UK’s Royal Society: geoengineering could be necessary

While the society found that geoengineering techniques are very likely to be technically possible and some may be useful, it raised concerns about major uncertainties regarding the techniques’ effectiveness, cost and environmental impacts.

"It is an unpalatable truth that unless we can succeed in greatly reducing carbon-dioxide emissions we are headed for a very uncomfortable and challenging climate future, and geoengineering will be the only option left to limit further temperature increases,” said John Shepherd, chair of the Royal Society’s geoengineering study. “Our research found that some geoengineering techniques could have serious unintended and detrimental effects on many people and ecosystems – yet we are still failing to take the only action that will prevent us from having to rely on them. Geoengineering and its consequences are the price we may have to pay for failure to act on climate change."

The report preferred carbon-dioxide-removal techniques over solar-radiation-management approaches because they address the root of the problem. That said, carbon-dioxide-removal techniques have not yet been demonstrated as effective at an affordable cost with acceptable environmental impacts, and only work over long timescales.

Solar-radiation-management techniques, meanwhile, can lower temperatures rapidly but do not reduce carbon-dioxide levels, meaning that problems such as ocean acidification remain. They would need to be deployed for a long time and, although relatively cheap, their regional consequences are uncertain. Solar-radiation-management techniques could be useful if a threshold is reached where action to reduce temperatures must be taken rapidly, concludes the report, but they are not an alternative to emissions reductions or carbon-dioxide-removal techniques.

“None of the geoengineering technologies so far suggested is a magic bullet, and all have risks and uncertainties associated with them,” said Shepherd. “It is essential that we strive to cut emissions now but we must also face the very real possibility that we will fail.”

According to Shepherd, if "Plan B" is to be an option, considerable research and development of the different methods, their environmental impacts and governance issues must be undertaken now. “Used irresponsibly or without regard for possible side effects, geoengineering could have catastrophic consequences similar to those of climate change itself,” he said. “We must ensure that a governance framework is in place to prevent this."

Shepherd and colleagues’ preferred method of geoengineering was carbon-dioxide capture from ambient air, as it effectively reverses the cause of climate change. Much more research and development is needed, however, and no cost-effective methods have been shown to date.

Of the other carbon dioxide removal techniques, the researchers concluded that enhanced weathering – using naturally-occuring reactions of carbon dioxide from the air with rocks and minerals – showed potential as a long-term option provided its environmental implications were considered, while land use management and afforestation could play a small but significant role although competing demands for land could limit its scope. The report raised significant doubts as to the effectiveness and safety of biochar techniques and recommended substantial research before their consideration for eligibility for UN carbon credits. Ocean fertilization, meanwhile, has not been proved to be effective and has high potential for undesirable ecological side effects.

If solar radiation management techniques were to become necessary, the researchers believe that stratospheric aerosols, space-based methods and cloud albedo approaches have the most potential. However, stratospheric aerosols may cause adverse effects such as depletion of stratospheric ozone, space-based methods are expensive, complex and would be slow to implement, and cloud albedo approaches such as cloud ships could have negative impacts on regional weather patterns and ocean currents, and their effectiveness is uncertain.

"If we are confronted with a climate emergency and decide we cannot tolerate any more warming, engineering some system to deflect more sunlight back to space would likely be the primary option available to cool the Earth quickly," said report co-author Ken Caldeira of the Carnegie Institution, US. "Of course, we need to make sure that tinkering with our environment in this way would not just cause bigger problems. We need to study these options now so that we can understand the pluses and minuses in case we need to deploy them."

The surface albedo approach to solar-radiation management, including white-roof methods, reflective crops and desert reflectors, was considered ineffective, expensive and in some cases likely to have serious impacts on local and regional weather patterns.