Solar radiation management (SRM) proposals are a type of climate engineering which would seek to reflect sunlight and thus reduce global warming. Proposed methods include increasing the planetary albedo, for example using stratospheric sulfate aerosols. Restorative methods have been proposed regarding the protection of natural heat reflectors like sea ice, snow and glaciers with engineering projects. Their principal advantages as an approach to climate engineering is the speed with which they can be deployed and become fully active, their potential low financial cost, and the reversibility of their direct climatic effects. Solar radiation management projects could serve as a temporary response while levels of greenhouse gases can be brought under control by mitigation and greenhouse gas removal techniques. They would not reduce greenhouse gas concentrations in the atmosphere, and thus do not address problems such as ocean acidification caused by excess carbon dioxide (CO2).
Climate engineering projects have been proposed in order to reduce global warming. 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. The annual cost of delivering a sufficient amount of sulfur to counteract expected greenhouse warming is estimated at to $8 billion US dollars.
Scientists are postulating various ways in which mankind can hinder the detrimental effects of climate change and the warming of the planet. Researchers have proposed that increasing the Earth's albedo, or the reflectivity of the Sun's infrared rays, by spraying aerosols such as sulfur dioxide into the stratosphere could help eliminate the greenhouse gas effect. This phenomenon of cooling the Earth's surface by emissions occurs naturally in the world by the eruption of volcanoes. Some scientists believe that artificially producing the same emissions of volcanoes could mitigate the warming of the planet.
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.
By modifying the albedo of the Earth's surface, or by preventing sunlight reaching the Earth by using a solar shade, the sun's warming effect can be cancelled out—although the cancellation is imperfect, with regional discrepancies remaining. SRM or albedo modification, is considered to be a potential option for addressing climate change. As the National Academy of Sciences states in its 2015 report: "The two main options for responding to the risks of climate change involve mitigation—reducing and eventually eliminating human-caused emissions of CO2 and other greenhouse gases (GHGs)—and adaptation—reducing the vulnerability of human and natural systems to changes in climate. 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. 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." In this context, solar radiation management is widely viewed as a complement, not a substitute, to climate change mitigation and adaptation efforts. As The Royal Society concluded in its 2009 report: "Geoengineering methods are not a substitute for climate change mitigation, and should only be considered as part of a wider package of options for addressing climate change." Or put another way: "The safest and most predictable method of moderating climate change is to take early and effective action to reduce emissions of greenhouse gases. No geoengineering method can provide an easy or readily acceptable alternative solution to the problem of climate change. Geoengineering methods could, however, potentially be useful in future to augment continuing efforts to mitigate climate change by reducing emissions, and so should be subject to more detailed research and analysis."
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.
The National Academy of Sciences describes several of the potential benefits and risks of solar radiation management: "Modeling studies have shown that large amounts of cooling, equivalent in scale to the predicted warming due to doubling the CO2 concentration in the atmosphere, can be produced by the introduction of tens of millions of tons of aerosols into the stratosphere. ... Preliminary modeling results suggest that albedo modification may be able to counter many of the damaging effects of high greenhouse gas concentrations on temperature and the hydrological cycle and reduce some impacts to sea ice. Models also strongly suggest that the benefits and risks will not be uniformly distributed around the globe."
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.
Various small-scale experiments have been carried out on techniques such as cloud seeding, increasing the volume of stratospheric sulfate aerosols and implementing cool roof technology.
SRM has been suggested to control regional climate, but precise control over the geographical boundaries of the effect is not possible.
SRM projects could, for example, be used as a temporary response while levels of greenhouse gases can be brought under control by greenhouse gas remediation techniques but would not reduce greenhouse gas concentrations in the atmosphere, and thus not address problems such as ocean acidification caused by excess carbon dioxide (CO2).
Solar radiation management has certain advantages relative to emissions cuts, adaptation, and carbon dioxide removal. 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. 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. This creates a different problem structure. Whereas the provision of emissions reduction and carbon dioxide removal present collective action problems (because ensuring a lower atmospheric carbon dioxide concentration is a public good), 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 timescales.
As well as the imperfect cancellation of the climatic effect of greenhouse gases, there are other significant problems with solar radiation management as a form of climate engineering. SRM is temporary in its effect, and thus any 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.
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. While not an argument against solar radiation management per se, this is an argument against reliance on climate engineering to the exclusion of greenhouse gas reduction.
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. In the cases of systems which involve tipping points, effects may be irreversible. Furthermore, most modeling to date consider the effects of using solar radiation management to fully counteract the increase in global average surface temperature arising from a doubling or a quadrupling of the preindustrial carbon dioxide concentration. Under these assumptions, it overcompensates for the changes in precipitation from climate change. Solar radiation management is more likely to be optimized in a way that balances counteracting changes to temperature and precipitation, to compensate for some portion of climate change, and/or to slow down the rate of climate change.
There may be unintended climatic consequences of solar radiation management, such as significant changes to the hydrological cycle that might not be predicted by the models used to plan them. Such effects may be cumulative or chaotic in nature. Ozone depletion is a risk of techniques involving sulfur delivery into the stratosphere. Not all side effects are negative, and an increase in agricultural productivity has been predicted by some studies due to the combination of more diffuse light and elevated carbon dioxide concentration. A recent (2019) study published in Nature Climate Change computer modeling tested results when solar geoengineering reduced by half the warming produced by doubling CO2 (half SG). The study concluded ". . . neither temperature, water availability, extreme temperature nor extreme precipitation are exacerbated under half-SG when averaged over any Intergovernmental Panel on Climate Change (IPCC) Special Report on Extremes (SREX) region." One study author, David Keith of Harvard University explains, "Big uncertainties remain, but climate models suggest that geoengineering could enable surprisingly uniform benefits."
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.
Leaders of countries and other actors may disagree as to whether, how, and to what degree solar radiation management be used, which could exacerbate international tensions.
In 1976, 85 countries signed the U.N. Convention on the Prohibition of Military or Any Other Hostile Use of Environmental Modification Techniques. The Environmental Modification Convention generally prohibits weaponising climate engineering techniques. However, this does not eliminate the risk. 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.
Carnegie's Ken Caldeira said, "It will make it harder to achieve broad consensus on developing and governing these technologies if there is suspicion that gaining military advantage is an underlying motivation for its development..."
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. It is believed that there would be a significant effect on the appearance of the sky from stratospheric aerosol injection projects, notably a hazing of blue skies and a change in the appearance of sunsets. Aerosols affect the formation of clouds, especially cirrus clouds.
These projects seek to modify the atmosphere, either by enhancing naturally occurring stratospheric aerosols, or by using artificial techniques such as reflective balloons.
Injecting reflective aerosols into the stratosphere is the proposed solar radiation management method that has received the most sustained attention. This technique could give much more than 3.7 W/m2 of globally averaged negative forcing, which is sufficient to entirely offset the warming caused by a doubling of CO2, which is a common benchmark for assessing future climate scenarios. Sulfates are the most commonly proposed aerosols for climate engineering, since there is a good natural analogue with (and evidence from) volcanic eruptions. Explosive volcanic eruptions inject large amounts of sulfur dioxide gas into the stratosphere, which form sulfate aerosol and cool the planet. Alternative materials such as using photophoretic particles, titanium dioxide, and diamond have been proposed. Delivery could be achieved using artillery, aircraft (such as the high-flying F15-C) or balloons. Broadly speaking, stratospheric aerosol injection is seen as a relatively more credible climate engineering technique, although one with potential major risks and challenges for its implementation. Risks include changes in precipitation and, in the case of sulfur, possible ozone depletion.
Various cloud reflectivity methods have been suggested, such as that proposed by John Latham and Stephen Salter, which works 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. 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 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. Such fertilization, especially in the Southern Ocean, would enhance dimethyl sulfide production and consequently cloud reflectivity. This could potentially be used as regional solar radiation management, to slow Antarctic ice from melting. Such techniques also tend to sequester carbon, but the enhancement of cloud albedo also appears to be a likely effect.
Painting roof materials in white or pale colours to reflect solar radiation, known as 'cool roof' technology, is encouraged by legislation in some areas (notably California). This technique is limited in its ultimate effectiveness by the constrained 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 small relative to the 3.7 W/m2 of positive forcing from a doubling of CO2. Moreover, while in small cases it can be achieved at little or no cost by simply selecting different materials, it can be costly if implemented on a larger scale. A 2009 Royal Society report states that, "the overall cost of a 'white roof method' covering an area of 1% of the land surface (about 1012 m2) would be about $300 billion/yr, making this one of the least effective and most expensive methods considered." However, it can reduce the need for air conditioning, which emits CO2 and contributes to 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 may be able to give globally averaged 1.74 W/m2 of negative forcing. Although insufficient to fully offset the 3.7 W/m2 of positive forcing from a doubling of CO2, this would still be a significant contribution thereto, and would 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, desert albedo modification would be expensive, would compete with other land uses, and would have strongly negative ecological consequences. Finally, the total area required during 2010–70 is larger than all non-polar deserts combined.
Oceanic foams have also been suggested, using microscopic bubbles suspended in the upper layers of the photic zone. A less costly proposal is to simply lengthen and brighten existing ship wakes.
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. Salt water could be pumped out of the ocean and snowed onto the West Antarctic ice sheet.
Reforestation in tropical areas has a cooling effect. Deforestation of high-latitude and high-altitude forests exposes snow and thus increases albedo.
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 (space sunshades) are seen by many commentators and scientists as being very expensive and technically difficult, with the Royal Society suggesting that "the costs of setting in place such a space-based armada for the relatively short period that SRM geoengineering may be considered applicable (decades rather than centuries) would likely make it uncompetitive with other SRM approaches."
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 United States dollar 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.
Climate engineering poses several challenges in the context of governance because of issues of power and jurisdiction. Climate engineering as a climate change solution differs from other mitigation and adaptation strategies. Unlike a carbon trading system that would be focused on participation from multiple parties along with transparency, monitoring measures and compliance procedures; this is not necessarily required by climate engineering. Bengtsson (2006) argues that "the artificial release of sulphate aerosols is a commitment of at least several hundred years". Yet this is true only if a long-term deployment strategy is adopted. Under a short-term, temporary strategy, implementation would instead be limited to decades. Both cases, however, highlight the importance for a political framework that is sustainable enough to contain a multilateral commitment over such a long period and yet is flexible as the techniques innovate through time. There are many controversies surrounding this topic and hence, climate engineering has become a very political issue. Most discussions and debates are not about which climate engineering technique is better than the other, or which one is more economically and socially feasible. Discussions are broadly on who will have control over the deployment of climate engineering and under what governance regime the deployment can be monitored and supervised. This is especially important due to the regional variability of the effects of many climate engineering techniques, benefiting some countries while damaging others. The main challenge posed by climate engineering is not how to get countries to do it. It is to address the fundamental question of who should decide whether and how climate engineering should be attempted – a problem of governance.
Solar radiation management raises a number of governance challenges. David Keith argues that the cost is within the realm of small countries, large corporations, or even very wealthy individuals. David Victor suggests that climate engineering is within the reach of a lone "Greenfinger," a wealthy individual who takes it upon him or herself to be the "self-appointed protector of the planet". However, it has been argued that a rogue state threatening solar radiation management may strengthen action on mitigation.
Legal and regulatory systems may face a significant challenge in effectively regulating solar radiation management in a manner that allows for an acceptable result for society. There are, however, significant incentives for states to cooperate in choosing a specific climate engineering policy, which make unilateral deployment a rather unlikely event.
Some researchers have suggested that building a global agreement on climate engineering deployment will be very difficult, and instead power blocs are likely to emerge.
There have been a handful of studies into attitudes to and opinions of solar radiation management. These generally find low levels of awareness, uneasiness with the implementation of solar radiation management, cautious support of research, and a preference for greenhouse gas emissions reduction. As is often the case with public opinions regarding emerging issues, the responses are highly sensitive to the questions' particular wording and context.
One cited objection to implementing a short-term temperature fix is that there might then be less incentive to reduce carbon dioxide emissions until it caused some other environmental catastrophe, such as a chemical change in ocean water that could be disastrous to ocean life.
Ever since the idea of artificial cooling of planet was proposed, there has been major backlash and skepticism. Many people oppose the suggestion, but a recent study from the journal of Nature Climate Change has shown the speculation that solar geoengineering could cause extreme temperatures and increase severity of storms is actually incorrect. This journal shows that only 0.4% of places on the Earth would experience worsened weather conditions. Although no action has been carried out for spraying these gases and clouds into the atmosphere, this discovery could have a major influence on the course of action humans choose to take for reducing the greenhouse gas effect.
Many critics and concerned scientists are whole-heartedly against the idea of solar geoengineering. A geophysics professor, Alan Robock, has reprimanded the Nature Climate Change journal for neglecting to mention other environmental effects that will occur from the atmospheric spray. Robock had said the choice to cool the Earth by artificial emissions would be very costly and it could serve a potential threat to different plant and animal species. Likewise, The Nature Ecology and Evolution journal predicted the use of aerosols would cause a quick transfer of temperatures from warm to cold which would not allow animals to move to a comfortable environment. Ultimately, it would cause a massive extinction of various animal and plant species.