Unintended consequences: could solar radiation management create new climate risks?

On January 1, 2020 the International Maritime Organization’s (IMO) much-vaunted Global Sulphur Cap regulation came into force. After years in the debating chamber and months of fervent preparation, the global shipping industry had agreed it would cap sulphur content in marine fuel at 0.5%, down from 3.5%. The aim was to cut particulate emissions and help prevent the thousands of premature deaths from pollution globally each year.

No one could know then this cap would turn out to be arguably the largest accidental experiment in geoengineering in human history.

Earlier this year, US scientists researching the sudden increase in global temperatures in 2022 and 2023, made a startling discovery: the 80% drop in sulphur dioxide from ships had created a so-called geoengineering “termination shock”.

Dimmer clouds over the ocean – caused by fewer sulphur aerosols in the atmosphere – meant less sunlight was reflected into space, they said. Overall, the IMO’s efforts to tackle global warming may have caused an additional rise of about 0.16°C in global average temperatures.

This research is still being corroborated by the global scientific community but, if proven, shows the tangible risks of tampering with Earth’s atmosphere. It also lays bare the unintended consequences of rushing to find solutions to climate change.

Solar radiation management

Solar radiation management (SRM) has long been mooted as a potential way to limit the impacts of climate change. Its proponents argue, if conducted on a temporary basis, SRM could buy us more time to cut greenhouse gas (GHG) emissions and reduce carbon dioxide in the atmosphere.

SRM encompasses a range of approaches that aim to limit global warming by increasing the amount of sunlight the atmosphere reflects back into space or by reducing how much outgoing thermal radiation is trapped by clouds.

Potential methods include injecting reflective aerosols into the stratosphere and brightening clouds over the ocean by spraying mist created from sea water from unmanned rotor ships.

No matter how it may be performed, SRM is a solution that is slowly garnering interest from governments and scientists alike. The UK and German governments already have an explicit position of support for SRM research and countries including the US, China, Finland, Norway and Japan have funded exploratory projects into SRM techniques.

However, Robert Muir-Wood, chief research officer at Moody’s RMS, argues SRM remains “extremely controversial, with a lot of opposition to even performing experiments”.

He says while the “theory of geoengineering is quite well developed, the practice of geoengineering has got absolutely nowhere”.

This is what makes experiments like the shipping industry’s action on sulphur dioxide so interesting, he adds.

“If anyone had suggested an experiment to see if the sulphur cap for ships’ fuel influenced the climate, it would never have been allowed. But because it wasn’t considered an experiment around geoengineering it happened and we can see the results,” Muir-Wood says.

Emerging understanding

This sentiment is shared by Sarah Kapnick, chief scientist at the US National Oceanic and Atmospheric Administration (NOAA), and her special adviser, Greg Frost. Between them they have more than 50 years of experience in climate science but acknowledge SRM is a politically sensitive and emerging field where practicalities are concerned.

“It’s right to say this is emerging. We’ve passed through theoretical and early research and it’s starting to be more applied, but we must create a model of the Earth’s atmospheric system to get there,” Kapnick says.

Robert Muir-Wood, Moody's RMS

The US Congress has injected $41m over five years into what Frost and Kapnick both call a “comprehensive research programme” for the NOAA.

“The NOAA is the only federal agency across the US that has funding from Congress to study SRM,” Kapnick says, adding a huge focus of this research will be aimed at understanding the scientific foundation of SRM approaches and their impacts.

It is these impacts that give Muir-Wood the most pause for thought.

“At present, everyone knows acts of God, hurricanes and droughts happen. But as soon as blame can be assigned, liability insurance comes into play. The question becomes, who might need to pay up because they can’t prove they weren’t responsible in some way?”
_{Robert Muir-Wood
Moody’s RMS}

“The insurance sector might not know it, but we’ve seen how inadvertent SRM can cause claims,” he says. He cites the eruption of Mount Pinatubo in the Philippines in 1991, which was the second-largest volcanic eruption of the 20th century. 

Scientists believe the volcano dumped more than 17 million tonnes of sulphur dioxide into the atmosphere, causing a global volcanic winter and an average decrease in global temperatures by 0.4°C. This had several knock-on effects, with stratospheric temperature increases purported to have contributed to the 1993 Storm of the Century in the US. The storm – also known as 93 Superstorm - caused $5.5bn of damage in 1993 ($12bn in today’s money).

“Mount Pinatubo proves the climate is very sensitive to injecting aerosols in the upper atmosphere. Between this and the Global Sulphur Cap, we have two pretty good inadvertent experiments,” Muir-Wood says.

Causal link challenge

But the 1991 example also shows the problem for insurers and policymakers alike when it comes to SRM – the still relatively limited understanding of interconnected atmospheric systems makes proving causal links extremely challenging.

Muir-Wood points to the example of the Braer storm of January 1993, which remains the most intense extratropical cyclone ever recorded over the northern Atlantic. The “explosively deepening” storm took its name from MV Braer – the stranded oil tanker the cyclone hammered near the Shetland Islands.

MV Braer was a landmark case in marine environmental protection and Muir-Wood says there is a possible link to the Pinatubo eruption two years earlier.

Sarah Kapnick, NOAA

“One theory is the exceptional low pressures [associated with the Braer storm] were a function of the cooling caused by Mount Pinatubo. Here we potentially have a cooling event that was linked to some other things that might not have happened otherwise… and SRM tries to imitate those cooling events.”

He is quick to add the standard of proof for these causal links would be very different. “To prove that link to the point you could actually use it to justify insurance payouts would be quite challenging,” he says.

This speaks to a wider point about why SRM should be on the radar for insurers. Muir-Wood says in court it would be very hard to prove claims-generating events were caused by SRM modifications.

“Clearly the only long-term solution to anthropogenic climate change is to reduce the amount of GHGs in the atmosphere. But that’s taking longer than we hoped… SRM is one way to cool the planet down very quickly and cut some of the known risks of climate change that are already being felt”
_{Sarah Kapnick
National Oceanic and Atmospheric Administration}

“At present, everyone knows acts of God, hurricanes and droughts happen. But as soon as blame can be assigned, liability insurance comes into play. The question becomes, who might need to pay up because they can’t prove they weren’t responsible in some way?” he asks.

Muir-Wood argues even if international co-operation on SRM could be agreed and a global body was created to manage the world’s geoengineering projects, it would be “incredibly burdensome” for an insurer to sit at the end of the process and underwrite based on potential liability.

He says even when running a hypothetical court case (a decade ago) on the attribution of climate change-related claims, it was not possible to get the “nature of proof” that would be used in an actual court of law.

The fundamental challenge, he says, is all atmospheric models have assumptions. “We can say that climate change might have increased the probability of a heatwave by a factor of 10, but to establish liability requires a level of proof beyond any scientist right now.”

For an SRM project, the challenge would be to prove the geoengineers were not in some way responsible for the occurrence of the latest climate extreme.

This is a sentiment the team at the NOAA shares. Much of their $41m in Congressional funding is being directed towards the challenge of atmospheric modelling, as a pre-requisite to any SRM project.

“How does one begin to even think about how to create the framework for creating this in the models?” Frost asks. “I think there won't be a point where we just know everything we need to know. The idea, hopefully, is eventually we narrow uncertainties… at first, the more we learn, the more uncertainties we might create.”

In other words, SRM may create more Global Sulphur Cap-style risks and unintended consequences. “There are uncertain but very real potential impacts of SRM on regional weather and extreme weather events. That uncertainty could lead to risk and that risk could lead to conflict,” Frost adds.

Winners and losers

They say SRM might be a game of winners and losers if it is deployed on a global scale. But the NOAA still believes it is worth pursuing because SRM can enable reduced heat stress, mitigated sea level rises, reduced impacts to agriculture and ecosystems and even diminished coral bleaching.

It is a textbook example of not letting perfect be the enemy of good and both Kapnick and Frost admit SRM is far from perfect.

“Let’s be clear: SRM does not deal with the underlying drivers of climate change. At best, it’s a band aid for its impacts. The best case of SRM is we treat it as a stop-gap to enable us to reduce emissions in real terms across the global economy,” Frost says.

Muir-Wood points out even projects created with the best of intentions may not make it out of the research lab.

“When it comes to SRM, we already see a lot of societal pressure. It’s like a ‘chemtrails conspiracy’ for the climate,” he says.

Resistance from citizens even to small-scale SRM projects in countries such as Sweden have served as the harbinger of the challenges to come. But Muir-Wood stresses the climate change science and policy community are also resistant to SRM projects because they distract from the real drivers of climate change – and potentially give bad actors a pass to continue their polluting ways.

Kapnick and Frost are sympathetic to that perspective, but still support SRM.

“Clearly the only long-term solution to anthropogenic climate change is to reduce the amount of GHGs in the atmosphere. But that’s taking longer than we hoped… SRM is one way to cool the planet down very quickly and cut some of the known risks of climate change that are already being felt,” Kapnick adds.

SRM, then, fits squarely on the Rumsfeld Matrix. Scientists argue the “known unknown” impacts of SRM are better than the “known known” impacts of climate change. Caught in the middle are citizens and, yes, insurers, who will have to watch from the sidelines as these projects are developed and debated in the coming decades.