This is a cross-post of 80,000 Hours' new problem profile on climate change. We're cross posting the first section, which is our research into the question of the extent to which climate change is a direct extinction risk. For more, including the indirect impacts of climate change, and career recommendations, see the full profile. 

Could climate change lead to the end of civilisation?

Across the world, over half of young people believe that, as a result of climate change, humanity is doomed.^[1] They feel angry, powerless, and — above all — afraid about what the future may hold.^[2] 

Climate change matters so much, to so many, not just because of the suffering and injustice it’s already causing, but also because it’s one of the few issues that has obvious potential to affect our world over many future generations. We think safeguarding future generations is a key moral priority, and should be a crucial consideration in prioritising problems on which to work.

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.

So – what does the science say?

The Intergovernmental Panel on Climate Change (IPCC) Sixth Assessment Report is, to our knowledge, the most authoritative and comprehensive source on climate change. The report is clear: climate change will be hugely destructive. We’ll see floods, famines, fires, and droughts — and the world’s poorest people will be affected the most.^[3]

But even when we try to account for unknown unknowns,^[4] nothing in the IPCC’s report suggests that civilisation will be destroyed.

This isn’t to say society shouldn’t do far more to tackle climate change.

That’s because climate change’s impacts will still be significant – it could destabilise society, destroy ecosystems, put millions into poverty, and worsen other existential threats such as engineered pandemics, risks from AI, or nuclear war. If you want to make climate change the focus of your career, we include some thoughts below on the most effective ways to help tackle it.

So yes, climate change is scary. And people are right to be angry that too little is being done.

But we’re not powerless.

And we’re far from doomed.

Summary

Climate change is going to significantly and negatively impact the world. Its impacts on the poorest people in our society and our planet’s biodiversity are cause for particular concern. 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. But because the worst potential consequences seem to run through those other sources, and these other risks seem larger and more neglected, we think most readers can have a greater impact in expectation working directly on one of these other risks.

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.

Our overall view

Sometimes recommended

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.

Scale  

We think work to materially reduce the probability of the worst outcomes of climate change would have a large positive impact. However, climate change seems hundreds of times less likely to directly cause human extinction than other risks we’re concerned about, like catastrophic pandemics. As a result, if climate change does have catastrophic and potentially long-lasting consequences for human civilisation, this will likely be through aggravating other problems, such as conflict between great powers. This indirect risk brings the scale of climate change as a problem closer to other extinction risks, although it still seems more than 10 times less likely to cause extinction than nuclear war or pandemics. Our guess is that more people should seriously consider aiming at those issues directly.

Neglectedness  

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.

Solvability  

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.

Could extreme climate change (directly) lead to the extinction of humanity?

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.

In this generally grim area, this is a piece of good news people don’t always appreciate.

That said, we shouldn’t be unconcerned about climate change — not only does it pose grave dangers short of extinction, we also think climate change indirectly raises the risk of extinction via making other threats worse, which we’ll cover in the next section.

How hot could it get?

The hotter the Earth gets, the worse we can expect effects from climate change to be.

So, to figure out whether climate change could cause extinction, we need to know how much we expect temperatures to rise. To figure that out, we first need to have some idea of how much greenhouse gas we’re going to emit, as well as how much of a temperature rise that will produce. We’ll look at each in turn.

How much greenhouse gas could we emit?

The IPCC Sixth Assessment Report considered many illustrative scenarios, including:

• The world meets the goals of the Paris Agreement from COP-21, to limit warming to 1.5°C. (SSP1-1.9)
• The world takes enough action to limit warming to 2°C. (SSP1-2.6)
• There are modest mitigation efforts, with slightly lower emissions than what current policies might suggest. (SSP2-4.5)
• There is a reversal of some current policies, increasing warming. For example, this could happen if countries are competing against each other for growth. (SSP3-7.0)
• There is significant policy reversal. The world decides to use fossil fuels to cause rapid development even if they are more expensive than renewable energy. (SSP5-8.5)

These are the most likely scenarios. But what about the most extreme scenario? What if we tried to burn literally all the fossil fuel in the ground?

The IPCC estimates that there are 18,635 gigatonnes of carbon in the Earth’s fossil fuel reserves.^[5]

Luckily, fossil fuel extraction methods don’t allow you to extract all the fossil fuels in a deposit — especially with coal. So the question is not how much fossil fuel there is, but how much might ultimately be recoverable with future technology.

The highest estimate we’ve seen on the quantity of recoverable fossil fuels is 2,860 gigatonnes of carbon.^[6]

Releasing 3,000 gigatonnes of carbon would take us to carbon dioxide concentrations in the atmosphere of around 2,000 parts per million (for comparison, pre-industrial carbon dioxide concentrations were 278 parts per million, and current concentrations are 415 parts per million).^[7]

How much warming could happen as a result?

The warming caused by our greenhouse gas emissions will occur in the decades and centuries after the emissions, and is effectively caused by the total amount of carbon emitted.

But actually predicting the total amount of warming caused by some quantity of greenhouse gas is difficult, because there are feedback loops.

Here’s an example of one of those feedback loops: when you heat a metal enough, it glows red. Things at much lower temperatures glow in infrared — that’s why you can see people at night using infrared cameras. The hotter something is, the more energy it releases through this glow (known as black-body radiation). So as the Earth’s temperature increases, it radiates more infrared radiation back into space. This reduces the effect of emissions on global temperatures.

There are other similar feedback loops that help reduce the effects of emissions. For example, as the oceans warm, they will be able to dissolve more carbon dioxide out of the atmosphere. Plants photosynthesise more at higher temperatures (up to a point), and this also reduces the effect of our emissions.

But there are also feedback loops that could make things worse, which we’ll go through here. In the worst cases, these are associated with tipping points, where once a certain amount of greenhouse gas has been released, it triggers some feedback loop that results in an extremely significant and permanent increase in temperature.

The runaway greenhouse effect

We could theoretically see very extreme temperature rises through a runaway greenhouse effect.

We know this is possible because it appears to have happened before: on Venus. Soon after it formed, Venus may have been habitable, with a large water ocean. But Venus formed closer to the Sun than the Earth did, and this slight increase in temperature led to the gradual evaporation of its ocean. Water vapour is a greenhouse gas, so this led to further heating, and further evaporation, eventually moving Venus from a habitable world to one where surface temperatures reach 462°C (864°F), hot enough to melt lead.

Luckily, most models suggest that it’s just not possible, even in principle, for anthropogenic carbon dioxide emissions to reach levels high enough to trigger a runaway greenhouse effect on a Venus-like scale.^[8]

And even if we did eventually lose all the oceans to space, this would take hundreds of millions of years. So we’d be very likely to be able to stop the process or find other ways to survive (if something else doesn’t kill us in the meantime).

Cloud feedbacks

One study found that if carbon dioxide concentrations in the atmosphere reach around 1,300 parts per million (which is unfortunately plausible under worst-case scenarios), clouds that shade large parts of the oceans and reflect light back into space could break up.

Many scientists think that the model involved is far too simple to be plausible.

However, there isn’t consensus on this. If the modelling in this study is right, cloud feedbacks would cause an extra 8°C of warming on top of the expected 6–7°C associated with that quantity of emissions. That would bring us into the extreme regions of the effects listed below.

Methane clathrates

Methane clathrate is a substance formed of methane within a crystal of solid water molecules — basically ice with methane trapped in it. There are large quantities of methane clathrates under the ocean floor.

When oceans warm, they could melt these clathrates, releasing further methane into the atmosphere.

The IPCC’s Sixth Assessment Report estimates that there is the equivalent of 1,500–2,000 gigatonnes of carbon dioxide trapped in methane clathrates (approximately twice as much as we have emitted so far). However, they expect any release of clathrates to occur over centuries or millennia, which would give us substantially more time to adapt to any changes.

That said, the IPCC thinks it’s unlikely that clathrate emissions will cause substantial warming in the next few centuries.^[9]

Research into methane clathrate release appears underdeveloped, so there’s a lot we don’t know — and this contributes to our overall uncertainty on how hot it will get.

Melting permafrost

There are permanently frozen layers in the Arctic and other cold regions of Earth. The IPCC estimates that there are 1,460–1,600 gigatonnes of carbon dioxide (or equivalent quantities of other greenhouse gases) stored in permafrost.^[10]

Some of this permafrost is already melting, releasing greenhouse gases.

Abrupt thawing could cause up to half of these trapped greenhouse gases to be released immediately, with the rest released more gradually over decades.

The IPCC report says that for each 1ºC of warming, permafrost emissions will increase by 18 gigatonnes of carbon dioxide, with a 5% to 95% range of 3 to 42 gigatonnes. At the upper end of models, we could see up to 600 gigatonnes of carbon dioxide released from permafrost, warming the planet by about an extra degree (on top of the 6ºC we’d already see from human emissions in these scenarios).

Summing up: what is the range of possible temperature increases from climate change?

Given some amount of greenhouse gas we emit, we need to know what the possible temperatures are that Earth could reach.^[11]

There’s clearly a minimum possible temperature change; it’s virtually certain that temperatures will go up.^[12]

The Sixth Assessment Report gives estimates of how much temperatures will go up under the emission pathways we looked at earlier (which range from meeting the terms of the Paris Agreement to an extreme fossil-fuel burning scenario).

While these estimates do incorporate uncertainty around which feedback loops are plausible (more on that below), they show 90% confidence intervals. That is, (roughly) there’s a 10% chance that temperature changes in each scenario could be higher than the top of the thin lines, or below the bottom of the thin lines (representing the confidence intervals).

In a scenario worse than the IPCC’s worst case, where all the recoverable fossil fuels are burned, there’s a 1 in 6 chance we’d see greater than 9ºC of warming by 2100.^[13] And it looks like there’s an extremely small, but real, chance that this would be enough to cause the worst-case cloud feedback loops.

In total, that would lead to something like 13°C of warming relative to pre-industrial levels. We’d reach that 13°C in the years or decades after we trigger the cloud feedback tipping point — and then there could be additional warming in the centuries and millennia after that. This 13°C of warming would be a humanitarian disaster of unprecedented scale.

As far as we can tell, reaching 13°C is very unlikely, and about as hot as our models suggest it could possibly get on timescales we might not be able to adapt to.^[14]

Now we’ll turn to whether this warming could directly cause extinction — via pure heat stress, sea level rise, or agricultural collapse.

Could climate change simply make it too hot for humans to survive?

On the hottest and most humid days, you’d walk outside and it felt immediately like someone pressed a hot wet towel, like you sometimes get on airplanes, over your entire head. I wear glasses, and they’d immediately fog up. You sweat instantly. People just avoid being outside in any way they can. In the summers, my friends and I would become nocturnal as a way to beat the heat.

John Hagner, on living in Dharan, Saudi Arabia (one of the most heat-stressed inhabited places in the world)

If temperatures rise high enough, it becomes too hot for humans to survive for more than a few hours — even in the shade. In places with high humidity, like the tropics, it’s harder to cool through sweating, so this effect is even worse.

This could make significant parts of the planet uninhabitable (at least outdoors, or without air conditioning) for significant portions of the year. This map shows the number of days per year we’d get surface temperatures greater than 35°C (95°F) in various regions on Earth, if we had around 7°C of warming. This is a good illustration of the sorts of areas that might become too hot for humans to survive with more than 7°C of warming.^[15]

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 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).

Moreover, it would be hard for this to lead directly to extinction even if we didn’t adapt, given that a large chunk of land on Earth would remain habitable, even with 13°C of warming. We would have to live in a much smaller area, but civilisation would survive.

Could the world sink under water?

The IPCC’s Sixth Assessment Report projects a sea level rise of around one metre by 2100 if we see 5°C of warming above pre-industrial levels — in worst-case scenarios, this could be up to two metres.

This map shows the areas that will be below high tide by 2100 if there were 5°C of warming, assuming a 95th percentile worst-case scenario. (That is, there are various proposed effects that could help reduce the amount the seas will rise, and this map assumes that these largely won’t occur.)

Modelling sea level rise is difficult, so there’s large uncertainty on how bad it could get. And over centuries, sea level rise could be much higher. The IPCC says that, in the worst emissions scenario we’ve considered with around 6°C of warming, “sea level rise greater than 15m [by 2300] cannot be ruled out.”

We haven’t seen any modelling on sea level rise with 13°C of warming. 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.

As with heat stress, we’ll probably be able to adapt to these changes, particularly through building new infrastructure like homes or flood defences. And the fact that it will take centuries for sea levels to rise completely means this adaptation will likely be much easier.

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.

We know this adaptation is possible because we’ve seen adaptation to rapid sea level rise before. Tokyo is sinking into the ocean, and experienced effectively four metres of sea level rise in the 20th century.^[17] This rise happened at a rate of about 40 millimetres per year, which is similar to what we’d expect with the IPCC’s worst-case projections.

So sea level rise will cause substantial disruption to our society, particularly in developing countries. And our adaptation will be expensive — the IPCC projects that, with 4–5°C of warming, we’ll be spending over 1% of our GDP adapting to floods.

But, as with heat stress, sea level rise does not pose an extinction risk.

Could climate change destroy global agriculture?

The IPCC’s Special Report on Climate Change and Land reports that hundreds of millions of additional people will likely be at risk of hunger by 2050 as a result of climate change.

On top of extreme events like hurricanes or droughts disrupting agriculture, we can expect temperature changes, changes in rainfall, and other weather-related changes to significantly harm our ability to grow food.

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.

With more extreme warming, higher temperatures will directly affect agriculture.

The chemical reactions plants need to survive (including photosynthesis and respiration) can’t operate if temperatures are too high.

As a result, more than 10°C of warming would be likely to destroy agriculture in India and regions with similar climates.

We could also see substantial changes in precipitation levels that, under extreme scenarios, would significantly harm agriculture. This map shows changes in precipitation by 2050 in various regions, under a high-emissions scenario.^[18]

(In general, predictions about precipitation and other weather changes, like the frequency of extreme weather events, are difficult to make and vary significantly between models — so take all these figures with a grain of salt!)

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 the Earth to warm significantly. 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 plausibly lead to humanity being unable to ever recover.

How would extreme climate change affect biodiversity?

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.

Estimates on the proportion of species that could go extinct from climate change vary, but in the worst cases, models predict up to 40% of species could be “committed to extinction” by the middle of the century.^[19]

So extreme climate change could have significant negative effects on biodiversity. What about the instrumental importance of biodiversity? Could reduced biodiversity exacerbate the effects of extreme warming on agriculture? In order for this to happen, we’d have to see something crucial to our food chain go extinct. One plausible possibility here is that pollinators — whose populations are already in decline — could go extinct. But models suggest our agricultural production would drop by only around 10% if we didn’t have pollinators.^[20] Kareiva and Carranza at the Cambridge Centre for the Study of Existential Risk looked into this in more detail and concluded that ecosystem collapse is extremely unlikely to pose risks to human existence.^[21]

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.

Summing up: why climate change almost definitely won’t directly cause human extinction

If we follow current policies, we’ll probably end up seeing around 2–3°C of warming. It’s also possible that we’ll see a reversal of current attempts to reduce emissions. This could happen if sectors of the economy that we can’t decarbonise grow rapidly, such as if we develop new technology that uses large quantities of energy, or if something like a large war incentivises high-carbon activities.

In a worse scenario, we’ll burn fossil fuels even though they’re more expensive than renewable energy. And in the worst-case, but extremely unlikely, scenario, we could burn all the recoverable fossil fuels and reach 7°C of warming.

There’s also a very small chance that in these unlikely scenarios where we rapidly burn far more fossil fuels than we are currently on track to, that cloud feedback tipping points could be reached. This could lead to something like 13°C of warming.

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.

But does this conclusion adequately take uncertainty into account?

After all, any time we try to use what we’ve discovered so far to make predictions about the future, we have to be aware that there could be things we don’t know which could make things worse than we expect.

We saw above that one source of uncertainty is the possible emission pathways that we will follow in the future. We tried to take that into account by considering a wide range of scenarios.

We’ve also seen structural uncertainty: that is, uncertainty in our predictions because there are things we don’t know about how the system works — for example, whether methane clathrates will cause substantial warming in the next few centuries.

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 extremely surprised if they point in one direction or the other.

There are a few caveats:

• The higher our emissions are, the further they get from the sorts of baseline assumptions the IPCC used to come to this conclusion. So if we’re really very wrong about the amount of carbon emissions we’re likely to emit, things could still get very bad (but it seems unlikely we’re very wrong about that).
• There is much more uncertainty about how other things will change. For example, it’s hard to predict how high sea levels will rise or how precipitation patterns will change (although even then we don’t think these things will change in ways that increase the direct risk of extinction).

But overall, despite our lack of knowledge about some relevant feedbacks, this makes for a very small chance that model uncertainty means things could be radically worse.

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.

For more, including the importance of indirect impacts of climate change, and our climate change career recommendations, see the full profile. 

If you have any feedback on this article – whether there’s something we’ve got wrong, some wording we could improve, or you want to tell us you loved reading it – we’d really appreciate it if you could tell us what you think using this form.

1. ^{^}

   Hickman et al., 2021, Climate anxiety in children and young people and their beliefs about government responses to climate change: a global survey. Hickman et al. surveyed 10,000 children and young people (aged 16–25 years) in 10 countries (1,000 participants each in Australia, Brazil, Finland, France, India, Nigeria, Philippines, Portugal, the UK, and the USA) between May 18 and June 7, 2021. 55.7% of respondents agreed that “humanity is doomed."

2. ^{^}

   Hickman et al., 2021. Here, we’re citing the following results from the survey: 56.8% reported having felt angry, 56.0% reported having felt hopeless, 61.8% reported having felt afraid, and 75.5% agreed that “the future is frightening.”

3. ^{^}

   More specifically, the report warns that, by 2050, without dramatic emissions reductions, we’ll see huge changes to the planet: hurricanes, storms, tsunamis, and sea level rise will threaten more than a billion people living in low-lying areas. Many of these people will be forced to move to higher ground, increasing the chances of conflict and migration crises. Up to 183 million additional people will become undernourished in low-income countries, and up to 4 billion people will experience chronic water scarcity.
   
   These climate change impacts are taken from question 3 of the FAQ on the IPCC’s Sixth Assessment Report: How will climate change affect the lives of today’s children tomorrow, if no immediate action is taken?
   
   Chronic water scarcity means, approximately, living in a region where, for at least one month a year, every year, more water is needed than is expected to be available in the region.

4. ^{^}

   See section 7.5.5 of the Sixth Assessment Report. Speaking about equilibrium climate sensitivity (ECS, which is approximately how much warming we can expect for some quantity of carbon emissions), the report says:

   In the climate sciences, there are often good reasons to consider representing deep uncertainty, or what is sometimes referred to as unknown unknowns. This is natural in a field that considers a system that is both complex and at the same time challenging to observe. For instance, since emergent constraints represent a relatively new line of evidence, important feedback mechanisms may be biased in process-level understanding, pattern effects and aerosol cooling may be large and paleo evidence inherently builds on indirect and incomplete evidence of past climate states, there certainly can be valid reasons to add uncertainty to the ranges assessed on individual lines of evidence. This has indeed been addressed throughout Sections 7.5.1–7.5.4. Since it is neither probable that all lines of evidence assessed here are collectively biased nor is the assessment sensitive to single lines of evidence, deep uncertainty it is not considered as necessary to frame the combined assessment of ECS.

5. ^{^}

   “Fossil fuel resources are 11,490 PgC for coal, 6,780 PgC for oil and 365 PgC for natural gas.” (Note 1 PgC is 1 petagram of carbon, which is equivalent to 1 gigatonne of carbon.) From Fig. 5.12 of Chapter 5 of the Working Group I contribution to the IPCC’s Sixth Assessment Report.

6. ^{^}

   This estimate is from Welsby et al., 2021. Other estimates for the total ultimately recoverable fossil fuel resources include: 1,200 billion tonnes (Ritchie & Dowlatabadi, 2017, adjusted for non-coal fossil fuels by John Halstead); 1,040 billion tonnes (Mohr et al., 2015, low estimate; 1,580 billion tonnes (Mohr et al., best guess); and 2,500 billion tonnes (Mohr et al., high estimate).

7. ^{^}

   Lord et al., 2016, fig. 2.

8. ^{^}

   "But could we bring on such a catastrophe prematurely, by our current climate-altering activities? Here, we review what is known about the runaway greenhouse to answer this question, describing the various limits on outgoing radiation and how climate will evolve between these. The good news is that almost all lines of evidence lead us to believe that is unlikely to be possible, even in principle, to trigger full a runaway greenhouse by addition of non-condensible greenhouse gases such as carbon dioxide to the atmosphere.” Goldblatt & Watson, 2012.

9. ^{^}

   The discussion on clathrates can be found in section 5.4.9.1.3, p. 740, Chapter 5 of the Working Group I contribution to the IPCC’s Sixth Assessment Report.

10. ^{^}

    Box 5.1, p. 726, Chapter 5 of the Working Group I contribution to the IPCC’s Sixth Assessment Report.

11. ^{^}

    This figure (how much temperatures will rise for some quantity of emissions) is known as climate sensitivity, and there are multiple different technical definitions.

12. ^{^}

    IPCC projections based on the assumption that the world will continue on its current emissions-curbing policies suggest we’ll get around 2.5 to 3.5°C of warming. This is clearly enough to have disastrous humanitarian consequences.

13. ^{^}

    This follows from the IPCC’s estimates of the transient climate response to cumulative CO2 emissions (TCRE), which can be found in section 3.2.1 of the technical summary of the Working Group I contribution to the Sixth Assessment Report.

14. ^{^}

    We could reach the tipping point for cloud feedbacks before we burn all the fossil fuels. There’s some chance that, despite having to deal with 8°C of warming in years or decades, we would continue to burn substantial quantities of fossil fuels. This means there’s some chance we end up with something more like 15°C of warming by 2100. We haven’t looked into this scenario as much, but we think this is extremely unlikely, and that the arguments for why 13°C of warming won’t cause extinction also apply with 15°C of warming.

15. ^{^}

    More precisely, the map shows the number of days per year with surface temperatures greater than 35°C (95°F) in various regions on Earth, under the RCP 8.5 emissions scenario, by 2100, and with temperature changes at the 95th percentile calculated using the Surrogate Model / Mixed Ensemble model in Rasmussen et al. (2016) (i.e. there is a 95% chance that under this emissions scenario, according to the model in Rasmussen et al., that yearly mean temperatures will be smaller than those required to produce the number of days shown in the map). This is approximately equivalent to 7°C of warming since pre-industrial levels.

16. ^{^}

    “Peak heat stress, quantified by the wet-bulb temperature T_W, is surprisingly similar across diverse climates today. T_W never exceeds 31 °C. Any exceedence of 35 °C for extended periods should induce hyperthermia in humans and other mammals, as dissipation of metabolic heat becomes impossible. While this never happens now, it would begin to occur with global-mean warming of about 7 °C, calling the habitability of some regions into question. With 11–12 °C warming, such regions would spread to encompass the majority of the human population as currently distributed. Eventual warmings of 12 °C are possible from fossil fuel burning.” Sherwood & Huber, 2010.

17. ^{^}

    “The research team assessed four components of relative sea-level change — climate induced sea-level change, the effects of glacier weight removal causing land uplift or sinking, estimates of river delta subsidence and subsidence in cities. Sea-level measurements were taken from satellite data. The team then weighted their results by population to show their importance to people. The overall analysis used the Dynamic Interactive Vulnerability Assessment (DIVA) model which is designed for understanding coastal management needs. They found that high rates of relative sea-level rise are most urgent in South, South East and East Asia as the area has many subsiding deltas and coastal flood plains, growing coastal megacities and more than 70 per cent of the world’s coastal population. They also found that over the 20th Century, the city of Tokyo experienced net subsidence of 4m, while Shanghai, Bangkok, New Orleans, and Jakarta, have experienced between 2m and 3m subsidence.” ScienceDaily, 2021.

18. ^{^}

    This map assumes the RCP 8.5 emissions scenario. (More details on the methodology.)

19. ^{^}

    Thomas et al., 2004 provides particularly pessimistic estimates: “Exploring three approaches in which the estimated probability of extinction shows a powerlaw relationship with geographical range size, we predict, on the basis of mid-range climate-warming scenarios for 2050, that 15–37% of species in our sample of regions and taxa will be ‘committed to extinction’.”

20. ^{^}

    Aizen et al., 2009 say: “The expected direct reduction in total agricultural production in the absence of animal pollination ranged from 3 to 8%, with smaller impacts on agricultural production diversity.” However, as pointed out by Hannah Ritchie [here], our dependence on pollinators is growing, so this may be more like 10% now.

21. ^{^}

    Kareiva & Carranza, 2018:

    One boundary often mentioned as a concern for the fate of global civilization is biodiversity (Ehrlich & Ehrlich, 2012), with the proposed safety threshold being a loss of greater than .001% per year (Rockström et al., 2009). There is little evidence that this particular .001% annual loss is a threshold—and it is hard to imagine any data that would allow one to identify where the threshold was (Brook et al., 2013; Lenton & Williams, 2013). A better question is whether one can imagine any scenario by which the loss of too many species leads to the collapse of societies and environmental disasters, even though one cannot know the absolute number of extinctions that would be required to create this dystopia. While there are data that relate local reductions in species richness to altered ecosystem function, these results do not point to substantial existential risks. The data are small-scale experiments in which plant productivity, or nutrient retention is reduced as species number declines locally (Vellend, 2017), or are local observations of increased variability in fisheries yield when stock diversity is lost (Schindler et al., 2010). Those are not existential risks. To make the link even more tenuous, there is little evidence that biodiversity is even declining at local scales (Vellend et al 2017; Vellend et al., 2013). Total planetary biodiversity may be in decline, but local and regional biodiversity is often staying the same because species from elsewhere replace local losses, albeit homogenizing the world in the process. Although the majority of conservation scientists are likely to flinch at this conclusion, there is growing skepticism regarding the strength of evidence linking trends in biodiversity loss to an existential risk for humans (Maier, 2012; Vellend, 2014). Obviously if all biodiversity disappeared civilization would end—but no one is forecasting the loss of all species. It seems plausible that the loss of 90% of the world’s species could also be apocalyptic, but not one is predicting that degree of biodiversity loss either. Tragic, but plausible is the possibility our planet suffering a loss of as many as half of its species. If global biodiversity were halved, but at the same time locally the number of species stayed relatively stable, what would be the mechanism for an end-of-civilization or even end of human prosperity scenario? Extinctions and biodiversity loss are ethical and spiritual losses, but perhaps not an existential risk.

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    “We assess evidence relevant to Earth’s equilibrium climate sensitivity per doubling of atmospheric CO2, characterized by an effective sensitivity S. This evidence includes feedback process understanding, the historical climate record, and the paleoclimate record. An S value lower than 2 K is difficult to reconcile with any of the three lines of evidence. The amount of cooling during the Last Glacial Maximum provides strong evidence against values of S greater than 4.5 K. Other lines of evidence in combination also show that this is relatively unlikely. We use a Bayesian approach to produce a probability density function (PDF) for S given all the evidence, including tests of robustness to difficult-to-quantify uncertainties and different priors. The 66% range is 2.6–3.9 K for our Baseline calculation and remains within 2.3–4.5 K under the robustness tests; corresponding 5–95% ranges are 2.3–4.7 K, bounded by 2.0–5.7 K (although such high-confidence ranges should be regarded more cautiously). This indicates a stronger constraint on S than reported in past assessments, by lifting the low end of the range. This narrowing occurs because the three lines of evidence agree and are judged to be largely independent and because of greater confidence in understanding feedback processes and in combining evidence.” Sherwood et al., 2020
    
    CarbonBrief has a good summary of the research written by the authors.

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    See, for example: “What if we stopped pretending?,” The New Yorker, September 2019; “How to cope with the climate apocalypse,” Financial Times, July 2021; “The climate disaster is here,” The Guardian, October 2021; and “Apocalypse when? Global warming’s endless scroll,” The New York Times, February 2022.