The technologies being proposed have been categorized as carbon dioxide removal (CDR) and solar radiation management (SRM). CDR would reduce the atmospheric concentration of CO2, allowing more heat to escape and thus cooling the Earth. For example, proposed CDR technologies include enhancing the uptake of CO2 in oceans and forests and capturing CO2 from air chemically for storage underground. SRM technologies would place reflective material in space or in Earth's atmosphere to scatter or reflect sunlight (for example, by injecting sulfate aerosols into the stratosphere to scatter incoming solar radiation or brightening clouds) or would increase the planet's reflectivity (for example, by painting roofs and pavements in light colors).
The GAO report examined a range of proposed geoengineering fixes to determine their “technological readiness” on a scale of one to nine. Direct air capture of carbon dioxide scored the highest at three. No solar resource management approach scored above two.
GAO found these technologies currently immature, many with potentially negative consequences. Some studies say, for example, that stratospheric aerosols might greatly reduce summer precipitation in places such as India and northern China. Many experts advocated research because of its potential benefits but also recognized its risks. For example, a country might unilaterally deploy a technology with a transboundary effect.
Highlights of the study may be read here, and the entire 135-page report is available at this site.