A recent modelling study has raised new concerns about the unpredictability of solar geoengineering, questioning our ability to reliably manage the outcomes of this approach in reducing the impacts of climate change.

Earth atmosphere

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As efforts to reduce greenhouse gas emissions are progressing more slowly than hoped, some are looking to geoengineering as a way to slow rising global temperatures

Solar geoengineering aims to cool the planet by reducing the amount of solar radiation reaching the Earth’s surface, reproducing the dimming effect observed after volcanic eruptions. This seems like a plausible solution on paper, especially given current efforts to curb greenhouse gas emissions are not going to be enough to avoid surpassing the 1.5°C threshold in the next decade. But since entering the mainstream scientific discourse, geoengineering approaches to climate change have been a divisive topic.

Depending on who you ask, solar geoengineering is seen as a threat to meaningful climate action, a remote fallback, or an essential piece of the puzzle. What most people can agree on is that we don’t yet understand its geophysical and political consequences well enough.

To address this, the team modelled regional effects of injecting sulfur aerosols into the tropical stratosphere. They used a global aerosol-chemistry climate model that links the chemistry and microphysics of sulfate aerosols within complex climate systems, specifically how aerosols absorb heat emitted by the Earth called longwave radiation.

‘We found that some detrimental effects of this injection are of a similar magnitude to those from climate change itself in some regions,’ the team writes. ‘This includes a strong warming 15km above the tropics, which alters large-scale weather patterns in the atmosphere … enhanced surface warming in the polar regions, and modification in regional precipitation patterns over land, therefore not completely alleviating the warming of the high-emission scenario in high northern latitudes.’

A key finding of the paper, says Helene Muri at the Norwegian University of Science and Technology who was not involved in the study, is that it is the absorption by the low-latitude aerosols of longwave radiation that contributes significantly to unintended or undesirable consequences. These consequences can include regional warming when aerosols are injected at lower latitudes.

‘The findings strengthen [the idea] that the uneven and unpredictable effects, and perceptions and misinformation about these, will make [stratospheric aerosol interventions] an unusually hot “hot potato”,’ says Olaf Corry, a political scientist at the University of Leeds who was not involved in the study.

The study’s authors suggest that using less absorptive materials, like diamond or alumina, or those that are chemically inert, like calcite, could alleviate some of the side effects, alongside more ‘complex’ injection strategies.

Andrew Lockley an independent researcher who was also not involved in this work, says the study adds further weight to a body of work suggesting aerosol placement and type will determine success. ‘I think what this shows is that, as we suspected for some time, stratospheric heating with sulfate aerosol injection is a significant issue. We need to make sure we are looking at using aerosols that are sized optimally and of a material that is designed to minimise that stratospheric heating,’ says Lockley. ‘More poleward distribution of stratospheric aerosol injections would be useful – not least because it would be less expensive, but additionally because it tends to correct the polar amplification of global warming.’

Lockley stresses that while modelling climate outcomes from interventions like this is valuable, connecting the results to real-world data is crucial for understanding the actual impacts. ‘It’s important to understand the fate of aerosols that we inject into the atmosphere,’ he says. ‘Integral to this is making sure we are using adequate, early experiments … to have a better understanding of the aerosol life cycle in the stratosphere and the consequential thermal and induced dynamical effects.’

Even if the atmospheric consequences of stratospheric aerosol injections were absolutely defined, there is an additional problem with modelling studies, argues Corry. They don’t necessarily consider the complexities of implementing large-scale interventions across national boundaries or the practical challenges involved.

‘These modelling approaches are done in a way that ignores all the messiness of the world and re-introducing that … is essential to avoiding an over-optimistic assessment of it,’ Corry says. ‘The latter poses risks of over-optimism that increases the risk that these ideas will sabotage desperately needed emissions reductions.’