The United Nations published an in-depth document outlining possible "Solar Radiation Modification" programs, otherwise describing a series of methods to cool the earth's temperature in emergency situations. More than one of these methods has been backed by Bill Gates. The document, titled "One Atmosphere: An Independent Expert Review on Solar Radiation Modification Research and Deployment," contains a lengthy disclaimer admitting that there is "little information on the risks of SRM" and that there needs to be "robust, equitable and rigorous trans-disciplinary scientific review process to reduce uncertainties." The first suggested method involves stratospheric aerosol injections, or "SAI," which involve "injecting highly reflective sub-micron-size particles into the stratosphere," requiring aircrafts that can release the particles at an altitude of 20-25 kilometers. The idea was theorized around 2017 and then subsequently experimented with by Bill Gates and Harvard University in 2018. https://www.theblaze.com/news/un-climate-emergency-earth-temp