need to be evaluated but should not be implemented without broad understanding of the direct effects and the potential side effects, the ethical issues, and the risks .". In July 2011 a report by the United States Government Accountability Office on geoengineering found that "(c) limate engineering technologies do not now offer a viable response to global climate change ." </P> <P> Carbon dioxide removal has been proposed as a method of reducing the amount of radiative forcing . A variety of means of artificially capturing and storing carbon, as well as of enhancing natural sequestration processes, are being explored . The main natural process is photosynthesis by plants and single - celled organisms (see biosequestration). Artificial processes vary, and concerns have been expressed about the long - term effects of some of these processes . </P> <P> It is notable that the availability of cheap energy and appropriate sites for geological storage of carbon may make carbon dioxide air capture viable commercially . It is, however, generally expected that carbon dioxide air capture may be uneconomic when compared to carbon capture and storage from major sources--in particular, fossil fuel powered power stations, refineries, etc . As in the case of the US Kemper Project with carbon capture, costs of energy produced will grow significantly . However, captured CO can be used to force more crude oil out of oil fields, as Statoil and Shell have made plans to do . CO can also be used in commercial greenhouses, giving an opportunity to kick - start the technology . Some attempts have been made to use algae to capture smokestack emissions, notably the GreenFuel Technologies Corporation, who have now shut down operations . </P>

Which geoengineering method for carbon sequestration has been enacted for several decades