Solar radiation management (SRM) is a type of climate engineering which seek to reflect sunlight and thus reduce global warming.

Background

The main options for responding to the risks of climate change involve-

1 . Mitigation – reducing and eventually eliminating human-caused emissions of CO2 and other greenhouse gases (GHGs)

2 . Adaptation – reducing the vulnerability of human and natural systems to changes in climate.

3 . Carbon dioxide removal (CDR) – A third potentially viable option, currently under development but not yet widely deployed, is carbon dioxide removal (CDR) from the atmosphere accompanied by reliable sequestration.

4 . Albedo modification –A fourth, more speculative family of approaches called albedo modification seeks to offset climate warming by greenhouse gases by increasing the amount of sunlight reflected back to space.

By intentionally changing the Earth’s albedo, or reflectivity, scientists propose that we could reflect more heat back out into space. We could also intercept sunlight before it reaches the Earth through a literal shade built in space. The effects are uncertain but it has been suggested that 2% albedo increase would roughly halve the effect of CO2 doubling.

Proposed forms of solar radiation management

1 . Stratospheric aerosols – Injecting reflective aerosols into the stratosphere is the proposed solar radiation management method that has received the most sustained attention. Sulfates are the most commonly proposed aerosols for climate engineering, since there is a good natural analogue with volcanic eruptions.

2 .Marine cloud brightening – Various cloud reflectivity methods have been suggested, which work by spraying seawater in the atmosphere to increase the reflectivity of clouds.The extra condensation nuclei created by the spray will change the size distribution of the drops in existing clouds to make them whiter.

3 . Ocean sulfur cycle enhancement – Enhancing the natural marine sulfur cycle by fertilizing a small portion with iron—typically considered to be a greenhouse gas remediation method—may also increase the reflection of sunlight.

4 . Cool roof – Painting roof materials in white or pale colours to reflect solar radiation, known as ‘cool roof’ technology, is another proposed method.

5 .Reflective sheeting – Covering deserts with reflective plastic sheets can help to reflect the Sun’s energy.

6 . Oceanic foams – have also been suggested using microscopic bubbles suspended in the upper layers of the photic zone.

7 . Ice protection – Arctic sea ice formation could be increased by pumping deep cooler water to the surface. Sea ice (and terrestrial) ice can be thickened by increasing albedo with silica spheres. Glaciers flowing into the sea may be stabilized by blocking the flow of warm water to the glacier.

8 . Forestry – Reforestation in tropical areas has a cooling effect. Deforestation of high-latitude and high-altitude forests exposes snow and this increases albedo.

9 . Grassland management – Changes to grassland have been proposed to increase albedo.

10 . High-albedo crop varieties – Selecting or genetically modifying commercial crops with high albedo has been suggested. This has the advantage of being relatively simple to implement, with farmers simply switching from one variety to another. This technique is an example of bio-geoengineering.

11 . Space mirrors – A percentage of sunlight can be deflected into space, using mirrors orbiting around the Earth.

12 . Moon dust – Mining moon dust to create a shielding cloud is also proposed .

13 . Dispersive solutions – Several authors have proposed dispersing light before it reaches the Earth by putting a very large diffraction grating (thin wire mesh) or lens in space, perhaps at the L1 point between the Earth and the Sun.

Advantages of solar radiation management

Solar radiation management has certain advantages relative to emissions cuts, adaptation, and carbon dioxide removal.

Firstly, its effect of counteracting climate change would be experienced very rapidly, on the order of months after implementation, whereas the effects of emissions cuts and carbon dioxide removal are delayed because the climate change that they prevent is itself delayed.

Secondly, some proposed solar radiation management techniques are expected to have very low direct financial costs of implementation, relative to the expected costs of both unabated climate change and aggressive mitigation.

Thirdly, whereas the provision of emissions reduction and carbon dioxide removal present collective action problems, a single country or a handful of countries could implement solar radiation management.

Finally, the direct climatic effects of solar radiation management are reversible on short time scales.

Limitations of solar radiation management

There are many significant problems with solar radiation management as a form of climate engineering.

1 . SRM is temporary in its effect, and thus long-term restoration of the climate would rely on long-term SRM, unless carbon dioxide removal was subsequently used. However, short-term SRM programs are potentially beneficial.

2 . Solar radiation management does not remove greenhouse gases from the atmosphere and thus does not reduce other effects from these gases, such as ocean acidification.

3 . Most of the information on solar radiation management is from models and computer simulations. The actual results may differ from the predicted effect. The full effects of various solar radiation management proposals are not yet well understood. It may be difficult to predict the ultimate effects of projects, with models presently giving varying results.

4 . There may be unintended climatic consequences of solar radiation management, such as significant changes to the hydrological cycle . Ozone depletion is a risk of techniques involving sulfur delivery into the stratosphere.

5 . If solar radiation management were masking a significant amount of warming and then were to abruptly stop, the climate would rapidly warm. This would cause a sudden rise in global temperatures towards levels which would have existed without the use of the climate engineering technique. The rapid rise in temperature may lead to more severe consequences than a gradual rise of the same magnitude.

6 . Leaders of countries and other actors may disagree as to whether, how, and to what degree solar radiation would be used, which could exacerbate international tensions.

7 . If perfected to a degree of controllability and accuracy that is not considered possible at the moment, climate engineering techniques could theoretically be used by militaries to cause droughts or famines. Theoretically they could also be used simply to make battlefield conditions more favourable to one side or the other in a war.

8 . Managing solar radiation using aerosols or cloud cover would involve changing the ratio between direct and indirect solar radiation. This would affect plant life and solar energy.

9 . SRM has been suggested to control regional climate, but precise control over the geographical boundaries of the effect is not possible.