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approach. It is now an entirely serious proposition that we should shift the sunshade provided by sulfur aerosols from the lower troposphere, where sulfur dioxide and other suspended particles damage the health of millions and cause acid rain, to the higher stratosphere, where sulfates can continue to cut incoming solar radiation without anyone having to breathe them in. The idea is called “solar-radiation management,” and is one of many possible approaches for what is generally termed “geoengineering.” In all likelihood, it is now technically and economically entirely feasible. But whether it is socially or politically desirable is another matter entirely.

One of the earliest and highest-profile proponents of solar-radiation management as a climate-change mitigation strategy was Paul Crutzen, the Nobel Prize–winning atmospheric chemist and a member of the planetary boundaries expert group. Professor Crutzen, whose eminence as a scientific authority about the atmosphere is unimpeachable, pointed out in a landmark essay in 2006 that if just 2–4 percent of the 55 million tonnes of sulfur humanity releases annually in the lower atmosphere were injected into the stratosphere instead, ongoing global warming could be substantially reduced.42 Although sulfur particles last only for a week or so in the active weather of the troposphere, once they are floating safely above even the highest storm clouds in the stratosphere their atmospheric lifetime extends to a year or more. We know the idea would work, because nature does it from time to time through big volcanic eruptions. The last major eruption, of Mount Pinatubo in the Philippines in 1991, cooled the planet by half a degree for over a year. 43

Intentionally altering the Earth’s albedo through geoengineering would be an enormous step for humanity to take. Consequently, the idea is vociferously opposed by many scientists as well as environmentalists. They have good cause for concern. The injection of aerosols into the stratosphere would likely reduce the strength of the African and Asian summer monsoons, potentially affecting water and food supplies for two billion people.44 The 1991 Pinatubo eruption led to dramatically reduced rainfall and runoff throughout the subtropics the following year. “The central concern with geoengineering fixes to global warming is that the cure could be worse than the disease,” warned climatologists Kevin Trenberth and Aiguo Dai following Crutzen’s essay.45 There is also the danger of “moral hazard.” This is the idea that if a technological fix for global warming were made available, then governments would avoid the more difficult challenge of reducing greenhouse gas emissions. “It’s like a junkie figuring out new ways of stealing from his children,” was the angry response of Meinrat Andreae, an atmospheric scientist at the Max Planck Institute in Germany.46

The problem with these very legitimate objections—and there are many more too numerous to list here47—is that they fail to address the central conundrum identified by Crutzen: that by removing the aerosol pollution sunshade that currently reduces global warming by 50 percent or more, humanity will expose itself and the planet to the full glare of the sun and consequent soaring temperatures.48 Keeping our cities smoggy is not an option, for the reasons already outlined. So why not take up Crutzen’s suggestion and simply move the sulfur sunshade higher up in the atmosphere? This challenge is one of the most critical questions facing humanity today, because it forces us to confront the unavoidable necessity of having to manage the planet intelligently. In my view, the arguments against intentional geoengineering are strong but run the risk of amounting to a demand that we should merely stick to inadvertent geoengineering—that we should, in other words, go ahead and remove the aerosol sunshade anyway, and then just see what happens. I cannot readily accept that accidental planetary management is necessarily better than deliberate planetary management, so I think it is premature to reject geoengineering