Radiative cooling through the atmospheric window: A third, less intrusive geoengineering approach

Geoengineering methods based on either direct carbon dioxide removal (CDR) from the atmosphere or solar radiation management (SRM) that curtails solar irradiation are campaigned for as technical solutions that would slow down the global temperature rise and climate change. Except for a few CDR methods, this does not receive much interest from policy-makers as a result of a lack of evidence on net advantages and decision-making challenges related to boundary-crossing effects, not to mention costs. An alternative, third geoengineering approach would be enhanced cooling by thermal radiation from the Earth's surface into space. The so-called atmospheric window, the 8–14 μm bandwidth where the atmosphere is transparent for thermal radiation indeed offers a “window of opportunity” for technology that enables sending out thermal radiation at rates that significantly exceed the natural process. This paper describes work that addresses this, with focus on technical devices that combine materials with the properties required for enhanced long wavelength (LW) thermal radiation heat transfer from Earth to space, through the atmospheric window. One example is a skylight (roof window) developed and tested at our institute, using ZnS windows and HFC-type gas (performing better than CO2 or NH3). Suggestions for several other system layouts are given.