November 26, 2018, by Tim Radford
With new aircraft, humans could potentially mimic volcanic action, dim the sunlight and slow global warming, showing that geoengineering works. It’s a controversial idea.
LONDON, 26 November, 2018 – Nobody knows for sure whether we shall ever see if geoengineering works. But now somebody knows how to do it.
Engineers have designed an aircraft that could lift a cargo of sulphur dioxide to an altitude of 20 kilometres and spray it into the stratosphere to darken the skies, dim the sunlight and damp down climate change driven by emissions from factory chimneys, power stations and vehicle exhausts.
The aircraft – already dubbed SAIL, or the stratospheric aerosol injection lofter, could cost no more than $2.35 billion a year for airframe and engine, and the first eight could be rolling down the runway 15 years from now to begin flying 4,000 missions a year. By the end of another 15 years, a fleet of 100 high-flying sulphate dumpsters could be in business, making 60,000 high altitude deliveries a year to combat global warming.
US scientists report in the journal Environmental Research Letters that they addressed the costs and practicalities of what is certainly the most-frequently invoked and hotly disputed form of climate engineering on a global scale.
It is considered necessary because, if humans go on burning fossil fuels at the present rates, greenhouse gas build-up in the atmosphere could increase planetary average temperatures to a catastrophic 3°C or more by 2100.
Darkened skies do lower planetary temperatures: violent volcanic eruptions have in recent history injected cubic kilometres of fine ash, smoke and sulphur into the upper atmosphere on scales that lower global average temperatures measurably.
For more than a decade, researchers have argued that – since humankind collectively still shows no great sign of drastically reducing greenhouse gas emissions – some radical form of solar geoengineering might be necessary. Others have opposed the case, citing possible unwelcome consequences.
But the first question was: could it be done at all? The latest answer is that it can, but not with existing hardware.
“This plan is a distraction that may well encourage weaker action on emissions reduction by governments in the hope they will no longer be necessary”
“While we don’t make any judgment about the desirability of SAI, we do show that a hypothetical deployment programme, starting 15 years from now, while both highly uncertain and ambitious, would be technically possible from an engineering perspective,” said Gernot Wagner, of Harvard University.
“It would also be remarkably inexpensive, at an average of around $2 bn to $2.5 bn per year over the first 15 years.”
His co-author Wake Smith, who moved from the aviation business to lecture at Yale College, and who led the study, said he had become intrigued by research that suggested that existing aircraft could be modified to lift huge quantities of sulphur dioxide to great heights, and then release it.
“Turns out that is not so,” he said. “It would indeed take an entirely new plane design to do SAI under reasonable albeit entirely hypothetical parameters. No existing aircraft has the combination of altitude and payload capabilities required.”
The scientists outlined a solution: it had the same weight as a large, narrow-bodied jet passenger aircraft. But to sustain level flight at 20kms, it needed roughly double the wing surface of such an airliner, and double the thrust, with four engines rather than two. “At the same time, its fuselage would be stubby and narrow, sized to accommodate a heavy but dense mass of molten sulphur rather than the large volume of space and air required for passengers,” Mr Smith said.
They then calculated the rate at which such planes could be built, and the numbers needed to make a significant difference to global warming. Global projects on such a scale need international agreement, and the two authors rule out the possibility that any individual nation could hope to secretly operate such a high-flying programme, involving so many flights, without detection. But they do not see it as excessively costly.
“Given the potential benefits of halving average projected increases to radiative forcing from a particular date onward, these numbers invoke the ‘incredible economics’ of solar geoengineering,” Dr Wagner said. “Dozens of countries could fund such a programme, and the required technology is not particularly exotic.”
But solar radiation management or SRM – the catch-all term for any plans to cool the world by dimming the sunlight, rather than reducing greenhouse gas emissions – remains politically fraught, and in any case an incomplete answer: it would, for instance, do nothing to slow the increasing acidification of the world’s oceans, and it could seriously affect rainfall patterns in so far unpredictable ways.
The world has already warmed by 1°C in the last century. In Paris in 2015 the nations of almost the entire world agreed to try to contain global warming to 1.5°C if at all possible. And the world now has only about a dozen years to make this happen.
“Why then set out a plan to implement solar radiation management from a date 15 years hence? This plan is a distraction that may well encourage weaker action on emissions reduction by governments in the hope they will no longer be necessary”, said Joanna Haigh, co-director of the Grantham Institute for Climate Change at Imperial College in the UK.
“Previously, proponents of SRM have suggested that it be used to delay the onset of the inevitable warming arising from human greenhouse gas emissions. This paper, however, seems to suggest that the implementation should be ongoing. Forever?” – Climate News Network
Tim Radford, a founding editor of Climate News Network, worked for The Guardian for 32 years, for most of that time as science editor. He has been covering climate change since 1988.