Saving humanity or taking an uncalculabe risk?

Why not look at the world from a completely different perspective? Instead of tackling climate change from the ground up by shutting down combustion engines and coal-fired power stations, we could also start at the top. Mirrors in space could deflect solar radiation. Or aeroplanes could spread a sulphur blanket in the stratosphere to artificially cool the atmosphere.

The debate about these and other 'geoengineering' methods flares up time and again. However, an SZ survey of leading climate researchers now shows that most scientists do not currently consider the use of geoengineering to be advisable. At the same time, most favour more research.

The SZ surveyed around 300 researchers specialising in climate change. Ninety-six of them, roughly one in three, completed the questionnaire anonymously, meaning it is not possible to trace who holds which position. Twelve participants stated that they themselves had been involved in solar geoengineering research projects in the past five years.

Solar geoengineering, also known as solar radiation modification, involves reflecting sunlight back into space to cool the Earth.

Since the technical hurdles for space mirrors seem insurmountable at present, and light colours for buildings are largely uncontroversial, climate researchers were only asked for their opinion on stratospheric aerosol injection and marine cloud brightening.

‘These techniques would have the greatest effect with relatively low technical hurdles,’ says James Haywood of the University of Exeter, who was not involved in the survey.

It is clear that solar geoengineering does nothing to address the causes of global warming, i.e. CO₂ emissions from the use of fossil fuels. The methods are only intended to mitigate the effects of greenhouse gases – just as a painkiller does not heal an injury, but makes it temporarily bearable.

The controversy is in full swing

Amidst ever-new temperature records and stagnating climate protection, calls for such symptomatic treatment have recently become louder. ‘If reflecting sunlight could save lives and protect the environment, it is at least worth discussing,’ argued US climate researchers Zeke Hausfather of Berkeley Earth and David Keith of the University of Chicago in the New York Times recently. ‘We know it can work.’ For example, when the Pinatubo volcano erupted in 1991, large amounts of sulphur dioxide were released into the atmosphere, cooling the atmosphere in the following years.

David Keith and 100 other experts have also signed an open letter calling for more ‘climate intervention research’ in response to a study published in the journal Frontiers in Science. In it, 42 climatologists clearly reject both the use and research of geoengineering. Of the five technologies considered in the study, including stratospheric aerosol injection, none are suitable for combating climate change. ‘It is clear to us that the approaches examined are not feasible,’ and further research would only waste time and resources. Furthermore, the ideas should not distract from the necessary reduction of greenhouse gas emissions.

So the controversy is ongoing, but what is the balance of power? The SZ survey paints a nuanced picture.

As a concrete climate protection measure, the vast majority of researchers currently reject geoengineering. Only six percent of participants are in favour of allowing cooling particles to be released into the atmosphere. Support for the use of marine cloud brightening is twice as high at 14 percent, but still far from a majority.

There is more agreement on whether further research should be conducted into these technologies. A total of 54 per cent of the scientists surveyed favour simulations and limited field trials to gain a deeper understanding of stratospheric aerosol injection, while one third are against it. 57 per cent of participants are also in favour of researching marine cloud brightening in pilot trials.

At the same time, three-quarters of respondents agreed with the statement that research into solar geoengineering increases the risk of watering down ambitious CO₂ reduction targets. To put it bluntly, many researchers see the risk of geoengineering being a distraction, but at the same time, most respondents would accept this in favour of gaining more knowledge.

Isolated experiments are already emerging. For example, the Aria research agency in the United Kingdom is funding five planned field trials to test approaches to cooling the climate. Mexico, on the other hand, has banned solar geoengineering experiments in 2023 after a US company released balloons filled with sulphur there without prior notification.

In another question, 18 percent of participants stated that their attitude towards the use of SRM had changed for the better since the Paris Climate Agreement. However, twice as many say they have become even more sceptical over the past ten years.

Those who now have a more positive attitude towards geoengineering cite rising CO₂ concentrations in the atmosphere or approaching tipping points for coral reefs, for example, as the main reasons for this. The increase in extreme events and the accelerated warming of the oceans have caused them to rethink their position, explained a climate researcher from the South Pacific; she now supports the use of marine cloud brightening.

Overall, however, comments on the survey are dominated by criticism of solar radiation modification. Many researchers fear that countries act unilaterally and that the effects of such interventions, for example on biodiversity or the water cycle, are unknown. ‘Geoengineering approaches are a fig leaf that serves to avoid taking action by promising “something” for the future,’ writes a climate researcher from northern Germany.

1,800 launches into the stratosphere per day

The British Royal Society recently compiled the current state of research. According to this, most of the findings on both aerosol release and marine cloud brightening come from computer-based climate models. Nevertheless, there is ‘reliable evidence that a globally coordinated use of SRM could reduce the global mean surface temperature and associated effects such as sea level rise, forest fires and extreme precipitation, thereby masking part of the anthropogenic climate change’.

The Royal Society estimates that 1,800 aircraft would have to take off every day to cool the world by one degree Celsius with sulphur particles. This amount is challenging, but technically possible: 650 aircraft take off every day from London Heathrow Airport alone. Since the particles are best distributed high up in the atmosphere, the planes would have to climb to at least eight kilometres, and even more than 20 kilometres over the tropics because the atmosphere bulges around the equator. ‘The sky would be a little less blue as a result,’ said climate researcher James Haywood at the presentation of the Royal Society study, but not as dimmed as it would be after a volcanic eruption.

According to estimates, marine cloud brightening with the same cooling effect would require around 27,000 jets spraying salt water into the atmosphere around the clock, each at a rate of around 100 litres per minute.

Would the monsoon suddenly fail to arrive in India? No one knows

However, even in the stratosphere, aerosols lose their effect after two years at the latest, while greenhouse gases such as CO₂ can heat the atmosphere for centuries. This means that geoengineering would probably have to be carried out for decades or even centuries. If artificial cooling ends, warming rises again abruptly, which is known as ‘termination shock’.

The Royal Society experts list further risks. For example, rainfall could be slightly lower in a world that relies on solar geoengineering. The interventions do nothing to help against ocean acidification caused by high CO₂ concentrations. It is currently difficult to estimate what effects the experiments would have on a regional level, such as whether the monsoon could suddenly fail to arrive in India. Other studies show that the use of SAI in the Northern Hemisphere alone could cause the Sahel region to dry up. Use in the tropics alone could lead to winter droughts in the Mediterranean region and on the Iberian Peninsula.

And yet, the Royal Society believes that ‘at some point, policymakers may consider SRM to be the lesser of two evils.’ After all, the risks associated with geoengineering remain more or less the same, while the consequences of global warming become increasingly serious the further it progresses.

In other words, time is running out for geoengineering. This controversial topic is unlikely to disappear anytime soon.