Scientists are preparing to test a radical new way of tackling climate change by deliberately dimming the sun.
A team of researchers from Harvard University plans to release a balloon into the skies over the southwest United States in the new year.
Once it reaches an altitude of 20km, the balloon will release a chalky material to bounce the sun’s heat back into space.
Only a tiny amount of material will be released, and the effects will be limited to a few square kilometres. Even so, the experiment has reignited concern about such “geoengineering” solutions to tackling climate change.
The fear is that such schemes may have unintended consequences, solving one problem only to create many others.
Yet there is also mounting concern that time is running out to deal with climate change.
At the UN climate change conference in Poland last week, scientists insisted that CO2 emissions need to be slashed by almost 50 per cent in barely a decade to avert disastrous consequence.
Four former UN climate conference presidents went even further, declaring that decisive action is need in just two years.
Scientists have been issuing such dire warning for decades without any obvious effect. Despite repeated calls on governments to take action on CO2, emissions are expected to reach record levels this year.
Meanwhile, evidence of the need for action continues to mount: 17 of the 18 warmest years on record have occurred since 2001, with the last four being the warmest of all.
The impact of the rise in global temperatures is also growing. According to the UN, the cost of climate-related disasters last year alone topped half a trillion US dollars.
In the face of such apocalyptic statistics, the reluctance of the major CO2 emitters – principally China, the US and India – to take action is prompting growing interest in radical measures.
And they don’t come much more radical than giving the Earth a sun-screen.
Scientists already know it can work, because Nature has done it countless times – via massive volcanic eruptions.
As recently as 1991, the eruption of Mount Pinatubo in the Philippines injected around 20 million tonnes of sulphur dioxide gas in the stratosphere.
This bounced back so much of the sun’s light that the Earth’s average temperature dipped by 0.5°C for around 18 months.
Given the colossal size of the atmosphere, the effectiveness of this way of cooling the planet is astounding. In effect, adding barely one part per billion of reflective matter to the atmosphere cancelled out over a century’s worth of global warming.
But that also highlights a key concern about geoengineering: the risk of falling foul of the Butterfly Effect.
First coined by a climate researcher in the 1960s, this describes how the atmosphere has a way of amplifying even small tweaks – like the flap of a butterfly’s wing - into big consequences.
The Butterfly Effect is known to stalk attempts to tinker with the amount of sunlight reaching the earth’s surface. During the 1980s, average temperatures over mainland Europe started to rise far more rapidly than expected using global warming models.
Scientists traced the surge to – ironically enough – the clean air campaigns which forced factories to cut back on the amount of gunk they belched into the air.
This also cut the amount of light-reflecting gas and dust they emitted, thus allowing more of the sun’s heat through, triggering a jump in temperature.
That in turn highlights the other concern about geoengineering: its ability to trigger what the Germans call a schlimmbesserung: a "worse bettering", in which a supposed solution actually makes things worse.
Nature again provides worrying insights into some of the potential unintended consequences of dimming the sun. Following the Mt Pinatubo eruption, the darker skies led to cuts in the yields of crops such as rice, wheat and maize.
Some consequences are less obvious. The sulphur dioxide spewed out by the volcano cut visible light levels, but also boosted levels of harmful ultraviolet light – because of its effect on the ozone layer, which normally protects us from this radiation.
The eruption may also have affected the behaviour of the jet stream – the fast-moving currents of air in the stratosphere which can unleash bizarre weather across entire continents.
The scientists behind the planned experiment are fully aware of these lessons from nature, and have taken every precaution to prevent nasty surprises.
The first tests will simply release ice into the stratosphere from the balloon, to check all the systems are working.
The team then plans to move to calcium carbonate dust, and possibly other materials to study their ability to bounce back sunlight.
The amounts involved are so small they will only affect the air around the balloon, and will vanish in a matter of hours.
But critics of the experiment fear the experiment will have a permanent impact, by legitimising geoengineering in the minds of politicians.
The attraction of this remedy for global warming has been boosted by a recent UN report suggesting that deliberate solar dimming could cut global temperatures by 1.5°C for as little as USD 1 billion a year.
The prospect of a cheap and quick remedy to the impact of CO2 on global warming is likely to be hard for politicians to resist.
Yet the very success of solar dimming could lead to perhaps the most disastrous unintended consequence of all – by convincing politicians they need do nothing about CO2 levels.
While their effect on global warming might be reduced, ever-greater levels of CO2 would end up in the world’s oceans, making them increasingly acidic – with as yet unknown consequences.
In the end, the only way to find out the truth is via carefully planned scientific experiments. On the face of it, that is exactly what the Harvard team is planning. And it's far preferable to politicians suddenly waking up to the threat and demanding an instant fix which proves anything but.
Robert Matthews is Visiting Professor of Science at Aston University, Birmingham, UK