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Solar radiation management (SRM) projects (proposed and theoretical) are a type of climate engineering which seek to reflect sunlight and thus reduce global warming. Proposed examples include the creation of stratospheric sulfate aerosols. They would not reduce greenhouse gas concentrations in the atmosphere, and thus do not address problems such as ocean acidification caused by these gases. Their principal advantages as an approach to climate engineering is the speed with which they can be deployed and become fully active, as well as their low financial cost. By comparison, other climate engineering techniques based on greenhouse gas remediation, such as ocean iron fertilization, need to sequester the anthropogenic carbon excess before they can arrest global warming. Solar radiation management projects can therefore be used as a climate engineering "quick fix" while levels of greenhouse gases can be brought under control by greenhouse gas remediation techniques.
The phenomenon of global dimming is widely known, and is not necessarily a climate engineering technique. 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. The majority of recent global dimming has been in the troposphere, except that resulting from volcanos, which affect mainly the stratosphere.
By intentionally changing the Earth's albedo, or reflectivity, scientists propose that we could reflect more heat back out into space, or intercept sunlight before it reaches the Earth through a literal shade built in space. A 0.5% albedo increase would roughly halve the effect of CO2 doubling.
These climate engineering projects have been proposed in order to reduce global warming. The effect of rising greenhouse gas concentrations in the atmosphere on global climate is a warming effect on the planet. By modifying the albedo of the Earth's surface, or by preventing sunlight reaching the Earth by using a solar shade, this warming effect can be cancelled out — although the cancellation is imperfect, with regional discrepancies remaining.
The applicability of many techniques listed here has not been comprehensively tested. Even if the effects in computer simulation models or of small-scale interventions are known, there may be cumulative problems such as ozone depletion, which become apparent only from large scale experiments.
As early as 1974, Russian expert Mikhail Budyko suggested that if global warming became a problem, we could cool down the planet by burning sulfur in the stratosphere, which would create a haze. Paul Crutzen suggests that this would cost 25 to 50 billion dollars per year. It would, however, increase the environmental problem of acid rain. However, this is now believed to be a minor side effect.
A preliminary study by Edward Teller and others in 1997 presented the pros and cons of various relatively "low-tech" proposals to mitigate global warming through scattering/reflecting sunlight away from the Earth via insertion of various materials in the upper stratosphere, low earth orbit, and L1 locations.
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 climate engineering; not least of these are effects on the global hydrological cycle and the inability of such techniques to reduce ocean acidification.
Particular to solar radiation management, a risk of abrupt cessation exists. If SRM were to abruptly stop while masking a high degree of warming, 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.
SRM has been suggested to control regional climate, but precise control over the geographical boundaries of the effect is not possible.
Stratospheric sulfate aerosols as proposed by Paul Crutzen, with the purpose to modify the Earth's albedo with reflective or absorptive materials spread over portions of its surface. This would typically be achieved using hydrogen sulfide or sulfur dioxide, delivered using artillery, aircraft (such as the high-flying F15-C) or balloons.  (Alternative approaches using photophoretic particles have been proposed.) Ozone depletion is a risk of such techniques, but only if high enough quantities of aerosols drift to, or are deposited in, polar stratospheric clouds before the levels of CFCs and other ozone destroying gases fall naturally to safe levels because CFCs can settle on larger sulfate particles, increasing their ozone destroying potential. This proposal, not unlike the others, carries with it considerable risks, including increased drought or acid rain.
Broadly speaking, this technique is seen as a credible climate engineering scheme, although not one without major risks, and challenges for its implementation. This technique can give >3.7 W/m2 of globally averaged negative forcing, which is sufficient to entirely offset the warming caused by a doubling of CO2.
Sulfate is the most commonly proposed aerosol for climate engineering, since there is a good natural analogue with volcanic eruptions. Explosive eruptions inject large amounts of sulfur dioxide gas into the stratosphere, which form sulfate aerosol. Studies of the Earth's climate have shown that such aerosol can achieve significant cooling. Sulfate aerosols have been shown to enhance ozone depletion. However, other aerosol types may be more efficient at cooling the climate or less damaging to the ozone layer. Such aerosols include the highly reflective titanium dioxide.
United States Patent 5003186 suggested that tiny metal flakes could be "added to the fuel of jet airliners, so that the particles would be emitted from the jet engine exhaust while the airliner was at its cruising altitude." Alternative proposals, not known to have been published in peer-reviewed journals, include the addition of silicon compounds to jet fuel to make silicon dioxide particles in the exhaust.
A more sophisticated approach, using multi-layered nanoparticles (consisting of aluminium and barium titanate), was published by David Keith in 2010. He suggests utilizing the effects of photophoresis to increase the amount of time the aerosols stay airborne.
David Keith has also developed an experiment involving the release of sun-reflecting sulfate from a balloon more than 80,000 feet above Fort Sumner in New Mexico to replicate the cooling effects of volcanoes.
In 1992, a report by the US National Academy of Sciences (NAS) on geoengineering noted that dust is a better choice compared to sulphur, because dust is from natural soil and so should have no noticeable effect on the ground as it gradually falls into the troposphere and rains out. It estimated that about 1010 kg dust would be required to mitigate the warming from a doubling of atmospheric CO
2 or about 1 kg dust per 100 t of carbon emissions.
An example of the effects of the imposition of aerosol particles in the atmosphere can be found in history. Comets have been blamed for the dramatic but brief cooling period which commenced in 1159 BCE, and resulted in widespread disruption to civilisations at the time. However, this mechanism, and even the involvement of a comet, is not universally accepted. If a comet was indeed to blame, the action of its aerosols could also have been by the mechanism of cloud condensation nuclei. Other examples of climate change events linked to comets include the famines around 536 CE.
Various schemes have been suggested, such as that proposed by John Latham and Stephen Salter, which works by spraying seawater in the atmosphere to increase the reflectiveness 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. The sprayers would use fleets of unmanned rotor ships known as Flettner vessels to spray mist created from seawater into the air to thicken clouds and thus reflect more radiation from the Earth. The whitening effect is created by using very small cloud condensation nuclei, which whiten the clouds due to the Twomey effect.
Enhancing the natural sulfur cycle in the Southern Ocean ocean by fertilizing a small portion with iron in order to enhance dimethyl sulfide production and cloud reflectivity. The goal is to slow Antarctic ice from melting and raising sea level. Such techniques also tend to sequester carbon, but in this specific project the enhancement of cloud albedo was both the desired outcome and measured result. An alternative technique proposes the vertical mixing of ocean water, to bring deep-water nutrients to surface plankton. This technique can give only 0.016 W/m2 of globally averaged negative forcing, which is essentially insignificant for climate engineering purposes.
These reflectors would be placed at a high enough altitude so that they do not interfere with air traffic. The cost estimate is about 20 times as much as the distribution of dust in the stratosphere, making these schemes economically nonviable. The large number of reflectors and the trash problem posed by their fall make the system unattractive.
Cloud seeding has been proposed using various methods to distribute the cloud-seeding materials, including airliners and ships or power plants. Reck (1978) studied the effect of increases in cloud cover and, using a radiative-convective atmospheric model, found that a 4 to 5 percent increase in low-level cloud cover would be sufficient to offset the warming predicted from a doubling of preindustrial CO2. "This value is in reasonable agreement with Randall et al. (1984), who estimated that a 4 percent increase was required in the amount of marine stratocumulus, which comprises the bulk of the low clouds on a global basis."
Painting roof materials in white or pale colours to reflect solar radiation, known as 'cool roof' technology, and encouraged by legislation in some areas (notably California). This is a benign technique, although limited in its ultimate effectiveness by the costrained surface area available for treatment. This technique can give between 0.01-0.19 W/m2 of globally averaged negative forcing, depending on whether cities or all settlements are so treated. This is generally insignificant when compared to the 3.7 W/m2 of positive forcing from a doubling of CO2. However, in many cases it can be achieved at little or no cost by simply selecting different materials. Further, it can reduce the need for air conditioning, which causes CO2 emissions which worsen global warming.
Adding reflective plastic sheets covering 67,000 square miles (170,000 km2) of desert every year between 2010 and 2070 to reflect the Sun’s energy. This technique can give globally averaged 1.74 W/m2 of negative forcing, which is insufficient to offset the 3.7 W/m2 of positive forcing from a doubling of CO2, but is still a very significant contribution and is sufficient to offset the current level of warming (approx. 1.7 W/m2). However, the effect would be strongly regional, and would not be ideal for controlling Arctic shrinkage, which is one of the most significant problems resulting from global warming. Furthermore, the total area required during 2010-70 is larger than all non-polar deserts combined.
An early geoengineering idea was to use pale coloured floating litter within certain stable oceanic gyres. This litter would tend to group into large and stable areas, such as the Great Pacific Garbage Patch.
Changes to grassland have been proposed to increase albedo. This technique can give 0.64 W/m2 of globally averaged negative forcing, which is insufficient to offset the 3.7 W/m2 of positive forcing from a doubling of CO2, but could make a minor contribution.
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. Temperate areas may experience a 1 °C cooling as a result of this technique. This technique is an example of bio-geoengineering. This technique can give 0.44 W/m2 of globally averaged negative forcing, which is insufficient to offset the 3.7 W/m2 of positive forcing from a doubling of CO2, but could make a minor contribution.
Space-based climate engineering projects are seen by many commentators and scientists as being far-fetched at present.
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. Using a Fresnel lens in this manner was proposed in 1989 by J. T. Early. Using a diffraction grating was proposed in 1997 by Edward Teller, Lowell Wood, and Roderick Hyde. In 2004, physicist and science fiction author Gregory Benford calculated that a concave rotating Fresnel lens 1000 kilometres across, yet only a few millimeters thick, floating in space at the L1 point, would reduce the solar energy reaching the Earth by approximately 0.5% to 1%. He estimated that this would cost around US$10 billion up front, and another $10 billion in supportive cost during its lifespan. One issue with implementing such a solution is the need to counteract the effects of the solar wind moving such megastructures out of position. 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.
There have been a limited number of studies into attitudes to solar radiation management. Notable examples include Mercer et al.