Geoengineering Strategy Reduce Solar Irradiance

Modeling studies have found that a reduction of a small percentage (estimated at 1.6-1.8%) in absorbed solar radiation would compensate for the temperature-warming induced by CO2 doubling from pre-industrial levels (280 ppm) to future elevated concentrations (560 ppm) [18, 19]. Proposed approaches to geoengineer climate by reflecting solar radiation all depend upon some form of scattering mechanism situated at locations ranging from Earth's surface to outer space, as illustrated in Fig. 9.1. Several geoengineering proposals seek to reduce the quantity of solar radiation entering the earth's atmosphere by placing light scattering objects either outside of Earth's atmosphere in low-earth orbit (LEO; 222 km) or at the Lagrangian 1 point (L1, 1.5 x 106 km from Earth) where the gravitational fields of Earth and the Sun are in balance and allow a small mass to remain stationary relative to Earth. This concept was first introduced by Early [20], who

Space-Based Reflection of Solar Radiation

Low Earth Orbit

Space-Based Reflection of Solar Radiation

Low Earth Orbit

Stratospheric

Aerosol

Injection

Tropospheric Cloud Seeding

Increase Surface Reflectivity

Fig. 9.1 Geoengineering proposals to deflect incident solar radiation: placing scattering materials in outer space; injecting aerosols into the stratosphere; brightening tropospheric clouds, and; increasing the reflectivity of Earth's surface

proposed to loft into outer space a 10 mm thick and 2,000 km in diameter shield constructed of lunar materials and locate it at the L1 point. A more recent study proposed to launch 800,000 m-sized reflective objects manufactured on Earth to the L1 point, creating a 100,000 km diameter reflective "cloud" to deflect solar energy [21]. Both studies discussed the challenge of maintaining the delicate balance between the opposing gravitational fields, centripetal acceleration from orbiting the sun, and the forcing associated with the deflection of solar photons. In the [22] report, the list of proposed objects to be placed into LEO included a large solar-reflective screen, thousands of mirrors, and clouds of dust. Due to instability in orbit, the NAS committee ruled out the dust cloud proposal as impractical.

A project to reduce solar flux situated outside of the Earth's atmosphere has advantages. The strategy avoids the need to disturb Earth's surface or the atmosphere, alleviating the concern for potential damage to the environment or human health. In addition, launching reflective objects into outer space requires less maintenance, as objects placed into outer space have lifetimes as long as decades. This strategy is therefore less vulnerable to the wide number of factors that may interrupt other geoengineering strategies, such as international financial crises or conflict.

Mitigation Option: Launch reflective material either into LEO or to the L1 point, where the reflective material would remain to block incident solar radiation for decades.

Feasibility: The technical challenges involved with sending reflective material to block solar radiation in outer space are enormous. Current proposals involve yet undeveloped technologies, such as using lunar material to construct reflective objects in outer space [20]. Currently, cost estimates for the various proposals are very preliminary and range from $1 to 10 trillion current USD.

A crucial design consideration for a system of space-based reflectors will be the need for "fine-tuning" the quantity of solar radiation blocked by the system, as information about the climate system response to reduced S0 emerges. Each device will require a remote control mechanism for reducing or increasing the active reflective area. This will add significantly to the cost of implementing the scheme. The remoteness of the reflectors makes removal or other significant modification difficult at best.

Co-benefits and undesirable consequences: Geoengineering a decrease in solar irradiance from outer space is the most "clean" option, involving no direct interaction with Earth's surface or atmosphere. However, as for other proposals to limit solar radiation, the outer space options would also likely reduce the amount of photosynthetically active radiation (PAR, or wavelengths of 400-700 nm) available to support photosynthetic organisms. This effect is of note both in terms of its impact on ecosystem viability but also as a climate feedback mechanism - to what degree might a suppression of photosynthesis induce a positive (i.e., warming) feedback on climate?

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