When the threat of climate change from greenhouse gas pollution was brought to the attention of United States president Lyndon Johnson in 1965, his scientific advisers didn’t recommend solving the problem by cutting emissions. They recommended littering the sea with floating glitter. Lots and lots of glitter.

The temperature rise could be offset, wrote the environmental pollution panel of the president’s science advisory committee, “by raising the albedo, or reflectivity, of the Earth”. This could be brought about, they informed the president, “by spreading very small reflecting particles over large oceanic areas”. And it would be cheap, relatively speaking – about $500 million a year.

Sounds crazy? The scientists were serious … and rather prescient. More than 51 years later, similar ideas to cool the planet are being considered to alleviate the symptoms of runaway climate change. One of the pioneering researchers, climate scientist Ken Caldeira, of the Carnegie Institution for Science in the US, told The Saturday Paper that we face a bleak choice: “Either the planet gets quite warm, or we do solar geoengineering.”

“Geoengineering” means a deliberate intervention in the climate system to counter, or offset, global warming. Critics have called it “climate-hacking”: a last-ditch effort to tweak the planetary thermostat back to a more benign setting.

Solar geoengineering is a subcategory of these outlandish proposals, and focuses on reflecting or diffusing sunlight. The most widely studied and controversial technique is spraying sulphate aerosols into the upper atmosphere. This idea comes from observing large volcanic eruptions, which can blast millions of tonnes of sulphur into the stratosphere, dimming the sun. When Mount Pinatubo in the Philippines erupted in 1991, it cooled the Earth by about a third of a degree over a year – and possibly more.

For decades the topic was taboo – scientists were concerned that mentioning they had a back-up plan for global warming would reduce motivation to cut emissions. That changed in 2006 when Nobel laureate Paul Crutzen published a seminal paper calling for more research. Since then, the idea has been explored in hundreds of research articles, reports of Britain’s Royal Society, hearings in the British parliament and the US congress, and a recent report of the US National Academy of Sciences.

Controversially, the latest Intergovernmental Panel on Climate Change (IPCC) assessment report mentioned geoengineering in the prominent final paragraph of its summary for policymakers. And in January this year, a key US government science agency recommended, for the first time, that federal agencies start researching direct intervention in the climate.

Why all the interest? One reason, harking back to Lyndon Johnson’s day, is it could be a cheap substitute for cutting emissions. Sending up a fleet of planes to continuously spray sulphate aerosols in the upper atmosphere may cost in the tens of billions of dollars, whereas replacing global fossil fuel infrastructure may cost trillions.

In the late 2000s, this caught the interest of some conservative think tanks, such as the American Enterprise Institute, which pushed geoengineering as the perfect solution to the climate change problem it had previously denied existed. In 2008 Newt Gingrich, a Republican stalwart turned Donald Trump booster, famously declared that “geoengineering holds forth the promise of addressing global warming concerns for just a few billion dollars a year”, before concluding, “Bring on the American Ingenuity. Stop the green pig.”

But climate scientists argue solar geoengineering could never be a substitute for cutting greenhouse gas pollution. Reflecting sunlight doesn’t reverse the warming, it just masks it temporarily. If we didn’t also cut emissions, we’d have to keep spraying the stratosphere with more and more sulphur.

What would that look like in practice? Offsetting the expected temperature rise to meet the Paris Agreement target, says University of Melbourne climate scientist David Karoly, would be “like having five Mount Pinatubo volcanic eruptions every year continuously for the rest of humanity”.

Then there’s the problem of hitting the off button. Carbon dioxide can persist in the atmosphere for centuries or millennia, but sulphate aerosols hang around in the stratosphere for only a few years. If we suddenly stop pumping the stuff up there, we could face a rapid onslaught of pent-up warming. This frightening prospect has been dubbed “termination shock”.

Physicist David Keith, of Harvard University, a leading researcher on solar geoengineering, has proposed a way to minimise this risk. Instead of offsetting all the warming, he argues, we could just offset half of it. The sulphate spraying could be ramped up and down slowly, avoiding abrupt termination.

“Every major climate model that’s been run shows that if you have a moderate amount of solar geoengineering, say enough to cut the rate of warming in half, then almost all regions show both temperature and precipitation getting closer to pre-industrial,” says Keith. “If it’s true, it suggests it could really reduce climate risks a lot this century.”

Other climate scientists aren’t convinced by Keith’s plan. Raymond Pierrehumbert, at the University of Oxford, says ramping up and down solar geoengineering would still take close to a century. And there’d still be a risk of uncontrolled termination for a range of reasons including “economic collapse, war and disagreement per where to set the thermostat”.

A second attraction of solar geoengineering is that it’s a “necessary evil” in case we don’t cut emissions quickly enough to avoid catastrophic warming.

There’s a resigned realism – some might call it pessimism – to this argument. Caldeira says power stations have long lifetimes, and energy transitions throughout history have generally been slow. On top of that, we’re trying to rapidly replace fossil fuel infrastructure at the same time that the poorest countries are seeking to industrialise.

He isn’t hopeful about our chances. “I think the idea that we’re going to reduce emissions fast enough to avoid warming of 2 degrees Celsius is a fantasy at this point.”

We’re certainly a long way from that goal. The 2015 Paris Agreement aims to keep the global temperature rise “well below” 2 degrees, and hopefully 1.5 degrees. But Karoly says the current commitments on the table would still lead to substantially more than 2.5 degrees, and “possibly even as much as 3 degrees of warming”. And that’s assuming countries actually meet their pledges.

There are strong arguments that geoengineering is looking more likely whatever happens with the Paris Agreement. Almost all modelling of the IPCC’s scenarios to limit warming to 1.5 degrees assumes some kind of planetary technofix to remove carbon dioxide from the atmosphere. If we’re fixated on a temperature target, solar geoengineering could be another “alternative” method to achieve it.

On the other hand, if US President Trump gets his way and Paris unravels, emissions and temperatures could rise even faster. As the only way to cool the planet quickly, geoengineering could be needed as an emergency response.

The biggest fear behind these “necessary evil” arguments is crossing irreversible climate change tipping points. An often-cited example is the thawing of perpetually frozen ground, called permafrost. If the Arctic permafrost melts, it could release tremendous amounts of greenhouse gases.

The problem is we don’t know exactly where these tipping points lie. There’s an argument that “moderate” solar geoengineering could reduce the risk of crossing them by lowering the peak global temperature rise this century, or slowing the rate of change.

But solar geoengineering also introduces entirely novel risks. David Karoly says spraying sulphate aerosols in the stratosphere would deplete the ozone layer, leading to more skin cancer and changed rainfall patterns over the tropics, causing drought. “Major volcanic eruptions like Mount Pinatubo did globally reduce rainfall.”

Despite these dangers, some climate activists are calling for more research into the technique. David Spratt, co-author of Climate Code Red, acknowledges all the concerns but still says we need to at least consider it. “There is a compelling need to cool the planet, and prevent us going past further significant tipping points,” he explains. “The brutal fact is there are no clear easy paths out of this, and there are no paths that won’t involve severe consequences.”

Back in 1965, the scientific advisers to President Johnson wrote that burning fossil fuels would cause significant rises in temperature. “Through his worldwide industrial civilisation, Man is unwittingly conducting a vast geophysical experiment.” In decades hence, will we choose to counteract that massive global experiment with another?

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