About 70,000 years ago, a super volcanic eruption in Indonesia released more than 2,000 cubic kilometres of material, blocking out sunlight and reducing global mean temperature by about four degrees. This rapid change in climate may have had a catastrophic impact on our ancestors, possibly cutting the human population to only 10,000 survivors. Although the academic debate surrounding this hypothesis has not yet been settled, there is strong evidence that rapid warming or cooling episodes have caused multiple mass extinctions of other species over the past 500 million years.

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The likelihood of human extinction due to climate change is an open question, and one that is missed in most climate discussions. We know that the two to four degrees of projected warming this century is likely to force the relocation of people from flooded cities, shift growing seasons, cut a substantial fraction out of our GDP, and have a disproportionate impact upon the most impoverished parts of the world – but it doesn’t stop there. Today’s climate models also suggest a small probability of exceeding four degrees of warming, and research has suggested that anything beyond 12 degrees would render most of the planet uninhabitable for humanity.



It is true that such dramatic warming would be unlikely. Even the most pessimistic scenarios from the Intergovernmental Panel on Climate Change (IPCC) predict less than a 30% chance of exceeding 6.4 degrees of warming this century, while Martin L Weitzman of Harvard ballparks the probability of human extinction-level climate change at around 1% – assuming nothing is done about it.



At first glance, these may seem like acceptable odds, but let’s put this in perspective; if planes crashed 1% of the time, there would be more than 2,000 plane crashes every day. Should we really have higher safety standards for our planes than our entire planet? As Gernot Wagner and Weitzman argue in their recent book Climate Shock, consideration of these low-probability, high-stakes risks urgently needs to be incorporated into our assessment of climate change.



After all, just because an event is unlikely doesn’t mean we can be dismissive of the possibility; this is why we buy insurance and wear seatbelts, it’s why we invest in diverse portfolios and countermeasures for rare national defence contingencies, and it’s precisely why we should be doing more to tackle climate change.



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It could be that humanity will finally start tackling climate change as it gradually becomes a much more severe problem, and hence we never reach the truly catastrophic scenarios. The most obvious and safe way to do this would be to cut our carbon emissions. This is difficult. As Wagner and Weitzman point out, entrenched economic interests mean the world subsidises fossil fuels to the tune of $500bn (£328bn) each year. Moreover, climate suffers a free rider problem – any country that unilaterally cuts emissions suffers the full economic ramifications of such a move, while the benefits only occur if all countries come to an agreement. This is slow and requires us to have foresight as to how our actions will impact the generations to come.



Alternatively, the easy and fast way to reduce global temperatures would be through geoengineering. Spraying sulphate aerosols into the atmosphere would mimic the reflective particles released from volcanic eruptions, cooling the planet and returning us to pre-industrial temperatures. Such a project would probably be cheap – one tonne of sulphur dioxide would be sufficient to cancel out the climate effects of almost 30,000 tons of carbon dioxide, and Wagner and Weitzman estimate that the total cost of lowering the entire planet’s temperature is likely to be less than $10bn per year. Due to the affordability of this project, any single country could unilaterally undertake a geoengineering project, bypassing the bureaucratic hurdles and perpetual gridlock of international agreements.



But the ease with which geoengineering could be used could actually be one of its biggest drawbacks. Governance difficulties multiply when every country and/or adventurous billionaire gains access to the world’s thermostat. And even if the technology were used responsibly, the aerosols would do nothing to halt ocean acidification and a myriad of other problems with carbon emissions. Furthermore, if the aerosol spraying was halted for some reason, the particles would be washed out of the sky within a month or so, and global temperatures would skyrocket faster than ever.



What could induce a sudden pause in geoengineering? Other risks to human civilisation such as pandemics, warfare, or geomagnetic storms could certainly make the maintenance of such a project much harder. As Seth Baum and his colleagues at the Global Catastrophic Risk Institute argue, some of the worst risks of geoengineering might be the link forged between climate catastrophes and other shocks to humanity. When the stability of our biosphere becomes dependent on the stability of our human infrastructure, we’re baking in an automatic climate crisis to the next catastrophe. We really shouldn’t place future generations in the position of facing two catastrophic risks at once. Climate change is bad enough, so let’s cut emissions before geoengineering becomes too tempting to resist.



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