The idea of cooling the climate with stratospheric sunshades that would shield the planet from the sun’s warming rays moved up the international agenda in March 2019, with mixed results. On the one hand, new research suggested that it is theoretically possible to fine-tune such a shield without some of its potentially damaging consequences. Publication of this work coincided with a proposal at the biennial UN Environment Assembly (UNEA), held in Nairobi, Kenya, for an expert review of such geoengineering methods. This was the highest-level discussion of the topic so far. On the other hand, the more than 170 nations involved could not arrive at a consensus. In a fitting illustration of the heat surrounding geoengineering, the proposal was withdrawn at the eleventh hour.

Under the Paris Agreement, governments have pledged to keep average global warming to “well below” 2°C above pre-industrial levels and to try to limit maximum warming to 1.5°C. Many see these targets as wishful thinking: the planet is already roughly 1°C warmer than it was in pre-industrial times, global greenhouse gas emissions are still on the rise and national pledges to cut them fall short of what is needed to hit the 2°C target, let alone 1.5°C.

Faced with this, some think there is a need to turn down the global thermostat using geoengineering. This encompasses a range of possibilities, including technologies that suck carbon dioxide out of the atmosphere and others that block incoming solar energy….  The unea resolution was tabled by Switzerland, and by the start of the week it had received support from most governments. It called for an expert review of the science of geoengineering,…Among the most controversial but also effective and affordable geoengineering options are planetary sunshades. By using high-flying aircraft, for instance, to spray a fine mist of mineral or man-made particles into the upper stratosphere, a portion of the sun’s incoming energy could be bounced back out into space before it gets a chance to warm the planet.  But there are challenges. Stratospheric particles eventually fall back to Earth in rain, so the effect is short-lived. A sunshade would need to be continually resupplied, which is one reason for an international governance framework. If a sunshade were allowed to dissipate while atmospheric CO2 concentrations remained high, global temperatures would rapidly shoot up, with devastating consequences in some regions of the world.  Another problem is the effect of solar geoengineering on the water cycle. Over the past decade, several studies have suggested that sunshades could disproportionately affect rainfall, bringing drought to some regions. But that argument may be oversimplified, according to the new study published in Nature Climate Change .

Switzerland’s proposal to study geo-engineering was blocked at the UNEA…Several delegates told the Economist that America and Saudi Arabia opposed the Swiss proposal to review geoengineering, preferring the issue to be assessed by the Intergovernmental Panel on Climate Change (IPCC), which is due to include something about the technologies in its next big report, expected in 2021. ..But the Swiss proposal was for a more comprehensive appraisal and one that would be delivered more quickly, by August 2020…. Indeed, there are concerns that some geoengineering methods could be unilaterally deployed by one or more nations, to the possible detriment of others.  The Americans, some said, did not appear to want to make room for conversations, let alone make decisions, about a framework for geoengineering that could restrict their future options.

Excerpts from  Sunny with Overcast Features: Geoengineering, Economist, Mar. 16, 2019