FT Rethink
From shielding the sun to catching carbon: the geoengineering solutions that could help limit global warming
In 1815, the most powerful volcanic eruption in recorded human history took place on the island of Sumbabwa in present-day Indonesia. For more than six months, ash poured from the Mount Tambora volcano and dispersed around the world, blocking sunlight and lowering global temperatures. In the year that followed – known as The Year Without A Summer – Europe saw its lowest ever recorded summer temperatures.
A similar story was told more recently when, in 1991, Mount Pinatubo erupted in the Philippines, blasting around 17 million tonnes of sulphur dioxide into the atmosphere. It is estimated that global temperatures fell temporarily by 0.5 degrees Celsius as a result.
As we face the prospect of a rapidly warming world, scientists are investigating whether geoengineering – intervening to alter Earth’s natural systems – could help keep a cap on temperatures. One proposal is to replicate volcanoes’ cooling effect artificially by releasing sulphur dioxide into the upper atmosphere.
Other forms of geoengineering are further along the development track – carbon capture and storage, for instance, which takes its inspiration from trees’ ability to absorb carbon directly from the air, is on the cusp of large-scale commercialisation.
Geoengineering advocates argue that such large-scale interventions in Nature will be essential if we are to hit the Paris temperature targets. We take a look at the leading geoengineering solutions, and explore some of the environmental and cost implications.
Watch our video in partnership with the FT and discover the geoengineering solutions that could help limit global warming:
Shielding the sun…or opening Pandora’s box?
In 2014, scientists at Harvard’s Solar Geoengineering Research Program published research on what they called “stratospheric controlled perturbation”1. The paper laid out their plan to release sulphate aerosols into the atmosphere to create a reflective shield around the Earth, reducing the amount of sunlight that reaches the planet’s surface.
Pascal Lamy, former director general of the World Trade Organisation and chair of the Climate Overshoot Commission
A decade on, the Harvard plan has stalled in the face of criticism over the risk of unknown consequences. The technology is relatively simple, however, and a number of small-scale, ‘unsanctioned’ experiments have already been conducted, most notably a Silicon Valley-backed venture, Make Sunsets, which has launched dozens of weather balloons, each containing a small sulphate payload2.
Solar geoengineering, or solar radiation management (SRM) as it is also known, has polarised the scientific community. Pascal Lamy, former director general of the World Trade Organisation and chair of the Climate Overshoot Commission, says that with the risk of overshooting the Paris temperature target growing “higher and higher every day”, governments should “open the box” on SRM research.
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Others fear it could turn out to be Pandora’s box, and argue that the international community must keep the lid firmly closed. Research by MIT suggests that large-scale aerosol-based SRM could alter wind streams and ocean currents and even lead to reduced regional rainfall3, while a 2022 study by the World Meteorological Organisation warns that the release of sulphates could damage the ozone layer4.
Aarti Gupta, professor of global environmental governance at Wageningen University in the Netherlands, has joined hundreds of academics calling for a global moratorium on sulphate-based SRM research. Warning of the unknowable systemic risks it could pose, Gupta says, “In essence this is an untestable technology, because to test it would be to deploy it.”5
Whiter clouds and thicker ice
Deploying sulphate aerosols is just one of a number of solar radiation management solutions. Scientific research is also focussing on whether we could boost the ‘albedo effect’ – the reflectivity – of clouds and polar ice.
April 2024 saw the first US test of a technology that aims to brighten marine clouds by spraying them with a fine mist of sea water6. The hope is that adding small, saltwater droplets to already existing clouds will increase their capacity to disperse sunlight and reflect it back out to space. Stephen Salter, Emeritus professor at the University of Edinburgh, a leading proponent of marine cloud brightening, believes that a fleet of 300 permanently deployed autonomous ships, each carrying specialised seawater pumps, could reduce global warming by 1.5 degrees Celsius, at an annual cost of just USD 100 – 200 million7.
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Like clouds, polar sea ice also plays an important role reflecting sunlight and cooling the planet8. After decades of Arctic ice loss9, and warnings that Arctic summers will be ice-free by 205010, scientists and entrepreneurs are exploring ways to make sea ice more resilient.
The leading solution, which is being tested by Dutch firm Arctic Reflections and UK-based Real Ice, is to pump seawater from below the ice and spray it onto the surface, where the water freezes and thickens the ice.11 Arctic Reflections estimates that just 100–1,000 seawater pumps could save as much as 100,000 km of Arctic ice from melting in summer.
Others are more sceptical. Researchers from Arizona State University conclude that to protect the entire Arctic Ocean in this way as many as 100 million pumps would be needed, requiring 1 billion tonnes of steel (two thirds of the world’s entire annual steel production), occupying half of the world’s entire container ship capacity, and costing USD 5 trillion.12
Pulling carbon from the air
To date, solar radiation management is largely confined to small pilot projects. Other geoengineering solutions are more advanced, however. For over fifty years13, some power plants have deployed carbon capture and storage (CCS) to “scrub” carbon dioxide from exhaust gases by passing them through carbon-trapping filters. More recently, the shipping industry has adopted the same technology to reduce the environmental impact of shipping’s notoriously dirty heavy fuel oil.
Now CCS is being deployed on a more ambitious scale. In May 2024, Swiss firm Climeworks opened the world’s largest direct air CCS facility in Iceland14 – once the ‘Mammoth’ plant has reached full capacity it will capture 36,000 tonnes of CO2 each year directly from the surrounding air, rather than from exhaust gases. Climeworks is working with Iceland’s Carbfix to inject the extracted carbon deep underground, and is deploying renewable geothermal energy to power the process from start to finish.
While solar radiation management remains controversial, carbon capture and storage has gained broader acceptance. According to a 2022 report by the International Panel on Climate Change, “The deployment of carbon dioxide removal to counterbalance hard-to-abate residual emissions is unavoidable if net zero carbon dioxide…emissions are to be achieved.”15
Investing in geoengineering
Much of this carbon removal will be achieved via natural climate solutions such as the restoration and management of forests and peatlands. However, direct air CCS is set to play a growing role. In 2022, Climeworks raised USD 650 million of investment, the largest ever investment in direct air CCS technology16, and a number of major clients, including Microsoft and JP Morgan Chase17, are lining up to pay for carbon removals to help them reach their own sustainability goals.
While solar radiation management is largely still in the testing phase, the International Energy Agency estimates that 130 direct air CCS facilities are either planned or already in development.18 For investors this represents an opportunity to be part of a growing sector while contributing to the transition to a net-zero world. In 2022, the broader CCS sector, which includes both carbon captured at the point of emissions and direct air capture, was valued at almost USD 3.3 billion, with an expected annual growth rate of 6.2% until 2030.19
Despite direct air CCS providing high-quality carbon dioxide removal, a key challenge lies in its costs. For it to be widely adopted, costs must fall from USD 500 -1,000 per tonne to less than USD 200 per tonne.20 Furthermore, although there is broad acceptance among policymakers and scientists, carbon capture and storage remains controversial with many activists, who fear that artificially removing CO2 merely masks our underlying problem, and could even reduce the incentive to cut overall emissions. However, with the UN warning that the window to reaching the Paris temperature goals is “rapidly closing”21, we may soon have little choice. Whether solar radiation management or direct air carbon capture, large-scale geoengineering could become an essential component of the transition to a sustainable future.
1 Stratospheric controlled perturbation experiment: a small-scale experiment to improve understanding of the risks of solar geoengineering | Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences (royalsocietypublishing.org)
2 Climate engineering: a quick fix or a risky distraction? (ft.com)
3 Study: Reflecting sunlight to cool the planet will cause other global changes | MIT News | Massachusetts Institute of Technology
4 Sunlight-Dimming Climate Schemes Need Worldwide Oversight | Scientific American
5 Climate engineering: a quick fix or a risky distraction? (ft.com)
6 To Slow Global Warming, Scientists Test Solar Geoengineering - The New York Times (nytimes.com)
7 How artificially brightened clouds could stop climate change - BBC Future
8 Sea Ice | CMEMS (copernicus.eu)
9 Six ways loss of Arctic ice impacts everyone | Pages | WWF (worldwildlife.org)
10 Observationally-constrained projections of an ice-free Arctic even under a low emission scenario | Nature Communications
11 Pumped up: will a Dutch startup’s plan to restore Arctic sea-ice work? | Polar regions | The Guardian
12 Arctic ice management - Desch - 2017 - Earth's Future - Wiley Online Library
13 You Asked: Does Carbon Capture Technology Actually Work? – State of the Planet (columbia.edu)
14 Climeworks opens world's largest plant to extract CO2 from air in Iceland | Reuters
15 Removing carbon from air vital to reach climate goals, IPCC says | Reuters
16 Direct Air Capture - Energy System - IEA
17 Climeworks' 2023 year in review
18 Direct Air Capture - Energy System - IEA
19 Why carbon-capture tech is key to reaching climate goals | World Economic Forum (weforum.org)
20 How to get direct air capture costs to under $150 per ton | World Economic Forum (weforum.org)
21Window to reach climate goals ‘rapidly closing’, UN report warns | UN News
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