Huge mirrors in space, petrifying carbon dioxide, algae farms, and lots of white paint - these are some of the ideas being proposed to offset global warming amid fears that temperatures might rise by more than two degrees Celsius by the end of this century.
Global warming is driven by an increase in human-generated greenhouse gases (GHG), such as water vapor, carbon dioxide, ozone and methane, by burning coal, oil and natural gas. The gases trap heat in the atmosphere, keeping the planet warmer than it would be otherwise.
With past attempts at persuading countries to commit to reducing their GHG emissions inconclusive, "some scientists are beginning to feel that they have an answer", says Richard Samson Odingo, a University of Nairobi lecturer and former vice-chairman of the Intergovernmental Panel on Climate Change.
"Whether or not we can tinker with science and intervene realistically is the issue."
The main technologies being considered are Carbon Dioxide Removal (CDR) and Solar Radiation Management (SRM).
CDR entails removing carbon dioxide, CO2, from the atmosphere via sequestration, capture or ocean fertilization; CO2 emissions are said to be the dominant cause of global warming.
- CO2 capture and storage - Scientists are considering capturing CO2 from industrial plants and burying it at the bottom of the ocean, in salt mines, disused oil wells or other geologically stable areas, for 1,000 years. The logistics and cost-effectiveness of this CO2 capture, compression (CO2 liquefies at pressure), transport and injection is an issue, as are potential leakages.
- Ocean storage - This is considered the largest potential sink for man-made CO2 but there are environmental concerns; evaluating the results of such an intervention is also a likely challenge.
- Ocean fertilization - scientists are considering the possibility of adding nutrients such as iron to promote the growth of microscopic algae in the ocean; the algae pull CO2 from the atmosphere. But there are concerns that this could generate other greenhouse gases and there are calls for bigger, longer studies. "Experiments are not without their own risks however, and the larger they are, the riskier they become," warns a recent Oxford University blog.
- Turning CO2 into stone - scientists are studying if it is possible to use natural chemical reactions underground to solidify excess CO2. Gases generated by a geothermal power plant outside Reykjavik in Iceland will in April be pumped back into the Earth and it is hoped that natural reactions with the basalt should turn the CO2 into a solid carbonate.
- Locking CO2 into rock - In Oman, geochemists are examining how the rock peridotite reacts rapidly with water and CO2 to form solid carbonates to find out if the same process can be harnessed for the storage of industrial CO2.
- Afforestation, reforestation and avoidance of deforestation - forests help in absorbing CO2 from the atmosphere but processes such as deforestation and land clearance for croplands and pasture add to GHGs.
- Biochar - as an alternative to letting biomass degrade naturally, in the process emitting CO2, processes that heat the biomass under oxygen reduction are being considered to produce a solid material that locks in carbon for years and can be added to soils to help them retain water and nutrients.
Solar Radiation Management aims at reducing the amount of solar radiation reaching the Earth and include:
- Increasing surface reflectivity - among the techniques are painting infrastructure white and covering deserts with reflective material.
- Increasing the reflectivity of clouds above oceans - there are suggestions that whitening clouds over parts of the ocean could help in cooling the Earth.
- Injecting chemicals into the atmosphere - it has been proposed that certain chemicals could be released into the second major layer of the Earth's atmosphere to help scatter sunlight back into space.
- Light shields/deflectors - scientists are exploring the use of shields placed in space to reflect or deflect solar radiation.
- Large-scale reforestation - Forests help in cooling the surface in the tropics and sub-tropics while warming the surface in higher latitudes as they are much darker than snow and thus absorb more solar radiation.
- While SRM techniques have been proposed as possibly useful in case of an emergency as they are fast acting, to avoid a climate "tipping point", there are concerns they could create other problems, such as changing precipitation patterns.
Odingo asks: "When you put reflectors in the atmosphere, how does that interfere with agriculture and forests? All of these need to be studied."
According to critics, there is a need to keep in touch with the emerging geo-engineering scientific debate and to comment on the feasibility of the same, bearing in mind that the science may be a result of over-confidence in modeling.
There remains a need for countries to increase efforts towards mitigating and adapting to climate change, and in particular to agreeing to global emissions reductions of at least 50 percent on 1990 levels by 2050 and more because, "Nothing now known about geo-engineering options [CDR and SRM] gives any reason to diminish these efforts", states the Royal Society.
But, geo-engineering may be needed if the apparent lack of political will to significantly reduce GHG emissions does not change, notes John Shepherd, a Fellow of the Royal Society.
There is thus a need to "work outside our national borders, bringing together interested parties from around the globe to debate the issues of geo-engineering, agree appropriate governance structures and ensure that any research is undertaken in a safe, transparent and socially acceptable manner", states Shepherd. "The question of whether solar geo-engineering will prove to be helpful or harmful will largely depend on how humanity can govern the issue and its political implications, and avoid unilateral action."