Solar geoengineering is a proposed method of climate engineering that aims to reduce global warming using an artificial ‘sunscreen’ of aerosols in Earth’s high atmosphere. As planning of the first field experiments gets under way, any potential risks associated with the technology must be transparently assessed and not downplayed or dismissed. One such risk of solar geoengineering is its ‘temperature debt’ — the planetary heating that would arise if maintenance of the artificial sunscreen was discontinued.
Modelling suggests that most of the world’s population could benefit from this temporary sunscreen, compared with the adverse effects of unabated climate change (a questionable reference state, in my view). However, models also reveal the alarming rise in temperatures that could occur if the screen of short-lived aerosols should suddenly cease to function for any reason in the presence of high concentrations of long-lived greenhouse gases (see H. D. Matthews and K. Caldeira Proc. Natl Acad. Sci. USA 104, 9949–9954; 2007). This rapid warming would pose a severe risk to ecosystems and society.
Even with the best planning to ensure steady operation of the technology, its continuous safe functioning and maintenance cannot be guaranteed. Yet it could take hundreds of years to safely phase out solar geoengineering and achieve the same degree of cooling by reducing greenhouse-gas concentrations. It is therefore imperative that, in the absence of a fail-safe mode for solar geoengineering, the temperature debt is fully accounted for in any assessments of this technology.
Nature 554, 423 (2018)