Exactly 200 years ago, Indonesia’s Mount Tambora volcano erupted, spewing particles into the atmosphere. The particles blocked incoming sunlight, causing Earth’s surface to cool by about half a degree Celsius. The year after the explosion, 1816, became known as the Year Without a Summer. Crops failed and famine ensued, fueling disease outbreaks and some violence.
Many years later, scientists became aware that global temperatures were rising. They recognized that greenhouse gas emissions were a contributing factor, and that the phenomenon posed dangers to humanity and the planet. Thus far, though, humans have been unable to stop putting carbon dioxide into the atmosphere at a high rate.
Frustrated by society’s failure to reduce emissions, a growing community of experts began considering a technological fix inspired by volcanic cooling. It may be possible to intentionally place particles into the atmosphere, blocking sunlight as an eruption would, to offset the warming caused by greenhouse gas emissions. This untried technology is one form of geoengineering, which is the intentional manipulation of the global environment. Since this method involves injecting particles into the stratosphere—the second layer up in the atmosphere—it’s known as stratospheric geoengineering (or sometimes stratospheric aerosol geoengineering).
Putting particles into the atmosphere may indeed be able to make average global surface temperatures as cool as we’d like them to be. Many organizations and governments around the world are taking steps toward trying to reduce greenhouse gas emissions, but given how extremely difficult this task may seem, requiring no less than restructuring the global industrial economy, stratospheric geoengineering is looking more and more attractive.
However, stratospheric geoengineering is not a perfect solution. It could fix the average global surface temperature, but there would still be regional variations in temperature and new precipitation patterns, so people wouldn’t be spared from adapting to change. And stratospheric geoengineering would do nothing to stop the ocean acidification that threatens marine life, which is caused by carbon dioxide entering the oceans.
These are all serious issues. On the other hand, they are probably no worse than what would happen without stratospheric geoengineering. So why not try it, and bring Earth’s temperature down?
Because there is at least one way that stratospheric geoengineering could be even more dangerous than regular global warming. After embarking on implementation, people might stop putting particles into the stratosphere for any number of reasons, from a change in political leadership to a disabling catastrophe such as a pandemic or nuclear war.
The particles that were put into the atmosphere would fall out after a few years. Without continuing injections, Earth’s temperatures would spike back up to wherever they would have been without any geoengineering. Warming that would otherwise take centuries could occur in just a few decades. Such rapid warming would be extremely difficult to adapt to. For example, under rapidly-changing conditions, it could be difficult to figure out what crops to grow. If agriculture couldn’t be maintained, then famine would ensue on a massive scale worldwide. The result would be a catastrophe larger than any caused by regular global warming, which occurs at a slower pace, allowing more time for adaptation.
The risk of a rapid temperature spike makes the stratospheric geoengineering decision very difficult, especially because nobody knows exactly what the impact would be of either such a spike or regular global warming. It’s not like we can run the science experiment to see how human civilizations manage.
Another challenge is that we don’t have a good understanding of how likely people would be to stop injecting particles into the stratosphere once they had started. One would like to think that they wouldn’t be so stupid, but maybe they would have no choice.
To resolve this dilemma, we need to ask: Which is more likely to cause a catastrophe that brings major permanent harm to humanity, implementing stratospheric geoengineering or abstaining? For moral reasons, we should consider the risk to the whole of humanity, including future generations.
It would be premature to reach any definitive conclusions on this question, but it does make for a good research agenda. The question doesn’t need to be answered immediately, because decisions on whether to do stratospheric geoengineering won’t be made for decades at least. My current best guess—and it is merely a somewhat informed guess—is that major catastrophe is more likely with stratospheric geoengineering than without it. It may be unlikely that people would stop putting particles into the stratosphere if they were to start, but the danger if they did stop would be so great that it would exceed the potential dangers of global warming on its own. Global warming is definitely happening, but it might not result in the sort of major catastrophe that a temperature spike would cause.
Thus, if the only two choices humanity faced were global warming and stratospheric geoengineering, the latter might not be worth the risk. It might be better to have humanity suffer through regular global warming, however painful it might be. Political leaders would be in the unenviable position of having to explain that avoiding stratospheric geoengineering was the best course. Worse, they might be pressured into pursuing it. But if stratospheric geoengineering is indeed not worth the risk, then political leaders should resist the pressure.
Of course, there is another option: reducing greenhouse gas emissions. Most actions to reduce emissions don’t pose any sort of catastrophic risk. A lot of them have major side benefits like saving money via energy efficiency or improving health as people walk more and eat less meat. To avoid the dangers of both global warming and stratospheric geoengineering, we should pursue emissions reductions with vigor.
The author elaborates on the issues addressed here in his paper “The great downside dilemma for risky emerging technologies,” published in the December 2014 issue of Physica Scripta. The views presented here are the author’s alone, and not those of the Global Catastrophic Risk Institute.
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