Termination shock could make the cost of climate damage even higher

Solar geoengineering could be even more costly than unabated global warming if it is cut off suddenly, leading to a “termination shock” of rapidly rebounding temperatures.

As the rate of greenhouse gas emissions continues to climb, interest is growing in solar radiation modification (SRM) to cool the planet, such as by spreading sulphur dioxide aerosols in the stratosphere to block sunlight.

But solar geoengineering would need to continue uninterrupted for centuries, or the warming that was “masked” would come roaring back at a faster rate. Known as termination shock, this rebound would give humans and animals little time to adapt to the heat and could trigger climate tipping points like ice sheet collapse.

Based on established relationships between temperature increases and GDP loss, Francisco Estrada at the National Autonomous University of Mexico and his colleagues modelled the risks of climate inaction compared with those of solar geoengineering.

If humanity fails to reduce fossil fuel emissions, temperatures could reach a median 4.5°C above pre-industrial levels by 2100, causing economic damages of $868 billion, the researchers estimate. A hypothetical stratospheric aerosol injection programme beginning in 2020 that kept temperature rise to about 2.8°C could halve those damages.

But if the aerosol programme was suddenly terminated in 2030 and temperatures rebounded 0.6°C over the next eight years, damages could top $1 trillion by the end of the century. While the numbers might be higher or lower in real life, “the message doesn’t really change”, says Estrada. “It would be much worse if we have a termination shock than if we did nothing, if we had unabated climate change.”

The study is innovative in estimating damages based not just on total warming but also on how fast it arrives, says Gernot Wagner at Columbia University in New York.

Solar geoengineering “is riskier than it looks at first glance”, he says. “That is the contribution here.”

The Silicon Valley start-up Make Sunsets has already released more than 200 balloons of sulphur dioxide into the stratosphere to sell emission offsets. That included a launch in Mexico that prompted the government to threaten a geoengineering ban.

The Israeli company Stardust has raised $75 million and lobbied the US government about solar geoengineering. Two-thirds of scientists expect large-scale SRM this century, according to a survey carried out by New Scientist last year.

To cool Earth by 1°C, at least 100 aircraft would have to spread many millions of tonnes of sulphur dioxide in the stratosphere every year without interruption by political disputes, wars, pandemics or other black swan events.

Today, major players like the US are actively undermining international cooperation on climate policy, but this kind of cooperation would be required to avoid termination shock and make SRM a net benefit, the researchers conclude.

Graphing different combinations of parameters, they found aerosol injection is only likely to reduce climate damages if the probability of its termination in any given year is just a few tenths of a per cent, or if that termination could be tapered off over more than 15 years.

If countries slash emissions and only a small amount of geoengineered cooling is needed, aerosol injection could be beneficial at termination probabilities up to about 10 per cent. Although a 10 per cent probability of termination in any given year means a 99.9 per cent chance of failure over the course of a century, the temperature rebound should be small in this low-emissions scenario.

This need for international climate cooperation reveals what Estrada calls the “governance paradox” of solar geoengineering. “The probability of failure must be very, very low; you have to be able to manage if things go wrong and importantly you have to have very good governance for mitigation,” he says. But “if you’re able… to manage the problem of global mitigation of greenhouse gases, then you wouldn’t really need SRM”.

These findings suggest research into solar geoengineering is not necessarily a “slippery slope” to its deployment, as some have argued, says Chad Baum at Aarhus University, Denmark. Funding for the new work came from The Degrees Initiative, which finances geoengineering research in more vulnerable low-income countries.

“You want to have all steps of the research… have more input from the communities affected,” says Baum, who also collaborates with Degrees.

But given that emissions and climate impacts are increasing, more research is still needed on the trade-offs of geoengineering, says Wagner. “We are forced against the wall,” he says.