On October 8 the United Nation’s Intergovernmental Panel on Climate Change released a major new report arguing that drastic actions must be taken soon to avert dangerous levels of climate change. According to the report, to keep global temperatures from rising more than 1.5 degrees Celsius above pre-industrial levels, greenhouse gas emissions must decline at least 45 percent by 2030.

Despite the urgency behind the new report, however, the IPCC is remarkably picky when it comes to which policies it supports in response. For example, nuclear power currently provides more low carbon energy than any other single source. But the IPCC report is rather negative on nuclear power, citing “negative environmental effects” and “the risk of proliferation.” Similarly, most economists view a price on carbon as the most efficient way to drive emissions reductions. Yet when two authors of the new IPCC report were asked whether a carbon price could quickly drive needed emissions reductions, their response was laughter.

The biggest omission from the new report, though, is the failure to consider a technological response to climate change known as geoengineering, or more specifically one type of geoengineering known as solar radiation management (SRM). SRM would involve introducing heat-reflecting particles into the atmosphere that would serve to offset the temperature gains from greenhouse gas emissions. SRM may sound like science fiction, but it has a scientific basis. The 1991 eruption of Mt. Pinatubo sent 20 million tons of SO2 into the atmosphere, resulting in a nearly one-degree drop in average global temperatures from 1991 to 1993. SRM also has two big things going for it: it’s cheap, and it can be deployed quickly. Injecting enough particles to stabilize temperatures would be in the range of a few billion dollars a year, easily within the ability of many countries or even wealthy individuals.

Few would claim that SRM is a panacea for climate change. SRM has some potential downsides. While it would keep average temperatures constant, the cooling would be uneven, and there could be unpleasant side effects, many of which are as yet poorly understood. It would also need to be continuous – if SRM was stopped, warming would resume again. The “unnaturalness” of this technology has also raised political opposition.

Still, the outright refusal of the IPCC to consider SRM as even part of a response is strange. According to the IPCC, SRM is not appropriate to consider because of “large uncertainties and knowledge gaps as well as substantial risks.” Yet as the IPCC itself recognizes, even a crash program of decarbonization is unlikely to reduce emissions fast enough to keep temperatures from rising more than 1.5C. To achieve that goal, some means of canceling out past emissions must be employed.

Instead of SRM, the IPCC advocates “negative emissions” technologies that would remove carbon dioxide from the atmosphere. Considering negative emissions as a potential policy response is perfectly appropriate. But it’s worth noting that negative emissions technologies also contain their share of knowledge gaps and risks. Removing enough CO2 to make a dent on global warming could require a massive new industry that could rival the current fossil fuel industry in terms of scale. Current carbon removal technologies are also very costly, and could require huge tracts of land.

Indeed, many of the arguments used against SRM would also apply to negative emissions.

One of the main arguments against consideration of SRM is that it would create “moral hazard.” If people know that the effects of emissions can be canceled in the future, that will reduce the urgency to reduce emissions now. This is a dubious line of argument, but it is one that if valid would apply to negative emissions as well.

Geoengineering raises a host of scientific, political, and even moral and philosophical issues. While some may find it a neat trick to solve the climate problem, others view it as strange and disturbing. But climate change isn’t going away, and geoengineering must be considered at least as a potential part of the solution. By not considering all options, the IPCC undercuts the moral urgency of its own case for action on climate.

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