Dark side of the Sun
But there is also a more radical non-carbon-dioxide based option. Solar geoengineering (also known as solar radiation management or modification) would try to cool the world off by cutting down the amount of sunlight that reaches the Earth’s surface; less sunlight, less warming. The most discussed method for achieving this involves putting particles into the stratosphere to bounce a little of the incoming sunlight straight back out into space. Such cooling is seen in action after very large volcanic eruptions; the huge amounts of sulphur they squirt into the stratosphere create tiny reflective particles of sulphate “aerosols”. Geoengineering would be much less spasmodic. A steady stream of sulphur would be sprayed into the stratosphere for decades, or even centuries.
According to the latest projections by UNEP, which are roughly in line with those made by others, if countries were to live up to all their most recent emissions-reduction pledges and, beyond that, those with notional net-zero targets actually hit them, warming should peak at about 1.8°C above the pre-industrial. Katharine Ricke, a researcher at the University of California San Diego who has done a lot of work on solar geoengineering, estimates that, if such a scheme were to be based on sulphate particles, the 0.3°C of cooling needed to bring a 1.8°C world down to a 1.5°C world would require something like 3m tonnes of sulphur a year delivered to the stratosphere. It would also need a new class of very high-flying planes to get the sulphur up there, a system for monitoring what exactly it was doing to the stratosphere to be set up, a world-girdling set of air bases and some chunky new supply chains.
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