Limiting global temperature rise to no more than 1.5˚C, or 2.7˚F, means reducing greenhouse gas emissions in the near term, according to the United Nations. But humanity had better have some aces up its sleeve in the event it is unable to do that in time. If climate scenarios devolve into truly apocalyptic realities, the world might have to resort to stopgap solutions that artificially slow the pace of warming by adjusting how much the sun heats the earth.

Solar geoengineering, also known as solar radiation modification (SRM), would be able to do just that. SRM consists of a speculative group of technologies that are, in theory, able to reflect some of the sun’s energy back into space, offsetting temperature rise on earth. Examples of the large family of proposed SRM technologies to cool the planet include injecting aerosol into the stratosphere to make it more reflective and placing massive mirrors in orbit to limit the amount of sunrays reaching the earth.

SRM technologies are far from economically feasible right now, and even climate scientists have urged caution over how they would alter the planet’s natural processes. The UN Environment Programme and an expert panel recently conducted a scientific review of SRM technologies and released their findings in a report published Monday. While the UN found that the technology carries a huge amount of risk and recommended against deploying it anytime soon, it also conceded that the situation could change quickly if attempts to reduce greenhouse gas fail.

“The expert panel considers that a near- and midterm large-scale SRM deployment is not currently warranted and would be unwise,” the report said, but added: “This view may change if climate action remains insufficient.”

Extreme measures

In recent years, governments and companies have made massive strides in decarbonization and reducing greenhouse gas emissions, and the worst-case climate scenarios seem less likely than ever.

But by the UN’s own admission, current efforts to reduce emissions are still not enough to avert a climate catastrophe.

Policies and pledges to cut back on emissions “remain insufficient to limit global temperature rise to 1.5˚C by the end of the century,” the UN announced in an October report. The UN said that current commitments are still “pitifully short” and put the world on track for 2.5˚C of warming by 2100, well above the threshold deemed acceptable or safe by the scientific community. And keeping temperatures to the UN’s 1.5˚C goal will likely be impossible without artificially removing carbon that is already in the atmosphere, researchers say.

If the climate warms to those extremes, reducing emissions may not be enough to avoid the natural disasters, mass migration, and widespread famine and poverty that accompany higher temperatures, and the world may have to consider drastic measures.

Research, not implementation

Solar geoengineering represents the “only known approach that could be used to cool the earth within a few years,” the UN report on SRM said, adding that the technologies to deploy SRM, while nonexistent today, are far from science fiction.

“No showstopping technical hurdles have been identified,” the UN said, adding that solar geoengineering technology could be “developed in under 10 years.”

But research into what side effects the technology could have on the climate and biological life on earth is scarce. Philanthropic billionaires including Bill Gates have been investing in geoengineering research for years, and the scientific community may be starting to come around as well. On Monday, the same day the UN report was released, a group of over 60 scientists from prominent institutions globally signed an open letter urging the international community to expend more resources researching the costs and feasibility of solar geoengineering, as emissions reduction goals seem unlikely to be met.

“The current level of knowledge about SRM interventions is not sufficient to detect, attribute, or project their consequences for climate risks,” the scientists wrote.

But like the UN, scientists are still wary of advocating these strategies, at least until more is known about their potential consequences.

In a statement accompanying Monday’s report, the UN said SRM is “no substitute for a rapid reduction in greenhouse gas emissions, which must remain the global priority.” One important barrier to SRM is its cost. Deploying solar geoengineering at scale would likely cost tens of billions of dollars a year for every degree of cooling, the UN found, an expense that may prove unsustainable as SRM technologies would likely need to be employed for decades or even centuries to achieve desired levels of cooling, without even considering the costs of mitigating SRM’s unintended side effects to the planet.

Those unknown consequences may be the biggest hurdle to deploying SRM at scale, which the UN said could “introduce new risks to people and ecosystems.” Potential effects include irreparable damage to the ozone layer, exacerbating the effects of climate change at local scale, and if the technology is abruptly disabled, lead to even higher degrees of heating.

The scientists who wrote the open letter advocating for more research stressed that they do not support implementing the technology yet, and are well aware of its potential consequences. But as more climate thresholds are surpassed and tipping points continue to fall, knowing more about the technologies at our disposal certainly can’t hurt.

“While we fully support research into SRM approaches, this does not mean we support the use of SRM,” the scientists wrote. “We support a rigorous, rapid scientific assessment of the feasibility and impacts of SRM approaches specifically because such knowledge is a critical component of making effective and ethical decisions about SRM implementation.”