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It's 2030 — a bit over 10 years from now — and a fleet of modified cargo planes take off carrying an unusual payload.

They're headed 20 kilometres up — way above where existing commercial aircraft fly — where they will spray tonnes of sulfate particles into the stratosphere.

By the end of the century special planes like this will be making 300,000 flights a year to deliver millions of tonnes of sulfate particles to reflect sunlight.

It's a last-ditch effort to save the world from dangerous warming because we haven't been able to get our greenhouse emissions under control.

You might think this giant planetary sunshade sounds far-fetched, but some scientists starting to research this technology think we may well need such "a brutally ugly technical fix".

However others argue that such a speculative technology — known as "stratospheric aerosol injection" — poses even greater risks than climate change itself.

Indeed, this technology was listed in a 2018 Global Catastrophic Risks report on the major threats against humanity.

But the report also listed climate change. And three years after the historic Paris climate agreement was signed we face an uphill battle to stop the world from dangerous warming.

So, could creating a global sunshade be used to avert a climate change disaster or make things worse?

So how's it supposed to work?

Stratospheric aerosol injection (SAI) is the most researched of a suite of technologies aimed at cooling the Earth known as "solar geoengineering".

Other forms of geoengineering aim to suck carbon dioxide out of the atmosphere — using anything from fans to forests.

SAI was inspired by the cooling effect of volcanic eruptions such as the 1991 explosion of Mount Pinotubo.

Volcanic eruptions shoot sulfur dioxide up into the stratosphere where it forms tiny droplets of sulfuric acid that float around the globe for a year or two acting like tiny mirrors to reflect sunlight.

Mount Pinotubo spewed out 20 million tonnes of sulfur dioxide causing a dip in global temperatures of about half a degree.

Proponents of SAI suggest we could deliberately inject sulfate aerosols into the stratosphere to achieve the same effect.

Assuming the pledges made under the Paris agreement were honoured, Dr MacMartin has estimated we would need to cut the amount of sunlight reaching Earth by 1 per cent to limit warming to 1.5 degrees.

To reach that target, he proposed the deployment of SAI would need to be ramped up from 2030 or 2040 so by the end of the century "roughly a Pinotubo's worth" of sulfate aerosols would be injected into the atmosphere every year.

Why would we want to use it?

The IPCC scenarios to limit warming rely on technologies to remove carbon dioxide, but do not include technologies like solar geoengineering.

According to the IPCC there is evidence SAI could theoretically be a relatively cheap ($US1 to 10 billion a year) way of limiting warming to below 1.5 degrees — "if side-effects are low or neglected".

However it concluded the use of the technology could be limited by factors such as "public resistance, ethical concerns and potential impacts on sustainable development."

But Dr MacMartin believes we can't gamble on IPCC strategies that rely on the success of carbon removal technologies.

"We don't really know how much it's going to cost and how fast we can develop it."

Right now, SAI only exists in computer climate models in which the amount of sunlight falling on Earth is tweaked.

But Dr MacMartin would like to see models developed that specifically include the process of stratospheric aerosol injection.

Meanwhile, a team at Harvard University wants to carry out a field experiment in which a small quantity of aerosols is injected into the stratosphere from a balloon.

The aim of the experiment, known as Stratospheric Controlled Pertubation Experiment (SCoPEx) is to study basic particle behaviour and chemistry to better understand the risks and benefits of SAI.

Lizzie Burns, managing director of Harvard's Solar Geoengineering Research Program, said the experiment "would not be harmful in a physical sense to humans or the environment."

While it was possible the team may test sulfates, in the first instance they want to use calcium carbonate aerosols — limestone dust.

This is theoretically less likely than sulfates to deplete ozone and warm the stratosphere, she said.

But not all experts are onboard with the idea of SAI.

What are the environmental risks?

Along with concerns that the technology could deplete ozone levels, it might also affect things such as ocean circulation, extreme weather, agriculture, biodiversity and disease patterns.

And it might lead to winners and losers.

For a start, the global thermostat would be set for a particular global average temperature.

And — as with global warming now — any given change in global average temperature is linked to a particular redistribution of heat around the globe, with some places warming more than others.

Because heat drives the water cycle — in which water evaporates, forms clouds and eventually returns as rain — there will be an accompanying shift in rainfall patterns, which has implications for agriculture.

While temperatures might fall, the climate could change in unpredictable ways, said Janos Pasztor of the Carnegie Climate Governance Initiative, which wants to see global agreements to control the technology.

Shuchi Talati, of the Union of Concerned Scientists' climate and energy program agreed.

"We don't know what the regional impacts will be," Dr Talati said.

"There have been different modelling papers showing different things."

Model results differ depending in part on how much cooling is assumed and for how long and at this stage there is a debate on how big the "winners and losers" problem will be.

Potential to inflame global tensions

Beyond the environmental risks, there is also concern that, like climate change itself, SAI could increase security risks.

Who controls the global thermostat and to what end?

Could, for example, the US seek to set the thermostat so it gives its Midwest farmers good rain, at the price of worsening drought in Africa?

Ungoverned deployment of the technology — which is cheap enough for an individual country or even rich individual to go it alone — could pour "oil onto the fire" of existing tensions, Mr Pasztor said.

"Imagine a situation where China unilaterally goes ahead [with SAI] and a couple of years later the monsoon fails in India and lots of people starve.

"Even if you cannot link the failure of the monsoon to the stratospheric aerosol injection activity, people in India might think it's because of the Chinese."

Indeed, one of the biggest uncertainties about SAI is how humans will behave.

If, for example, those with the hands on the levers decided to switch SAI off, or were forced to because of a terrorist attack, this could result in a massive rebound of global temperatures.

The likelihood of this so-called "termination shock" has been debated but if it were to occur the sudden increase in temperature could lead to more rapid climate change than we might otherwise have had.

Mr Pasztor said the results of ungoverned use of the technology could rival climate change as a threat.

"If you make use of this technology and do it badly or ungoverned then you can have different kinds of global risks created that can have equal, if not even bigger, challenges to global society."

'Temperature debt ... a massive gamble'

Mr Pasztor, who helped advise on the Paris agreement while at the UN, emphasised solar geoengineering should not be used as a replacement for cutting emissions but as an "insurance" in case we overshoot our temperature goals.

Others fear reliance on SAI — and indeed some less proven carbon removal technologies — could undermine efforts to reduce greenhouse emissions in the first place.

"To a degree my concern is around the moral hazard induced by these technologies," said Mike Hulme, former IPCC climate scientist now at the department of geography at Cambridge University.

He likened the reliance on "speculative" geoengineering technologies to combat climate change to the risky financial strategy that led to the sub-prime mortgage scandal.

Just like low-income American mortgage holders were allowed to build up debt, solar geoengineering could create a "temperature debt" that would only be repaid if large scale deployment of carbon removal technologies was successful, Professor Hulme said.

"It is a massive gamble," he said. "Far better is not to build up this debt in the first place.

Professor Hulme and climate policy expert Jeremy Baskin of the University of Melbourne both think SAI, in particular, should not even be on the table.

Dr Baskin, who has studied the politics of geoengineering, said some of the risks would be "unknowable" until it is too late, and the technology was not only "unreliable" but "ungovernable".

"Even the most avid supporters of this technology would concede the global security consequences are really really troubling."

But beyond this, Dr Baskin, said, there is a prior question: should we do it?

He pointed to a previous proposal to genetically engineer smaller less resource intensive humans to deal with the problem of climate change.

"This was laughed out of court but conceptually it's the same idea," said Dr Baskin

'The idea is terrifying, but so is climate change'

Harvard's Ms Burns was adamant about the need to press ahead with solar geoengineering research to try and better understand the risks and help inform decisions on whether to use it or not.

"The idea is terrifying but so is climate change," she said.

The team is taking things slowly with the SCoPEx experiment, which was first proposed in 2014.

A similar field experiment by UK researchers — called Stratospheric Particle Injection for Climate Engineering (SPICE) — was cancelled in 2012 amid claims researchers had a conflict of interest.

Dr Talati said the problem at the time was a lack of sensitivity to concerns about where such experiments were headed.

"The fact that you're working on something that's going to affect the entire world I think that's a really scary thought to people," she said.

The Union of Concerned Scientists would like to see "meangingful civil society input" into what happens next.

"Right now there really aren't that many people at the table," Dr Talati said.

Her organisation is among those helping the Harvard team to set up an independent advisory committee that will determine if and how the field experiments go ahead.

Meanwhile, support for research on solar geoengineering is building up with scientific institutions like the US National Academy of Sciences taking an interest and private funding from the likes of Bill Gates.

It's clear, though, it won't be just a question of what science is done — but how.

"There has to be room for people who want to say no," Dr Talati said.

"Because this is a global endeavour and the only way it can really be legitimate is if we have all voices in the conversation."

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