• The “speculative group of technologies” that involve reflecting sunlight away from the Earth and back toward space, often called solar radiation modification, or more broadly solar geoengineering, should not be used now, the United Nations said, but they should be studied more rigorously.
  • Reflecting sunlight away from the Earth is dangerous, but it is also doable and quick, which means that if climate change-mitigation strategies continue to be insufficient, it could become a viable option.
  • There also needs to be international governance rules established for any possible use of sunlight-reflection technology, especially because the relatively low cost and relatively simple technology make it possible for a “rogue deployment.”
People photographed in Lower Saxony, Germany, on July 19, 2022. A number of European countries were affected by a heatwave last month.
People photographed in Lower Saxony, Germany, on July 19, 2022. A number of European countries were affected by a heatwave last month.


Global efforts to respond to climate change are so far insufficient, making it time to begin studying technologies to reflect sunlight away from the Earth to cool it down temporarily, said a new report from the United Nations published on Monday.
Reducing greenhouse gas emissions is the only way to permanently slow global warming, but worldwide efforts to reduce greenhouse gas emissions are currently “not on track to meet the 1.5° Celsius Paris Agreement goal,” the U.N. Environment Program said in a written statement accompanying the release of the report.
With the world not responding to climate change urgently enough, a “speculative group of technologies” to reflect sunlight back away from the Earth have been getting more attention recently, UNEP said in a written statement accompanying the report. This category of technologies is often called solar radiation modification (SRM) or more broadly solar geoengineering.
The report on these technologies, written by an expert panel brought together by the U.N. program, advised that it’s currently not a good idea to use them in an effort to respond to climate change.
However, “this view may change if climate action remains insufficient,” the report said, signaling that it’s time for rigorous study of both the technologies and the potential international governance.
A similar message came from a group of more than 60 scientists in an open letter that was also (coincidentally) published on Monday.

Fast and doable, but potentially dangerous

Solar geoengineering “is the only known approach that could be used to cool the Earth within a few years,” the U.N. report said, and would cost tens of billions of dollars per year per degree of cooling.
While the technology to inject large quantities of aerosols into the upper atmosphere does not exist today, it’s not seen as being terribly complicated: “No show-stopping technical hurdles have been identified,” the U.N. report said, and it could be “developed in under ten years.”
Scientists know it works quickly, citing the drop in the global average temperature after large volcanic eruptions have spread large quantities of aerosols into the upper atmosphere. These observations of volcanic activity provides “strong evidence that a deliberate injection of large amounts of reflective particles into the stratosphere would cool the Earth rapidly,” the U.N. study said.
“If global warming at some point produces outcomes widely seen as intolerable (e.g. widespread famines, mass migration, mass mortality and destruction of infrastructure) an operational SRM deployment as part of a ‘planned’ emergency response might be able to alleviate some of this suffering within a few years,” according to the report.
But the techniques can also be dangerous.
For example, sulfur dioxide is commonly proposed as an aerosol, but that practice would result in acid rain, the report warned. It also could increase ozone depletion. Specifically, “Antarctic ozone hole recovery could be delayed by a couple of decades and the ozone hole could become deeper in the first decade of SAI [stratospheric aerosol injection] deployment,” the U.N. report said.
So solar geoengineering could be considered a one-time shot to mitigate extreme suffering and death caused by climate change.
Or sunlight-reflection technology could become part of a “phased” longer-term strategy to buy more time to aggressively and permanently reduce greenhouse gas emissions.

The risk of rogue actors

Regardless, experts concede that we just don’t know enough about the side effects of these technologies.
“We only have one atmosphere. We cannot risk further damaging it through a poorly understood shortcut to fixing the damage we already caused,” wrote Inger Andersen, the executive director of UNEP, in a forward to the study.
And right now, there is not enough reliable information to make an informed decision.
“The review finds that there is little information on the risks of SRM and limited literature on the environmental and social impacts of these technologies,” Andersen wrote. “Even as a temporary response option, large-scale SRM deployment is fraught with scientific uncertainties and ethical issues. The evidence base is simply not there to make informed decisions.”
In addition to needing rigorous scientific study, the report added there needs to be a globally coordinated governance strategy for any potential use of solar-geoengineering technology.
But the relatively low cost — it can be deployed for as little as $20 billion per 1 degree Celsius of cooling per year — means it is “within reach” of many countries and organizations, opening the possibility of a “rogue deployment,” the report said.
The United Nations could be a leader in global discussions of solar geoengineering conversations, the report said, noting that not having international cooperation and governance is potentially dire.
“One can assume that there will never be universal consensus in the broader community on an SRM deployment, which means that communities, nations and societies opposed to SRM deployment would be exposed to its effects against their wishes, raising ethical and legal concern,” the study said.


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