The simplest notion is a large occulting disc or “parasol”. This would be placed at a carefully chosen position between the Earth and the Sun, in order to produce a permanent partial solar eclipse. The ideal location would be Lagrange Point 1, where the gravitational pulls of the Sun and Earth are balanced. The disc would need to be fitted with steerable rocket thrusters to maintain its position: it could also be manoeuvred into different orientations, giving a measure of control.27
Researchers have also considered a number of alternative reflectors for reducing the radiation incident on Earth.28 These include Fresnel lenses, diffraction gratings and mirrors.29 In theory, all of them could produce sufficient cooling to offset the warming effect of our greenhouse gas emissions.30 The key considerations are the robustness of the design to meteoroids and other threats, and the mass of the structure — all of which must be carried into space by rocket, or else manufactured in space, adding to the cost.
All these technologies face considerable technical and economic barriers.31 For example, an occulting disc at Lagrange Point 1 would need to have a surface area of millions of square kilometres: no structure remotely close to such a scale has ever been constructed in space. Furthermore, such projects arguably also create a dangerous single point of failure in our climate mitigation strategies: in contrast to Earth-based forms of SRM, the scale of investment and hardware deployment required for a space-based reflector would mean putting all our eggs in one basket, with catastrophic risks if the project failed.