Climate geo-engineering options
Copenhagen - Following is a snapshot of the verdict handed out on geo-engineering schemes in an 81-page report released in September by the Royal Society, Britain's de-facto academy of sciences.
- Carbon removal projects
These are schemes that remove carbon dioxide (CO2), the principal greenhouse gas, from the atmosphere.
Projects that are shown to be "safe, effective, sustainable and affordable" should be deployed alongside cleaner energy and other conventional methods to reduce carbon emissions.
Among those highlighted in the report:
Planting trees: Afforestation would suck CO2 out of the atmosphere through the natural process of photosynthesis. For: Safe, easy, swift and cheap to deploy, good for biodiversity. Against: Only limited potential for carbon removal, potential conflicts over land use (forests vs. food crops).
Bio-energy: Use trees, shrubs and other vegetation as an energy source, such as bio-mass and charcoal. For: Affordable and safe. Against: Slow to reduce global temperatures, potential conflicts over land use.
Enhanced weathering: CO2 is removed from the atmosphere over thousands of years by a natural process involving the weathering, or dissolution, of carbonate and silicate soils. Enhanced weathering would accelerate the process by adding silicates to certain soils. For: High potential for storing CO2 in the soil. Against: Expensive, slow to take effect and impact on soil acidity and vegetation unclear.
Carbon scrubbers: Build hi-tech towers around the world to capture CO2 molecules from the air. For: Safe, technically feasible and very high cleanup potential. Against: Costs unknown but likely to be high, need for infrastructure to store the carbon collected by the towers.
Ocean fertilisation: Sow the open seas with iron nutrients to encourage the growth of marine plants called phytoplankton that suck up CO2 at the surface through photosynthesis. The phytoplankton die and sink to the ocean floor, effectively storing the carbon forever. For: Technically feasible, not too expensive. Against: May not work, given complex ocean currents; slow to reduce global temperatures; very high potential for damaging the marine ecosystem.
Oceanic upwelling: Place huge vertical pipes in the sea to pump water from the depths to the surface and from the surface to the depths. For: Would boost the efficiency of the ocean as a means of storing CO2. Against: Unfeasible, would only reduce atmospheric CO2 by a tiny fraction, environmental impact unknown.
- Solar radiation management
These are schemes that would cool the planet by reducing heat from the Sun rather than by curbing fossil-fuel pollution.
Some of these could have a quick cooling effect, but would not address CO2 build-up, which causes ocean acidification and other problems. They may also have a potential for causing massive environmental problems.
As a result, solar radiation management is less preferable than carbon dioxide removal, says the report. It should only be applied in an emergency and for a limited time, and in any case should accompany reductions in carbon emissions.
The principal schemes:
Albedo (reflective materials): Cover desert areas with reflecting film or generate white clouds over parts of the oceans through spray generators aboard "cloud ships". For: Quick to implement and rapidly effective. Against: Desert albedo would have a major impact on desert eco-systems, ocean albedo could affect weather patterns and ocean currents. Both very expensive.
Stratospheric aerosols: Mimicking the dust spewed from volcanoes, these would be fine, white particles of sulphate that would be scattered into the stratosphere to reflect sunlight. For: Technically feasible, highly effective (could start to reduce temperatures within one year), can be deployed quickly and at low cost. Against: Possible impact on ozone layer, high-altitude clouds, may disrupt regional rainfall patterns.
Space sunshade: Place reflectors in orbit that would reduce the amount of solar radiation reaching Earth by one or two percent. For: Highly effective, and no theoretical limit on potential cooling. Against: Would take decades to deploy; huge cost; potential effects on regional climate; impact of reduced sunlight on ecosystem unknown.