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a mountain white may, however, produce enough ice in the next couple of years to buy the villagers time to adapt to a different livelihood.

      Attempts to whiten the Earth’s surface to increase its reflectivity are also being considered on a far greater scale elsewhere. Methods to reduce the amount of the sun’s energy that heats the planet – called ‘solar radiation management’ – have the potential to rapidly counteract regional or even global warming. With global temperatures almost certain to exceed the 2°C of warming this century that scientists consider ‘safe’ for humanity, quick-cooling options look increasingly attractive. Deflecting the sun’s energy back into space would do nothing to counteract the ocean acidification effect of atmospheric carbon dioxide – which I’ll come to later – but it is a valuable way of buying time while societies decarbonise, adapt to warmer conditions and new climates, and figure out an effective and efficient way of removing the carbon dioxide we’ve put into the atmosphere.

      Some engineers are proposing erecting Earth-orbiting space mirrors that would bounce sunlight back out before it even enters our atmosphere. Terrestrial proposals include whitewashing roofs of houses and public buildings, planting lighter and more reflective crops (perhaps using genetically modified varieties), and covering deserts or ocean in reflective materials. With enough paint and willpower, strategic mountaintops could be sprayed white from the air, perhaps.

      Since the 1980s, Almería in southern Spain has developed the greatest concentration of greenhouses in the world, covering 26,000 hectares. Dubbed the ‘sea of plastic’, this Anthropocene landscape is remarkable not only because Europe’s driest desert now produces millions of tonnes of fruit and vegetables, but also because the greenhouses reflect so much sunlight back into the atmosphere that they are actually cooling the province. While temperatures in the rest of Spain have climbed faster than the world average, meteorological observatories located in the plastic expanse have shown a decline of 0.3°C per decade.⁵ It turns out that the plastic acts like a mirror, reflecting sunlight back into the atmosphere before it can reach and heat up the ground. At a local level, the plastic greenhouses offset the global greenhouse effect.

      More controversially, filling the atmosphere with airborne particulates would also cool the Earth by shading it from sunlight. This happens naturally after a volcano erupts, such as Pinatubo in 1991, which lowered global temperatures by more than half a degree for two years after the event.⁶ In the deep past, supervolcano eruptions threw the planet into ice ages, causing mass extinctions. The same effect, albeit on a far lesser scale, can also be seen on shipping lanes because ships typically burn heavy fuels that issue smoky sulphurous emissions that seed measurably colder airstreams across the oceans. Sulphur particles – like the ones found in the Asian brown haze pollution – have a shading effect that reduces the amount of sunlight reaching the Earth’s surface by as much as 15%, and are masking humanity’s warming by as much as 80%.

      No one would suggest increasing industrial air pollution as a solution to global warming, of course; instead, engineers are looking at other atmospheric ways of reflecting the sun’s energy. Injecting salt particles into low-level, stratocumulus clouds could make them brighter and more reflective, providing a local cooling effect. Clouds of perfect reflectivity and altitude for this occur naturally in semi-permanent sheets in three places: off Chile–Peru, Namibia–Angola and North America. Jim Haywood, an expert on brown haze at the UK Met Office Hadley Centre for Climate Research, has carried out modelling studies on the Chile–Peru stratocumulus clouds, which indicate that modifying the cloud would produce a significant cooling effect. However, modelling also reveals other potential consequences: spraying the West African clouds appears to reduce rainfall over the Amazon, which would be bad; spraying salt into the clouds off Chile seems to increase rainfall over arid Australia, which could be useful. British engineer and inventor Stephen Salter, who pioneered wave-energy technology, thinks that rather than focusing on the three main clouds, it would be better to monitor oceanic warming zones in strategic locations around the world and spray salty nuclei into the air above them to create cooling reflective clouds and help modify dangerous weather. ‘Typhoons like Haiyan [which devastated the Philippines in November 2013] could have been significantly dampened before it made landfall by spraying the cloud,’ Salter says. He has designed a fleet of floating towers that could pump seaspray into clouds to whiten them, and he calculates the total cost of deployment on a scale that reduced global temperatures by half a degree per year would be less than that of a single international climate conference.

      Meanwhile, other scientists are looking at what effect sulphur particles would have if they were pumped tens of kilometres into the stratosphere, mimicking a volcanic eruption but on a lesser, though more long-term, scale. Their experiments, restricted so far to laboratories, are looking at how reflective of the sun’s heat different particles would be, and whether the cooling particles would have any unwanted side effects, such as destroying the ozone layer.

      The changes humans have made to mountains in the Anthropocene have largely been driven by temperature or precipitation – both of which we still have the power to alter, whether by reducing our greenhouse gas emissions or by reducing the sun’s power to heat us. The Anthropocene could become a time of more nuanced climate change, where temperature and precipitation are modulated to humanity’s needs, where weather is planned. It’s an extraordinary idea.

      Humans have always modified their environment – it is only because of our exquisitely adapted brain that we thrive worldwide, essentially by insulating ourselves against the natural environment. Whether we raise average global temperatures by two, four or even six degrees, enterprising members of our species will no doubt adapt successfully. Given centuries, the entire human population would likely manage to live comfortably under such conditions. The problem is that the rate at which we are warming the atmosphere is too rapid for humans to adapt. Nevertheless, the concept of artificially cooling the atmosphere is highly controversial, given the atmosphere is a global commons. Perhaps it is because the intent is so explicit; although humans are artificially warming the atmosphere with greenhouse gas emissions, the intent behind burning fossil fuels has always been to produce energy, not to warm the planet. Some argue that even research in this area should be banned because it implies intent to carry out the practice; others say that it draws effort away from climate-change mitigation – from decarbonising our energy production. But surely freedom of inquiry should be preserved – carrying out scientific research into whether something would work and what its consequences might be does not make a scientist an advocate for deployment, and there are scientific questions that need to be answered, such as the impact on rainfall, and whether or not it would even be technologically possible, before society can start to decide whether or not to deploy such techniques.

      This new, Anthropocene field of geoengineering is a fascinating area of research and the scientists working in it are some of the most remarkable and thoughtful people I’ve encountered. It is eerily reminiscent of the atomic research carried out in the 1940s – today’s geoengineers are working at the cutting edge in an exciting, entirely new science, spending their lives making discoveries and designing amazingly powerful technologies that they fervently hope will never be deployed. Each speaks sincerely about the risks involved with deployment, and reiterates that slashing emissions of warming gases is the best way of dealing with the problem. There may be very real and serious consequences of using reflectors. Models indicate that cooling the northern hemisphere (to slow catastrophic ice melt in the Arctic) would slash rainfall in poor countries in the southern tropics; one solution would be to simultaneously deploy reflective coolants above the southern hemisphere. Another problem is the so-called ‘termination’ issue. In order to keep global temperatures down and counteract future warming, these reflectors would need to be sprayed continuously and perhaps in greater amounts. If the spraying programme was terminated, global temperatures would rise very suddenly by whole degrees – this would be much more dangerous for humanity than gradual global warming caused by our rise in emissions.

      However, the predicament that humans currently find themselves in – facing catastrophic climate change and yet increasingly reliant on the fuels that exacerbate the problem – means that planetary cooling techniques are likely to be seriously considered. It is, after all, what humans have always done when presented with a challenge – engineer a way through it. Proponents of solar radiation management, such

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