If climate change could lead to the end of civilisation, then that would mean future generations might never get to exist – or they could live in a permanently worse world. If so, then preventing it, and adapting to its effects, might be more important than working on almost any other issue.
But even when we try to account for unknown unknowns,4 nothing in the IPCC’s report suggests that civilisation will be destroyed.
Looking at the worst possible scenarios, it could be an important factor that increases existential threats from other sources, like great power conflicts, nuclear war, or pandemics.
We think your personal carbon footprint is much less important than what you do for work, and that some ways of making a difference on climate change are likely to be much more effective than others. In particular, you could use your career to help develop technology or advocate for policy that would reduce our current emissions, or research technology that could remove carbon from the atmosphere in the future.
We’d love to see more people working on this issue, but — given our general worldview — all else equal we’d be even more excited for someone to work on one of our top priority problem areas.
Overall, climate change is far less neglected than other issues we prioritise. Current spending is likely over $640 billion per year. Climate change has also received high levels of funding for decades, meaning lots of high-impact work has already occurred. It also seems likely that as climate change worsens, even more attention will be paid to it, allowing us to do more to combat its worst effects. However, there are likely specific areas that don’t get as much attention as they should.
Climate change seems more tractable than many other global catastrophic risks. This is because there is a clear measure of our success (how much greenhouse gas we are emitting), plus lots of experience seeing what works — so there is clear evidence on how to move ahead. That said, climate change is a tricky global coordination problem, which makes it harder to solve.
We’re going to review the three most common ways people say climate change might directly cause human extinction: high temperatures, rising water, and disruption to agriculture.
Worst case climate scenarios look very bad in terms of lives disrupted and lost. We’re focusing on extinction because, for reasons we discuss here, we think reducing existential threats should be among humanity’s biggest priorities – in part due to their significance for all future generations.
In short, most scientists think it’s pretty close to impossible for climate change to directly cause the extinction of humanity.
If there were 12°C of warming, a majority of land where humans currently live would be too hot for humans to survive at least a few days a year.16 An increase of 13°C would make working outdoors impossible for most of the year in the tropics, and around half the year in currently temperate regions.
But even with the cloud feedback loop, it would take decades for global temperatures to reach this level, and while this worst-case scenario would cause extraordinary suffering and death, it seems very likely that we could adapt to avoid extinction (for example, by building better buildings and widespread air conditioning, as well as building more in the cooler areas of the Earth).
As an upper bound, we can consider what would happen if the polar ice caps melted completely. The highest estimate we’ve seen is that this would produce sea level rise of around 80 metres. Fifty of the world’s major cities would flood, but the vast majority of land would remain above water.
It seems that a one-metre sea level rise would, without adaptation, displace around half a billion people from their homes. But with adaptation (like building flood defences), the number of people displaced would be much smaller: the IPCC estimates that hundreds of thousands of people would in reality be displaced due to a two-metre sea level rise, far fewer than half a billion.
But, as with heat stress, sea level rise does not pose an extinction risk.
There may also be some positive effects of climate change on agriculture — for example, we’ll be able to grow crops in areas that are currently too cold. It’s possible that these effects would be enough to completely mitigate the negative effects on agriculture.
But even with all these likely disruptions, we should still be able to adapt — due to increasing agricultural productivity. Over the past few centuries, food prices have fallen as technology makes it cheaper and cheaper to produce large quantities of food.
So it is against this backdrop of rapidly improving productivity that climate change will act — and even if temperatures rise a lot, it’ll take some time (decades or maybe centuries) for that to happen. As a result, the IPCC expects (with high confidence) that we’ll be able to adapt to climate change in such a way that risks to food security will be mitigated.
One expert we spoke to did say that their best guess is that a 13°C warmer world would lead — through droughts and the disruption of agriculture — to the deaths of hundreds of millions of people. But even this horrific scenario is a long way from human extinction or the kind of catastrophic event that could directly lead to humanity being unable to ever recover.
It’s possible that climate change could lead to ecosystem collapse. Many ethical views put intrinsic value on biodiversity — and even if you don’t, ecosystem collapse could affect people and nonhuman animals in other ways.
There are, of course, many other benefits to biodiversity, like the development of new medicines. But overall, biodiversity loss seems like it won’t cause the collapse of civilisation.
Though this would be a humanitarian disaster of unprecedented proportions, humanity would still have land cool enough to live on, it won’t all be submerged in the ocean, and we will still be able to grow food in many places, though not all. In other words, humanity would survive.
The IPCC’s Sixth Assessment Report, building on Sherwood et al.’s assessment of the Earth’s climate sensitivity attempts to account for structural uncertainty and unknown unknowns. Roughly, they find it’s unlikely that all the various lines of evidence are biased in just one direction — for every consideration that could increase warming, there are also considerations that could decrease it.22
This means we should expect unknowns mostly to cancel out, and be surprised if they point in one direction or the other.
As a result, it’s extremely unlikely (we’d guess less than a 1 in 1,000,000 chance) that we’ll see the temperature changes necessary for climate change to have the kinds of effects that would directly lead to extinction.
It’s often claimed that displaced populations can increase resource scarcity and the risk of conflict in countries that they move to. Forced displacement also arguably increases the spread of infectious diseases and general political tensions. But it’s very difficult to estimate the size of these effects — and from there, to estimate the implications of these effects for the rest of society.
There’s also the possibility of much larger wars. If climate change significantly affects the fortunes of Russia, China, India, Pakistan, the EU, or the US, this could cause a great power war. Migration crises, heat stress, sea level rise, changes to agriculture, or broader economic effects on these countries could all contribute to the chances of conflict.
We haven’t thought about this possibility as much, but the same reasons we think climate change won’t lead to extinction suggest it won’t lead to a catastrophic event of this size. In short: even in the worst-case warming scenarios, a lot of humans will still be able to live on the land and grow food.
Even in the top 1% of worst scenarios, our guess is that it is extremely unlikely for premature deaths due to climate change to exceed a billion people, and this loss would likely be gradual (e.g. over a century) and due to things like declining economic productivity, rather than an all-at-once catastrophic collapse.
Moreover, if climate change gets very bad, that probably means we burned through our fossil fuel reserves. This isn’t an effect of climate change per se, but rather an effect of us not doing enough to prevent it by reducing fossil fuel use. Besides causing climate change and everything that that entails, using up our fossil fuel reserves would mean that if humanity does suffer a (different) global catastrophe that leads to a civilisational collapse, it might be harder to rebuild.
There are lots of global issues that deserve more attention than they currently get. This includes climate change, but also others that seem to pose a more material risk of extinction — like catastrophic pandemics or nuclear war.
Climate change seems unusually solvable for a global issue: there is a clear measure of our success (how much greenhouse gas we are emitting), plus lots of experience seeing what works — so there is clear evidence on how to move ahead.
And working on clean energy tech also seems neglected relative to its importance for solving the problem, though it still gets a lot of resources.
Other existential threats seem considerably greater
Experts studying risks of human extinction usually think nuclear war, great power conflict in general, and certain dangerous advances in machine learning or biotechnology all have a higher likelihood of causing human extinction than climate change.
Second, solutions that require coordination are difficult to achieve. This is true on both an individual level and a country level.
For this reason, focusing on developing and deploying new technology seems more likely to succeed (and has fewer downsides, and faces fewer coordination issues) than seeking to encourage individuals to voluntarily reduce their energy consumption. This is because it doesn’t cost the innovator much; they can benefit from selling their inventions.
For example, emissions from cars are only about four times higher than emissions from cement, but there’s much more than four times the focus on electric cars. That means there could be better opportunities to move the needle by greening cement production. We think that means working on the latter could plausibly be better
There’s also value in technology that increases energy efficiency, for example by reducing the costs of building better-insulated buildings.
The other primary form of geoengineering is solar geoengineering (deliberately deflecting sunlight away from Earth to cool the planet down). Solar geoengineering poses potential risks to humanity in itself, given the unprecedented scale of the intervention and the fact that, once in use, solar geoengineering can’t be left untended without disastrous effects.
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