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"Reducing our greenhouse gas emissions is more important than ever," said coauthor Ken Caldeira of the Carnegie Institution's Department of Global Ecology, "but even with our best efforts, the Earth is likely to continue warming throughout this century due to inertia in the climate system. Cutting emissions can reduce but cannot eliminate the risk of a climate emergency." Possible climate emergencies include rapid collapse of the Greenland ice sheet into the sea causing major sea level rise, a shift in rainfall patterns causing massive global crop failures, or melting Arctic permafrost causing catastrophic release of the powerful greenhouse gas methane.

Professor John Shepherd, who chaired the study said, "It is an unpalatable truth that unless we can succeed in greatly reducing CO2 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. 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 assesses the two main kinds of geoengineering techniques-Carbon Dioxide Removal (CDR) and Solar Radiation Management (SRM). CDR techniques address the root of the problem rising CO2-and so have fewer uncertainties and risks, as they work to return the Earth to a more normal state. They are therefore considered preferable to SRM techniques, but none has yet been demonstrated to be effective at an affordable cost, with acceptable environmental impacts, and they only work to reduce temperatures over very long timescales.

SRM techniques act by reflecting the sun's energy away from Earth, meaning they lower temperatures rapidly, but do not affect CO2 levels. They therefore fail to address the wider effects of rising CO2, such as ocean acidification, and would need to be deployed for a very long time. Although they are relatively cheap to deploy, there are considerable uncertainties about their regional consequences, and they only reduce some, but not all, of the effects of climate change, while possibly creating other problems. The report concludes that SRM techniques could be useful if a threshold is reached where action to reduce temperatures must be taken rapidly, but that they are not an alternative to emissions reductions or CDR techniques.

"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 Caldeira. "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."

Professor Shepherd added, "None of the geoengineering technologies so far suggested is a magic bullet, and all have risks and uncertainties associated with them. It is essential that we strive to cut emissions now, but we must also face the very real possibility that we will fail. If 'Plan B' is to be an option in the future, 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. We must ensure that a governance framework is in place to prevent this."

Carnegie Institution