‘Solar radiation modification’ will have its moment in the sun — an approach that includes polluting the stratosphere with sulphur dioxide to block out heat
By Danny Fortson | 17 September 2023
At the Rockefeller Foundation in New York, a group of the world’s top climate experts last week give oxygen to a proposal that has long been seen as taboo: injecting more pollution into the stratosphere.
Not just any pollution, but sulphur dioxide, which in large quantities can block enough of the sun’s rays to temporarily cool the planet. The notion of so-called solar radiation modification (SRM), the blanket term for techniques that seek to lessen the power of the sun’s rays, is not new. About 2,000 research papers have been produced on how we can block the sun — from giant mirrors in space to marine cloud brightening, which involves seeding clouds with seawater to make them more reflective.
So far, such ideas have largely been dismissed as outlandish or ignored entirely by policymakers and executives. Momentum for more radical solutions, however, is growing as climate change arrives in visceral ways — from extreme droughts and heatwaves to flooding and wildfires.
SRM’s moment may be hoving into view. It got its most prominent airing yet last week at the Climate Overshoot Commission when it convened in New York. The organisation was set up in 2022 on the assumption that the world will blow past the 1.5C temperature rise that countries agreed to target in the 2015 Paris climate accord.
Pascal Lamy, chairman of the commission and former director-general of the World Trade Organisation, laid out its recommendations on not only adapting to a warming world, but on how technology might muffle the most extreme consequences.
The commission called for more urgent research into SRM’s potential benefits and risks, but also asked for a near-term moratorium amid worries that countries or companies alarmed by the quickening pace of climate catastrophes will act unilaterally before exploring the downsides.
“This is the first major political intervention that would put geo-engineering on the table as mainstream policy,” said Joshua Horton, a Harvard fellow and research director at the commission. “My hope is that this report creates space, and a sort of acceptance of talking about it in a serious way — and not get treated as radioactive or something you can’t talk about in polite company.”
Pascal Lamy is head of the commission pushing for SRM — and calling for societies to adapt to the worsening climate.
Perhaps the most realistic technique is stratospheric aerosol injection — putting sulphur dioxide into the stratosphere. It holds promise as a potential stop-gap measure to give more time to decarbonise industry and suck CO2 out of the atmosphere.
The science is alluring because, as climate mitigation efforts go, it would be relatively cheap and potentially hugely effective, cooling the planet by 1C or more for as little as $20 billion (£16 billion) a year. Remaking the global energy system, by comparison, will cost trillions of dollars.
This cooling effect would probably be achieved by building a fleet of aeroplanes capable of reaching altitudes of about 12 miles — roughly twice the cruising altitude of airliners. These planes would regularly deposit large amounts of SO2 into the stratosphere, creating a blanket of aerosols that ever so slightly shades the planet. Harvard professor Wake Smith said such planes could be easily developed as “novel assemblages of well-established technologies”.
In June, the White House laid out the case for a federally funded SRM research programme, and San Francisco start-up Make Sunsets is developing balloons to send sulphur dioxide into the stratosphere.
Emmi Yonekura says space mirrors are no longer sci-fi.
Opposition to such ideas, however, is founded on a number of concerns. Some say it is a prime example of “moral hazard” — that cooling the planet this way would be a licence to keep burning fossil fuels. It is also unclear what large-scale sulphur injection would do to the environment. Given the altitudes at which it would be injected, it is not expected to cause acid rain, though it could influence regional weather patterns in unpredictable ways. Sulphur dioxide also falls out of the stratosphere after about a year, so it would need steady replenishment.
Horton likened the measure to “a giant band aid, which could be really useful, but it’s not actually addressing the underlying problem”.
It also raises the spectre of “termination shock”. Solar geo-engineering merely masks the effects of emissions, so if it were ever to stop — say, due to war or unforeseen environmental impacts — the effects of warming would come roaring back.
Those challenges have led some to argue for research into even more radical ideas, such as space mirrors. A fleet of satellites about 1.5 million kilometres from Earth, with enough mirrors to cover the landmass of Brazil, could cool the planet by about 1.5C, experts estimate. Emmi Yonekura, a scientist at the Rand Corporation think tank, last year penned an article titled “Why Not Space Mirrors?”, claiming that huge advances in materials science and plunging rocket launch costs meant the idea deserved proper scientific scrutiny.
“Decades ago, this was sort of proposed as a sort of sci-fi idea and was ruled out quickly. But as the space industry has been ramping up in recent years, it’s got to a point where we should actually consider a feasibility study,” she told The Sunday Times.
“It’s not necessarily that we’ve decided solar geo-engineering is on the table and we’re going for it, but there is this growing sense that our mitigation efforts to reduce emissions ... won’t be enough.”