The controversial concept of purposely deflecting the sun’s rays to cool down an overheating Earth should be further studied, according to a group of scientists headed by James Hansen, the renowned former Nasa climate researcher. An open letter from more than 60 scientists across the US, Canada and Europe warns that it is “increasingly unlikely” the world will remain below 2C of heating beyond pre-industrial times, due to a failure to slash greenhouse gas emissions, requiring a “rigorous, rapid scientific assessment” of previously outlandish proposals for solar geoengineering to provide rapid cooling. The letter’s signatories include Hansen, the veteran climate scientist credited with alerting the world to the dangerous escalation of global temperature in the 1980s. While they make clear that cutting emissions is the primary priority, the scientists argue that the full ramifications of the geoengineering, also called solar radiation management (SRM), needs to be understood before it is tried in desperation by a country. “Since decisions on whether or not to implement SRM are likely to be considered in the next one to two decades, a robust international scientific assessment of SRM approaches is needed as rapidly as possible,” the letter states. There are a range of different potential climate interventions to try to artificially curb global heating, such as the brightening of clouds to make them more reflective of sunlight, but the option considered most likely by scientists is the spraying of aerosol particles, such as sulphur, into the stratosphere. These particles would deflect the sun’s rays and rapidly cool the planet, by 1C or maybe even more, although they would only linger temporarily, requiring a constant series of trips by aircraft to spray more aerosols and replenish the reflective material. The basic mechanism behind this is well understood – volcanic eruptions similarly cause sunlight to dim – but solar geoengineering has never been tested fully and faced severe opposition when this has been attempted, due to fears of unknown environmental knock-on impacts and concerns over the lack of governance surrounding the practice. However, with governments still failing to cut emissions quickly enough to avoid disastrous climate change, support has among some powerful entities to research, if not fully deploy, solar geoengineering. The US government has already kicked off a research review of climate interventions and, on Monday, the United Nations’ Environment Programme (Unep) also released a report calling for further study of the options. The report states that spraying reflective particles “is the only known approach that could be used to cool the Earth within a few years” and that it would cost tens of billions of dollars a year, ongoing, to achieve a 1C reduction in global temperatures. But it also acknowledges a long list of potential dangers, such as damage to the ozone layer, possible power imbalances and conflicts between countries and the risk of “termination shock”, whereby a sudden halt of spraying the particles would unleash a burst of pent-up global heating. “Make no mistake: there are no quick fixes to the climate crisis,” wrote Inger Andersen, executive director of Unep, in the report’s foreword. “Yet current efforts remain insufficient. As a result, increasing voices are calling for and preparing alternative ‘emergency’ options to keep global temperature rise in check.” Andersen wrote there is little research on the large-scale adoption of solar geoengineering and that it is “fraught with scientific uncertainties and ethical issues”. This “speculative technology” should not be a considered a substitute for cutting emissions and does not remove carbon from the atmosphere, merely masking the warming caused by the burning of fossil fuels, she added. Opponents of solar geoengineering said it was concerning to see the apparent momentum gathering behind its progress and urged governments to follow the lead of Mexico, which recently banned experiments of the technology. More than 400 scientists have put their name to a document calling for a solar geoengineering non-use agreement. “There are definitely a handful of proponents actively pushing for a normalization of solar geoengineering as a climate response option [and] a few serious scientists who are coming to this from a point of despair,” said Lili Fuhr, deputy director of climate and energy at the Center for International Environmental Law. “The idea that we could take control of the global thermostat and dial down temperature levels to a desired state has been debunked by the scientific community again and again. But it is a very attractive idea for big polluters and governments who are unwilling to invest in the radical system change transformation that is so urgently needed.”