• More than sixty scientists from prominent institutions are advocating for rigorous study into reflecting sunlight away from the Earth to mitigate the effects of climate change.
• Air pollution is currently "masking" as much as a third of the impacts of global warming, and as regulations around the world tighten, the amount of global warming will be "unmasked."
• As the effects of climate change become more acute, pressure to use sunlight-reflection technologies will rise, so scientists argue we need international study now.

More than sixty scientists from prominent institutions are advocating for rigorous study into reflecting sunlight away from the Earth to mitigate the effects of climate change.

None of the scientists are endorsing the strategy, which is sometimes referred to as "solar geoengineering" and could have significant negative side effects.

Rather, they are advocating for a careful and coordinated study of the idea before the effects of climate change become so obvious and urgent that citizens and governments demand action. They believe this could happen in the next 10 to 20 years given the current trajectory of CO2-reduction plans.

"Climate change is causing devastating impacts on communities and ecosystems around the world, posing grave threats to public health, economic security, and global stability," the scientists wrote in their open letter published on a website made expressly for the purpose of publicizing their scientific viewpoint. "Natural systems are approaching thresholds for catastrophic changes with the potential to accelerate climate change and impacts beyond humans' ability to adapt."

The scientists also emphasize that solar geoengineering is not a solution. Greenhouse gas emissions must be immediately and urgently reduced, as that's the only permanent way to limit global warming.

The signatories come from prestigious institutions around the world, including Columbia University, Harvard University, the Massachusetts Institute of Technology and NASA. There is no organization or institution behind the letter, however. It was published as an independent effort by scientists.

Air pollution is masking some of the warming

The landmark Paris Climate Accord, signed in 2015, aimed to keep global warming "well below" 2 degrees Celsius compared with pre-industrial levels, and ideally below 1.5 degrees.

But the scientists believe that goal won't be met because so many greenhouse gases have already been emitted over the last century and a half, and remain in the atmosphere long after they are released.

"Even with aggressive action to reduce GHG emissions it is increasingly unlikely that climate warming will remain below 1.5-2°C in the near term," the scientists wrote. "This is because reversing current warming trends will require a significant reduction in the atmospheric concentrations of greenhouse gases, which significantly lag behind reductions in emissions due to their long atmospheric lifetime."

Adding to the concern, air pollution is working to counteract global warming. Particles of air pollution, also released by human activity, mix into clouds and reflect sunlight away from the Earth for the time they remain in the atmosphere.

"Aerosols from human activities are currently estimated to be offsetting about a third of greenhouse gas climate warming," the scientists wrote.

But governments are cracking down on air pollution, and as the air gets cleaner, it will lay bare the full amount of anthropogenic global warming.

"Reductions in aerosol emissions in the coming few decades will rapidly 'unmask' a significant but very uncertain amount of climate warming," the scientists warned.

Meanwhile, technologies to remove CO2 from the atmosphere are too early along to help the world stay below 1.5 degrees of warming, experts say.

"There are substantial environmental, technical, and cost challenges in using carbon dioxide removal (CDR) at the scale needed to significantly reduce global warming. While using CDR to remain below 1.5°C may be physically possible, these challenges and the slow response of the climate system make it unlikely that CDR could be implemented rapidly enough or at sufficient scale to entirely avoid dangerous levels of climate warming in the near term," the scientists wrote.

Reflecting sunlight away from the Earth is possible and can work quickly. Given our collective global warming status, it's time to take it seriously, the scientists argued.

Three techniques for global cooling

The scientists break down solar geoengineering into three categories:

• Stratospheric aerosol injection, SAI, which involves putting aerosols such as sulfur dioxide into the atmosphere to reflect sunlight.
• Marine cloud brightening, MCB, which involves putting sea salt aerosols into low-hanging marine clouds.
• Cirrus cloud thinning, CCT, which involves putting aerosols into cirrus clouds to reduce the amount of infrared radiation that the Earth retains.

The first two, SAI and MCB, are generally referred to as solar radiation modification, or SRM.

Right now, there isn't sufficient research to know exactly how these three techniques would affect the planet. In some cases, they might be less harmful if used in some combination, the scientists wrote.

But as the effects of global warming become more acute, "there will be increasing pressure" to use one of these adaptation techniques. That's why the scientists say it is urgent to study them now, as "the current level of knowledge about SRM interventions is not sufficient to detect, attribute or project their consequences for climate risks," the scientists wrote.

Peer-reviewed scientific research should include computer model simulations, observations, analytical studies, and small-scale field experiments, the scientists added. And all of the research should be open and transparent with open access to data.

Using any of these techniques should be subject to a global, international cooperation and decision making in a framework similar to the Montreal Protocol, which the scientists laud as a highly successful case of environmental global policy.

In addition, these kinds of sunlight reflection technologies should not be bought and sold in a commercial marketplace as a way to offset carbon emissions, the scientists wrote.

"The state of scientific knowledge about SRM is also currently insufficient for it to be included as part of a climate credit system or other commercial offering, as some have started to propose. Even for stratospheric aerosol injection (the most well-understood SRM approach), the amount of cooling achieved by the injection of a given mass of material and how SAI will affect the climate system are still highly uncertain," they wrote.

"Even with improved understanding of these effects, since SRM does not address the cause of climate change, nor all of the effects of increased greenhouse gas concentrations, it likely will never be an appropriate candidate for an open market system of credits and independent actors."