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Global efforts to cut greenhouse gas emissions are not on track to meet the 1.5°C Paris Agreement goal. Climate change is worsening and scientists are searching for emergency technologies in response. But the UN Environment Programme (UNEP) chief scientist Andrea Hinwood strikes a note of considerable caution.
“Climate change is taking the world into uncharted lands, and the search is on for all viable solutions,” she says. “However, all new technologies must be clearly understood, and potential risks or impacts identified before being put into use.”
Once dismissed as science fiction, geoengineering, or large-scale interventions in the Earth’s climate system, is being explored as a solution to global warming. However, scientists and environmental campaigners warn they will not fix the crisis by themselves.
One such idea is solar radiation modification (SRM), a speculative group of technologies that reflect sunlight back into space to reduce global heating and cool the planet. Hinwood is very cautious.
“The private sector and regulators need to address the basic uncertainties surrounding these technologies, answer some fundamental questions about safety and employ the precautionary principle before SRM can even be contemplated,” she says.
In 2023, an expert panel convened by UNEP found that SRM was not yet ready for large-scale deployment and no substitute for a rapid reduction in greenhouse gas emissions, which must remain the global priority.
So what is SRM? And has anything changed since the expert panel’s findings?
Several SRM techniques have been proposed. They include cirrus cloud thinning and space mirrors. But these have lower levels of effectiveness to generate large-scale cooling, or much higher technical barriers to deployment. However, two SRM techniques have grabbed the attention of scientists: stratospheric aerosol injection (SAI) and marine cloud brightening (MCB) (see Frontrunners, below).
The Royal Society is the latest heavyweight organisation to put a spotlight on SRM, concluding it is not a silver bullet. There are still many questions and no quick answers. SRM techniques “could only mask, not fix”, the effects of warming caused by greenhouse gas emissions, it warns. However, while SRM would not tackle the root cause of climate change or its associated impacts, it could be one of several tools used to reduce climate-related risks.
The key will be to deploy SRM “in a globally informed way”. This would help mitigate many of the adverse impacts of climate change, such as sea level rises and wildfires, and global rainfall would be likely to drop more under SRM than for an equivalent temperature reduction from emission cuts. But SRM would not tackle ocean acidification.
A much bigger problem, says the Royal Society, is deploying SRM “without due diligence”. SRM could exacerbate regional climate change. For example, deploying SAI in only the northern hemisphere could lead to drought in Africa’s Sahel region. Using it in the southern hemisphere only could mean more frequent and intense North Atlantic hurricanes. In short, the uneven impact of any cooling would create winners and losers – and, with them, even more geopolitical tensions.
There are also limits to the extent to which climate models can confidently predict regional climate change with or without SRM. A further factor is that the duration of SRM deployments that are required to reduce global temperatures to a given target level would be unknown. This would depend on future greenhouse gas mitigation measures and uncertain aspects of the climate system, “but could be many decades or even centuries”.
Moreover, if SRM were ever deployed and then stopped abruptly while greenhouse gas concentrations remained high, global temperatures could increase by 1-2°C within a couple of decades. Known as the “termination effect”, this phenomenon would be likely to have major consequences, with ecosystems and populations unable to adapt quickly. Any consideration of SRM would, therefore, require long-term international commitments. The track record for such commitments is not a good one.
“Unless there is a significant shift in our mitigation strategies, we are on track to breach the 1.5°C Paris Agreement warming goal in the near future,” says the University of Reading’s regius professor of meteorology and climate science, Keith Shine, who chaired the report’s working group. “Both SRM and unmitigated climate change carry significant risks, and the key challenge is to understand those risks in detail and assess them side by side, not in isolation.
“This is not a question of whether SRM is safe, as it is clearly not without risks. However, there may come a point where those risks are seen to be less severe than the risks of insufficiently mitigated climate change.
“If policymakers did take the decision to deploy SRM, a scientifically informed, globally coordinated and internationally agreed upon strategy would be essential both to achieve global cooling and avoid undesirable regional climate impacts.”
Frontrunners
Two SRM methods have so far received the most scientific attention
Stratospheric aerosol injection (SAI) is a proposal to inject aerosol particles into the stratosphere to enhance the reflection of sunlight back into space, thereby cooling the planet. SAI model simulations most commonly assume that sulphate aerosols would be used. These would be emitted by custom-made aircraft.
Marine cloud brightening (MCB) is a proposal to inject salt spray into shallow marine clouds to brighten them, increasing their reflection of sunlight and reducing the amount of heat absorbed by the water below. Delivered primarily by specialised ships, scientists think a targeted approach to MCB, rather than routine spraying under all conditions, might have a higher probability of success.
“We would have to get the right-sized particles into receptive clouds at the right times of day and seasons, and over large-enough areas to shade large areas of ocean,” says Graham Feingold, a research scientist at the National Oceanic and Atmospheric Administration’s Chemical Sciences Laboratory. “It’s a major challenge.”
Of the two proposed techniques, SAI is considered more technically feasible to scale up, and how it impacts the climate is currently better understood than MCB.
However, the Royal Society warns policymakers that many other issues would need to be considered before taking any decisions to implement SRM. These include engineering feasibility, costs, public perception, transparency, ethics and inclusivity.
Indeed, the Center for International Environmental Law (CIEL) lambasts geoengineering generally as “highly speculative techno-fixes” fraught with risks, while the UN Human Rights Council’s advisory committee says the technologies will have a disproportionate impact on those who have done the least to cause the climate crisis, including communities in the Global South.
The Royal Society argues that there are serious governance issues that need to be resolved if geoengineering is ever to become an acceptable method of moderating climate change. The CIEL puts this more bluntly, arguing that with the large scale of projects, “there is nothing in human history to suggest that deployment could ever be fairly or safely governed”. SAI is likely to require the constant injection of sulphur compounds in the upper stratosphere “over multiple generations if not centuries – a feat that only powerful nation-states or military regimes would be able to attempt”, it adds.
At some point, policymakers may deem “SRM to be the less bad of two bad options”, the Royal Society admits, with the risks associated with its implementation seen as lower than the risks of not doing so. It also points to findings by the Intergovernmental Panel on Climate Change, which argued that, if it is ever deemed necessary, “SRM cannot be the main policy response to climate change and is, at best, a supplement to achieving sustained net-zero or net-negative CO2 emission levels globally”.
Huw Morris is a freelance journalist
