Solar Radiation Management (GeoEngineering)

Solar radiation management (SRM) projects are a type of geoengineering which seek to reduce the amount of sunlight hitting the Earth and thus counteract global warming. The phenomenon of global dimming as a side-effect of fossil fuel use is widely known, and is not necessarily a geoengineering technique, as it occurs in normal conditions, due to aerosols caused by pollution, or caused naturally as a result of volcanoes and major forest fires. However, its deliberate manipulation is a tool of the geoengineer.

Solar radiation management projects do not reduce greenhouse gas concentrations in the atmosphere, and thus do not address problems such as ocean acidification caused by these gases. Their principle advantage as an approach to geoengineering is the speed with which they can be deployed and become fully active. By comparison, other geoengineering techniques based on greenhouse gas remediation, such as ocean iron fertilization, need to sequester the carbon excess before they can arrest global warming. Solar radiation management projects can therefore be used as a geoengineering 'quick fix' while levels of greenhouse gases can be brought under control by greenhouse gas remediation techniques.

By intentionally changing the Earth's albedo, or reflectivity, we could reflect more heat back out into space, or intercept sunlight before it reaches the Earth through a literal shade built in space. Examples of proposed solar radiation management techniques include the production of stratospheric sulfur aerosols, space mirrors, and cloud reflectivity enhancement. Most techniques have at least some side effects.

Various small-scale experiments have been carried out on techniques such as cloud seeding, increasing the volume of stratospheric sulfur aerosols and implementing cool roof technology. However, the applicability of many techniques has not been comprehensively tested. Even if the effects of small-scale interventions are known, there may be cumulative problems such as ozone depletion, which only become apparent from large scale experiments.

As well as the imperfect cancellation of the effect of greenhouse gases on global warming, there are other significant problems with solar radiation management as a form of geoengineering, not least of these are effects on the global hydrological cycle. Some like the burning of sulfur in the stratosphere would in fact increase the environmental problem of acid rain, while increased dust or soot particles may cause increased health concerns. Many would have interactions with global warming that would be complex and that may change with changing conditions.

Particular to solar radiation management, a risk of abrupt cessation exists. If SRM 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 geoengineering technique. The rapid rise in temperature may lead to more severe consequences than a gradual rise of the same magnitude.

A number of geoengineering solutions have been suggested. Most of these suggestions could not counter current global warming but could make a minor contribution to it.

Atmospheric projects
These projects seek to modify the atmosphere, either by enhancing natural processes such as the sulfur cycle, or by using artificial techniques such as reflective balloons.

  • Stratospheric sulfur aerosols modify the Earth's albedo with reflective or absorptive materials spread over portions of its surface.
  • Reflective aerosols or dust methods would use tiny metal flakes or silica compounds added to the fuel of jet airliners, so that particles would be emitted from the jet engine exhaust.
  • Cloud whitening / marine cloud brightening / cloud reflectivity enhancement projects recommend that ships spray seawater into the air to create clouds, shielding the earth from the sun.
  • Cloud seeding proposes the use of airliners to distribute the cloud-seeding materials or the burning of sulfur in ships or power plants to form sulfate aerosol in order to stimulate additional low marine clouds to reflect sunlight.
  • Ocean sulfur cycle enhancement would enhance the natural sulfur cycle in the southern ocean by fertilizing a small portion with iron in order to enhance dimethyl sulfide production and cloud reflectivity. The goal would be to slow Antarctic ice from melting and raising sea level.
  • Reflective balloons suggests billions of aluminized, hydrogen-filled balloons be placed in the stratosphere to provide a reflective screen.
  • Low stratospheric soot could be created by decreasing the efficiency of burning in engines of aircraft flying in the low stratosphere to maintain a thin cloud of soot to intercept sunlight.

Terrestrial albedo modification projects
These projects aim to enhance the reflectivity of the earth's surface reducing the amount of heat retained. Some examples are:

  • Cool roof - Painting pavements and roof materials in white or pale colours to reflect solar radiation, known as 'cool roof' technology, is encouraged by legislation in some areas. This is a benign technique, and it can reduce the need for air conditioning.
  • Reflective plastic sheeting covering 67,000 square miles of desert, to reflect the Sun’s energy could offset the current level of warming. However, the effect would be strongly regional.
  • Ocean litter - An early idea was to use pale coloured floating litter within stable oceanic gyres.
  • Farming, forestry, and land management - Reforestation in tropical areas has a cooling effect. Deforestation of high-latitude and high-altitude forests exposes snow that increases albedo. Changes to grassland could increase albedo, and selecting or genetically modifying commercial crops with high albedo has been suggested.

Space projects
Space-based geoengineering projects are seen by many as being far-fetched at present.

  • Mirrors in space could deflect a percentage of solar sunlight into space.
  • Mining moon dust could create a shielding cloud.
  • Dispersive solutions - Proposes to disperse light before it reaches the Earth by putting a very large diffraction grating or lens in space. Side-effects include that, if this lens were built and global warming were avoided, there would be less incentive to reduce greenhouse gases, and humans might continue to produce too much carbon dioxide until it caused some other environmental catastrophe, such as a chemical change in ocean water that could be disastrous to ocean life.

 Links

NOVA - Dimming the Planet

CBC Doc Zone - Playing God with Planet Earth

A Small Step Forward for Geoengineering - blog