Two years ago this month, in a disused Norfolk airfield, a small group of scientists were preparing to undertake one of the more controversial experiments in British scientific history. What little equipment it needed – a B&Q pressure washer, 1km of hydraulic hose and an 8m air balloon – had been bought or loaned. A truck was ready. Once in the air, the dirigible balloon would spray 120 litres of fine water droplets into the East Anglia sky, a miniaturised test for a much larger system that would eventually pump out chemical particles to reflect sunlight and, so the scientists calculated, cool the planet. It was to be a momentous day.

Geoengineering – as defined by the Royal Society in 2009 – is the large-scale, technological manipulation of the climate (some call it "planet hacking"). After decades of theorising, the Cambridge group was going to be the first in the West to take research out of doors. But shortly before lift-off, they aborted. There was, they feared, no way of knowing who could use their research, or in what way, and the Stratospheric Particle Injection for Climate Engineering (Spice) team did not want to open a door that might be impossible to close.

Little has changed in practical terms since 2011. The Spice balloon has been shipped back to its owners; the pressure washer is back in use spraying down cars. Yet, since the end of last month, the prospect of geoengineering has cast a giant shadow over the world of environmental campaigners and climate scientists. On 27 September, the Intergovernmental Panel on Climate Change (IPCC), the world's most respected authority on global warming, acknowledged for the first time the need to consider it as a weapon against rising temperatures.

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Roughly every six years the IPCC lays out the latest climate science in layman's terms, so policy-makers around the world can understand – and base policies on – accurate information about mankind's effect on rising temperatures. The 2013 report consolidated the belief that we are the dominant cause of warming to 95 per cent certainty. It acknowledged that the rate of surface-temperature rise had slowed in the past 15 years – but suggested that heating continues unabated in the oceans, and made it clear that short-term variations do not reflect long-term trends. Sea levels are rising faster than previously projected. Ice is melting faster, too. It is "very likely" that extreme weather events will soon strike more regularly. And what we are doing now, the IPCC stated, is "irreversible on a multi-century to millennial timescale".

After the deluge of bad news came the first mention of geoengineering. The technology is discussed in the final paragraph of the report, in a profoundly cautious tone: "Modelling indicates that [certain geoengineering methods] if realisable, have the potential to substantially offset a global temperature rise." Following the report's release, the Coalition's chief scientific adviser on climate change, David MacKay, called for more investment in research and development. "I am going to recommend that this is a public priority for the long-term," he said. And a number of people started to panic.

Geoengineering technologies are the stuff of Hollywood disaster movies. Researchers have suggested sending a giant glass sunshade into space to reflect light; the eruption of artificial volcanoes, or spraying of sulphate aerosols into the stratosphere; dissolving mountains and putting remains in the sea; and, least dramatically, perhaps, filing the ocean with iron filings to stimulate algae blooms. "People are right to be revolted and alarmed," says Matthew Watson, the head of Spice. "That's a good thing. It should not sound easy. If we do this, it will be the clearest indication we have failed as planetary stewards. It will be a desperate thing to do."

Already, a protest movement of sorts has formed. At the end of August, a ragtag group of "chemtrails" protesters met outside Downing Street, wielding placards with pictures of dirty skies and slogans such as, "Our weather is being engineered." They believe the government began manipulating the climate years ago, through plane-exhaust trails. (One told me that the only reason the sky above was blue and clean that day was that the Government had made it so.) But the fear of conspiracy theorists such as these is shared on a more rational level by scientists as much as environmental activists, for reasons that stretch back to the Vietnam War.

Between 1967 and 1972, the US air force carried out "Operation Popeye", the first use of weather as an instrument of war. Almost 3,000 flights were sent into the skies above the Ho Chi Minh Trail, where planes seeded clouds with silver iodide particles, causing storms and extending the monsoon season. "Popeye" turned the strategic pass into a bog – and appalled the international community. In 1977, the Enmod (Environmental Modification Convention) treaty outlawed weather warfare.

Since the IPCC statement, the spectre of Enmod has been invoked again. "The UN General Assembly should dust it down," says Jim Thomas of the ETC group, an organisation that advocates sustainable development. "We need to start to put in place real stops on geoengineering." The action group on Erosion, Technology and Concentration views any technological solution to climate change as a "dangerous distraction", taking focus away from conventional CO2 reduction methods, such as renewable energy sources. The nod to geoengineering in the IPCC report, argues Thomas, is enough to give polluters licence to continue as before, waiting for a technological silver bullet that may never arrive. A "moratorium should be called" on research, even in the lab, he says.

Except on that critical point, the line between backers of geoengineering and those who want it banned is surprisingly slim. Both camps largely agree that burning fewer fossil fuels is priority number one. While a handful of scientists think we must geoengineer or face disaster, they form a minority. One, the Russian Yuri Izrael, carried out a sulphate aerosol experiment in 2009; another, Russ George, dumped 100 tonnes of iron sulphate off the coast of Canada in 2012. It was wacky research such as this that the Spice team had in mind when they called off their own experiment. Geoengineering is "the wrong answer", says Spice's Matthew Watson, who blogs as the Reluctant Geoengineer. "The right answer is to decarbonise. But that takes a long time, and isn't happening at the moment."

From computer modelling, we know something of how wrong an answer geoengineering could be. Spraying large amounts of sulphate aerosols into the stratosphere would, for example, blur the presence of the sun's disc. Rainfall patterns could also go haywire, the ozone layer break up. Dump iron filings into the sea and you might well kill off swathes of marine life.

Equally terrifying are the geopolitical implications. Steve Rayner, who co-directs Oxford's Geoengineering programme, uses the example of the India/Pakistan conflict. Should India at any stage inject sulphate aerosols into the stratosphere, and should those aerosols disrupt the Asian monsoon, Pakistan could then blame any flooding on its old enemy. The chances of war, notes Rayner, escalate rapidly. This is why his team has foregone practical research and is, for the moment, focusing exclusively on governance issues. "I'm not for geoengineering, and I'm not against it," he says. "Let's get a better idea of the implications, and find a way of ensuring that research is conducted responsibly."

A similar sense of caution is shared by most geoengineering researchers. In one area only do they become aggressive: as things stand, we are not doing enough to reduce the threat of global warming. It is "beyond optimistic", says Rayner, to hope that treaties such as the 2009 UN Framework on Climate Change will stabilise our impact on the climate at a level below 2C – the convention's stated aim, and a figure beyond which environmental catastrophes might become norm rather than exception.

And there are geopolitical ramifications, too: an Alaskan village on the verge of being "wiped out" by rising sea levels has already filed a $400m lawsuit against oil and coal companies. Failing to slow climate change, says Watson, will hurt the least responsible nations the most. Britain might watch the effects of climate change for the next 50 years. Pakistan, Bangladesh and Jamaica are less likely to have that luxury.

COULD IT WORK? FIVE GEOENGINEERING PROJECTS

STRATOSPHERIC AEROSOLS

Heat could be stopped from reaching the Earth's atmosphere by reflecting some of the sun's rays back into space. Volcanic eruptions, which release a huge amount of sulphate particles, have led to periods of cooling: the eruption of Mount Pinatubo in 1991 saw global temperatures fall by ½C for 18 months, the released sulphates reflecting light.

Drawbacks: Though this would be among the fastest fixes to global warming, it is also among the riskiest. Global weather patterns could be drastically affected.

Practicality: 5/10

SPACE REFLECTORS

Assembling a giant glass sunshade in space then firing it into orbit could reflect a small proportion of the sun's rays, according to astronomer Roger Angel. He claims a reduction of just 2 per cent in sunlight reaching the Earth could make warming more manageable.

Drawbacks: Asteroids, primarily. Glass discs would be vulnerable to space debris, and maintenance of a "space umbrella" requires time, expertise and money.

Practicality: 4/10

OCEAN FERTILISATION

Dropping iron filings into the ocean can generate blooms of carbon-absorbing plankton. The plankton then take in CO2 at the surface, then carry it with them as they drift to the ocean floor after death, creating "carbon sinks".

Drawbacks: It takes a long time for comparatively little effect, and the impact on marine life could be devastating. Where plankton blooms in contained water sources, fish can suffocate. Fertilising the oceans would also marginally acidify them.

Practicality: 2/10

ARTIFICIAL TREES

Technology already exists to extract CO2 from the air. Klaus Lackner of the Columbia University Earth Institute has developed machinery that can extract 1,000 times more CO2 a day from the atmosphere than a natural tree. Building these structures around the world could add muscle to nature's blueprint.

Drawbacks: High cost: direct air-capture measures like this would cost a minimum of $600 per tonne of CO2 removed.

Practicality: 6/10

AFFORESTATION

Planting real trees has the aesthetic advantage of creating forests, and would satisfy those who believe that "engineering" the climate is madness. Painting the roofs of houses white or covering tranches of desert in reflective material could also bounce back unwanted radiation.

Drawbacks: Geoengineers argue that man is already engineering the climate with pollution from fossil fuels. Also, these methods alone would not be enough to avert the worst of climate change.

Practicality: 1/10