|Schematic overview of the climate geoengineering proposals considered. From Vaughan and Lenton (2009). Click to enlarge.|
Researchers at the University of East Anglia (UEA) have carried out the first comprehensive assessment of the relative merits of different geoengineering schemes in terms of the climate cooling potential. Their paper appears in the journal Atmospheric Chemistry and Physics Discussions.
Climate geoengineering proposals seek to combat the effects of climate change—in particular to counteract the effects of increased CO2 in the atmosphere. There are two basic approaches proposed: reducing the atmospheric absorption of incoming solar (shortwave) radiation, or removing CO2 from the atmosphere and transferring it to long-lived reservoirs, thereby increasing outgoing longwave radiation.
|“The realization that existing efforts to mitigate the effects of human-induced climate change are proving wholly ineffectual has fuelled a resurgence of interest in geo-engineering.”|
—Prof. Tim Lenton
A number of schemes have been suggested including nutrient fertilization of the oceans, cloud seeding, sunshades in space, stratospheric aerosol injections, and ocean pipes.
The critical metric for a geoengineering scheme is its effectiveness in cooling the climate; Tim Lenton and Nem Vaughan at UEA quantified that effectiveness in terms of radiative forcing potential.
Among their findings:
Enhancing carbon sinks could bring CO2 back to its pre-industrial level, but not before 2100—and only when combined with strong mitigation of CO2 emissions. Carbon cycle geoengineering carries less risk associated with failure.
Stratospheric aerosol injections and sunshades in space have by far the greatest potential to cool the climate back to pre-industrial temperatures by 2050. However, they also carry the most risk because they would have to be continually replenished and if deployment was suddenly stopped, extremely rapid warming could ensue.
Existing activities that add phosphorous to the ocean may have greater long-term carbon sequestration potential than deliberately adding iron or nitrogen.
On land, sequestering carbon in new forests and as bio-char (charcoal added back to the soil) have greater short-term cooling potential than ocean fertilization as well as benefits for soil fertility.
Air capture and storage shows the greatest potential, potentially combined with afforestation/reforestation and bio-char production.
Increasing the reflectivity of urban areas could reduce urban heat islands but will have minimal global effect.
Other globally ineffective schemes include ocean pipes and stimulating biologically-driven increases in cloud reflectivity.
The beneficial effects of some geo-engineering schemes have been exaggerated in the past and significant errors made in previous calculations.
Without mitigation, anthropogenic climate forcing could reach ~7W m-2 on the century timescale and remain greater than ~7W m-2 on the millennial scale. Even in a strong mitigation scenario, anthropogenic forcing will remain >1W m-2 for the rest of the millennium, exceeding 3W m-2 on the century timescale.
Climate geoengineering is best considered as a potential complement to the mitigation of CO2 emissions, rather than as an alternative to it. Strong mitigation could achieve the equivalent of up to -4W m-2 radiative forcing on the century timescale, relative to a worst case scenario for rising CO2. However, to tackle the remaining 3W m-2, which are likely even in a best case scenario of strongly mitigated CO2, a number of geoengineering options show promise.
...If our estimates are even remotely accurate, recent interest in ocean carbon cycle geoengineering seems a little misplaced, because even the more promising options are only worth considering as a millennial timescale activity. Perhaps the most surprising result is that activities that are already underway, particularly inadvertent phosphorus addition to coastal and shelf seas, may have greater long-term carbon sequestration potential than the much-studied iron fertilization. Some other suggestions that have received considerable media attention, in particular “ocean pipes” appear to be ineffective. The real value of such suggestions has been to redirect attention to the whole topic area. We hope that the present contribution provides a useful quantitative first step that can inform the prioritization of further research into various climate geoengineering options, and provide a common framework for the evaluation of new proposals.—Lenton and Vaughan (2009)
Tim Lenton and Nem Vaughan (2009) The radiative forcing potential of different climate geo-engineering options. Atmos. Chem. Phys. Discuss., 9, 1–50,