The world may increasingly look to geoengineering in the wake of the latest UN climate report, which says it could be adopted as a temporary “remedial measure” if the world heads towards dangerous levels of warming.

The authors of the new 1.5C study by the Intergovernmental Panel on Climate Change say there is high agreement that the injection of millions of tonnes of sulphur dioxide into the stratosphere could help limit temperature rises to the most ambitious target of the Paris accord.

But the authors warn there are major uncertainties about the social, environmental and ecological impacts, which mean the world would be far better off if policymakers strengthened natural cooling systems such as forest cover and accelerated efforts to reduce carbon emissions.

The lengthy document – which was approved at the weekend by all 195 nations in the UN – mentions several options for man-made interference in climate systems, including ocean fertilisation, carbon dioxide removal, marine cloud brightening, cirrus cloud thinning and ground-based albedo modification.

Quick Guide What difference would restricting warming to 1.5C make? Show A key finding of the new IPCC report is the dramatic difference that restricting warming to 1.5C above pre industrial levels would have on the global environment. The scientists found: • By 2100, global sea level rise would be 10cm lower with global warming of 1.5C compared with 2C. • Extreme heatwaves will be experienced by 14% of the world's population at least once every five years at 1.5C. But that figure rises to more than a third of the planet if temperatures rise to 2C • Arctic sea ice would remain during most summers if warming is kept to 1.5C. But at 2C, ice free summers are 10 times more likely, leading to greater habitat losses for polar bears, whales, seals and sea birds.

• If warming is kept to 1.5C, coral reefs will still decline by 70-90% but if temperatures rise to 2C virtually all of the world's reefs would be lost



But it focused most on stratospheric aerosol injection, a technique that essentially mimics the effect of a volcano by pumping gas into the sky that turns into aerosols, which reflect part of the sun’s heat.

Although the authors do not include such strategies in their pathways to 1.5C above pre-industrial temperatures, they raise the possibility that it could be used as a supplementary measure if this target is missed.

“If mitigation efforts do not keep global mean temperature below 1.5C, solar radiation modification can potentially reduce the climate impacts of a temporary temperature overshoot, in particular extreme temperatures, rate of sea level rise and intensity of tropical cyclones, alongside intense mitigation and adaptation efforts,” the report observes.

A search for palliatives will be necessary as the world is almost certain to miss the 1.5C goal. Current national pledges are forecast to lead to at least 3C of warming by the end of the century – and that is if governments keep their commitments.

The IPCC is clearly hesitant to endorse such emergency measures in part because this could allow government leaders to continue pushing problems into the future, but also because of the immense risks involved.

The report notes that the injection of sulphur dioxide would change rainfall patterns and weather circulation as well as disrupting stratospheric chemistry and ice formation. It could also result in more ultraviolet light exposure, which would have a negative impact on human health.

Ethical and institutional questions also arise over who would oversee such operations and which areas would be affected. The report suggests a number of UN organisations as possible supervisory bodies. But authors also observe that there are scarcely any laws or regulations to stop any country that wants to push ahead by itself. The only guideline cited was the Convention on Biodiversity which states “no climate-related geoengineering activity that affects biodiversity may take place.”

There are doubts also over effectiveness. While the aerosols might constrain temperature rises, they would not stop the accumulation of carbon dioxide in the atmosphere and the acidification of the oceans. What happens when this “temporary measure” is halted is also an area of concern, as the planetary system might suddenly be hit by a surge in temperature.

The IPCC says these uncertainties constrain the ability to implement solar radiation management in the near future. But with the 1.5C target current on course to be overshot at some point between 2030 and 2052, the urgency is likely to grow.

Johan Rockström, coauthor of the recent Hothouse Earth study, said the IPCC report was likely to stimulate discussion of these extreme emergency measures.

“I think this will raise solar radiation management to the highest political level. We currently have no framework for this,” he said. “I’m very scared of this technology but we need to turn every stone now.”

James Hansen said the tipping point in public opinion was more likely to come at a slightly higher temperature, but by then it may already be too late.

“2C would force geoengineering on today’s young people. Geoengineering, if global temperature passes 2C, would start, at the latest, once ice sheet collapse begins,” he told the Guardian. “Unfortunately, because of the inertia of the system, geoengineering then would probably be too late to prevent locking in the eventual loss of coastal cities.”