Let’s imagine the most optimistic climate change scenario for a century from now, without going completely bonkers. To do that, let’s first split up greenhouse gas emissions (CO₂-equivalent or “CO₂e”) into non-residual and residual emissions. The latter are emissions that are difficult or impossible to (fully) eliminate due to technological or other limitations. In our scenario, non-residual emissions — that is, emissions that we can actually reduce with current technology and without disastrous socio-economic impacts — continue to grow a little bit for a while (as they do in reality), but the curve starts bending down soon (which is a bit more doubtful, given current fossil-fuel-related plans). Most carbon-neutrality plans that were all the rage a decade ago have been quietly shelved, so reduction won’t be as fast as in an ideal scenario, but again, this is a merely maximally optimistic scenario without going bonkers.
Last year, I estimated residual emissions to be about 36% of current emissions. In the article about this estimate, I went through various emission categories and estimated how much they can be reduced with current technology. This is what lead to the aforementioned percentage. To write the present article, I went through the list again (several times, actually), to see what can be reduced and by how much if we assume moderate science fiction technology. With “moderate” I mean that we’re not assuming Star Trek stuff, but things that might become possible this century. Even when I was very optimistic about technological progress and the adoption rates of new technology (probably ridiculously optimistic even), I could not get residual emissions below 22%, but let’s be super-optimistic and say that we can reduce residual emissions to 20% of current total emissions within the next 100 years.
In addition to these optimistic assumptions about emission reduction, we need even more optimistic assumptions about the development and adoption of direct air capture of carbon (DAC) to reduce CO₂ in the atmosphere. Development of DAC is excruciatingly slow and its adoption is even slower. Moreover, getting CO₂ out of the atmosphere costs a lot of energy, which means that DAC competes with other uses of energy, which makes it rather unlikely that DAC will ever play a significant role. But let’s ridiculously optimistic again (actually, I fear we may pass into bonkers territory here, but let’s ignore that), and assume that DAC will become sufficiently widespread to completely remove all residual emissions from the atmosphere.1
The following graph illustrates this scenario. The thin black line represents residual emissions and the thin blue line carbon capture from the atmosphere (i.e., DAC). As just mentioned, these two lines converge. The thin dark red line shows total CO₂e emissions. If DAC is subtracted from that, you get net emissions, represented by the thick orange line. The latter is all we need to know to estimate average global warming, which is shown in the graph by the dotted yellow/orange line (on the right y-axis; all other graphs are on the left y-axis).
So, this maximally optimistic (but not completely bonkers) scenario results in almost 4°C of average global warming, which would make much of planet Earth effectively uninhabitable. And that is before tipping points and other hard-to-model feedback effects are taken into account. At 4°C of warming, these may add another 2°C, although there are very large uncertainty margins. This brings the total average global warming to 6°C, which is sufficient to make human extinction a serious risk, and which would most certainly destroy anything resembling civilization. But even 4°C itself would already be quite terrible. (More about that below.)
One obvious omission in this optimistic scenario, which is starting to look a lot less “optimistic”, is economic damage due to climate change and its effects (such as “natural” disasters). This is an important omission, because changes in CO₂e emissions have a very high correlation with economic growth, and consequently, significant economic damage is likely to bring down emissions. A very recent paper by Timothy Neal, Ben Newell, and Andy Pitman on this topic suggests that at 4°C of average global warming, global GDP loss will exceed 60%.2 However, economic damage doesn’t just influence emissions, but also investment in green technology (including DAC), and so forth. A severely stressed economy (due to disasters etc.) will switch to carbon-neutral technologies at a lower rate (or not at all), invest less in technological innovation, and will have less money available to run costly DAC machines. If all of these effects are taken into account, it turns out that the net result is almost the same as the scenario without economic damage, as illustrated by the following graph. The black lines are net emissions and temperature anomaly (dotted line) from the previous graph (were they were orange and dotted yellow/orange, respectively). The two red lines represent the variant scenario with economic damage based on the estimates by Timothy Neal and colleagues.
The graph relating economic damage to average global warming in the paper by Timothy Neal and colleagues is shown below. The red line in the graph is their estimate. (The black line is the absurdly optimistic estimate of an older model.) Regardless of whether it is right (or even plausible) that 4°C of average global warming will reduce global GDP with more than 60%, it is quite debatable whether numbers like these are actually all that meaningful. They seem to suggest a world much like ours, just with more poverty, but at 4°C of warming, we’ll have entered an entirely different world. And even at 2°C of average global warming, which we’ll probably reach around the year 2040, we are well on our way toward this new world. Unfortunately, “new”, here, does not mean “better”.
In 2020, Mark Lynas published an “update” (i.e., entirely new version) of his 2007 book, Six Degrees.3 Like the earlier version, Our Final Warning (i.e., the 2020 update) summarizes the effects of global warming at 1°C, 2°C, and so forth, up till 6°C (with one long chapter per degree).4 For those with little time or patience, the foreword offers a nice summary:
we are already living in a world one degree warmer than that inhabited by our parents and grandparents. Two degrees Celsius, which will stress human societies and destroy many natural ecosystems such as rainforests and coral reefs, looms on the near horizon. At three degrees I now believe that the stability of human civilisation will be seriously imperilled, while at four degrees a full-scale global collapse of human societies is probable, accompanied by a mass extinction of the biosphere that will be the worst on Earth for tens or even hundreds of millions of years. By five degrees we will see massive positive feedbacks coming into play, driving further warming and climate impacts so extreme that they will leave most of the globe biologically uninhabitable, with humans reduced to a precarious existence in small refuges. At six degrees we risk triggering a runaway warming process that could render the biosphere completely extinct and for ever destroy the capacity of this planet to support life.5
And the back cover of the edition of the book I have is even more succinct:
1 degree: Today’s world. Australia aflame. Rain at the North Pole.
2 degrees: The Paris target. Arctic sea ice gone. Worldwide drought.
3 degrees: Current emissions path. Global food crisis. Amazon rainforest collapse.
4 degrees: Much of China and India uninhabitable. Mountain glaciers gone.
5 degrees: Mass wipeout of life. Humans reduced to polar refugees
6 degrees: Possible human extinction.6
However, while passages like these make it clear that we’re in very deep trouble at 4°C of global warming or beyond, they fail to convey how bad 2°C already will be. There is a tendency in predicting the effects of global warming to focus on one kind of effect at the time, but the problem is that there are very, very many kinds of effects, which are all bad, and that these various effects interact and reinforce each other, at least in some sense. The problem is that 2°C of warming may very well be the civilizational equivalent of death by a thousand cuts.7
The most obvious direct effect of global warming is heat. Climate change makes heat waves longer, more frequent, and hotter. This is already happening; heat kills more people than all other “natural” disasters combined. Heat can kill fast or slowly. It kills fast when it’s just too hot to keep your body temperature down sufficiently; it kills slowly by causing kidney damage and other detrimental health effects. Furthermore, heat also kills livestock and plants, so during an intense heatwave, food production is hit in (at least!) two ways: not only does it become impossible to work outside, but crops and livestock are dying as well.8 Even if heat doesn’t kill crops, it severely reduces crop yields for all major food crops. At 2°C of average global warming, parts of Central America, Africa, and Asia (including densely populated parts of India, China, and Indonesia) will effectively become uninhabitable due to frequent deadly heatwaves destroying food production and much other economic activity. (At 4°C, pretty much everything between roughly 30° north and 30° south becomes effectively uninhabitable for this reason.)
Drought further exacerbates the problem. Much of the planet is already drying out due to climate change. At 2°C, between a fifth and a quarter of the global population will be directly affected by (severe) aridification. Hence, if in some region heat doesn’t kill crops and livestock, drought might. And water shortage doesn’t just affect agriculture (and thus, food), of course, but also limits drinking water and the availability of water for all kinds of other socially and/or economically necessary uses (including electricity production).
Heat doesn’t just affect land, by the way, but oceans as well. Furthermore, oceans are also affected by two other effects of pollution: acidification and the spread of anoxic “dead zones”. Acidification is caused by CO₂ and destroys mollusks, krill, and corals (among others). Anoxia is lack of oxygen, caused by heat or mostly agriculture-related pollution and kills almost everything (except algae) in the areas affected by it. At 2°C, virtually all coral reefs will be dead (mostly due to heat, but also due to acidification and sea level rise), which is problematic as about a quarter of animal species living in the oceans are directly or indirectly dependent on coral reefs. The loss of krill and some kinds of plankton removes the base of much of the rest of the marine food pyramid, further decimating life in the oceans. Growing anoxic dead zones kill everything within them. And a further problem for marine ecosystems is that changing water temperatures changes the geographic range of species, causing ecological collapse when invasive species destroy eco-systems that were previously outside their range. Because of all of these effects, even at 2°C, the oceans will probably no longer be a significant source of food, while currently more than a sixth of animal proteins consumed by humans comes from the oceans. (And at 3°C or 4°C of warming, the oceans will probably be mostly dead.)
Hotter oceans also lead to more and stronger cyclones (hurricanes and typhoons), causing storm damage and floods. Flooding is also caused by more extreme weather in general. Unfortunately, occasional heavy rain does little to mitigate drought, as water doesn’t soak into parched soil and just runs off (taking some of the soil with it). Flooding, storm, and other extreme weather will cause ever more damage. At 2°C, the areas affected by hurricanes or typhoons will be larger (or in other words, there will be parts of the world that have never experienced a hurricane yet, that will experience occasional hurricanes in the 2°C world), but the worst effects will be in regions that already experience such storms. Some places (including coastal cities) will be (effectively) abandoned after getting hit by severe storms several times. Or if they are hit by other kinds of “natural” disasters. In the 2°C world, with the economy in tatters due to drought, heat, refugees, disasters, and so forth, there will be no money available for rebuilding what has been destroyed by a storm, a flood, a forest fire, a tornado, or some other kind of disaster.
A further effect of rising average temperatures and ecosystem collapse due to heat and drought worth mentioning is a rise in zoonotic diseases and insect-borne diseases (such as malaria and dengue fever) and associated epidemics and pandemics. The hotter it gets, the higher the chance of serious pandemics (which can be far, far worse than COVID-19). And the hotter it gets, the further malaria, dengue fever, zika, and so forth, spread outside their traditional regions.
At 2°C of average global warming the vast majority of people will be affected by deadly heatwaves almost every summer. In some countries, these may only cause mass mortality among the elderly, but there are many currently densely populated areas where the effect of heatwaves will be much more severe. In parts of India and China, the two most populous countries in the world, every year, during part of summer, working outside (or even going outside for more than a short time) will become deadly, regardless of how healthy you are. With summers lengthening and heatwaves growing worse every year, areas like these will no longer be able to sustain significant populations at some point. We’re already getting closer to that point (although we’re not nearly there yet, fortunately). 2°C is most likely enough to pass it. And this is not just true for parts of India and China, but also for other regions in South and South-East Asia, the Middle East, Africa, and Central and Northern South America. It is hard to say how many people exactly will be directly affected, but a billion is not an unreasonable guess. (Although the uncertainty margins are large, it may be only half a billion if we are lucky, but it could also turn out to be two billion.)
At 2°C, drought and heat will devastate agriculture, causing food riots and famines worse than anything humanity ever experienced. Food insecurity and rising food prices are a common source of civil unrest or even violence. The worse the food situation, the worse the sociopolitical situation. Add to this problem the ever-growing increase in frequency and severity of “natural” disasters liker hurricanes, floods, extreme cold spells in winter (which are also caused by global warming), and so forth. And let’s not forget the spread of malaria, dengue, and the occasional epidemic (or even pandemic) of some disease spreading to humans from animals that have been pushed out of their collapsing ecological niche.
Almost every part of the planet will face several problems like these at once: deadly heat, crop failures and food shortages, drought, epidemics, storms, floods, and so forth. Local and national governments will increasingly fail to (sufficiently) alleviate the devastation, let alone fix the damage. Sooner or later, governments and other institutions stop functioning and civil society breaks down.
Facing an assemblage of problems, some caused directly by climate change, others by secondary disasters such as societal collapse and civil war, even at a mere 2°C of warming, there will be hundreds of millions of people – possibly even more than a billion – who face the choice of staying put and dying or trying to get to safer or more inhabitable regions. But those “more inhabitable” parts of the world will face similar problems, although possibly not as severe yet, and will try to put up barriers to keep climate refugees out. This won’t work for long, of course, because there just will be too many refugees and too many problems “at home” to keep (all) refugees out.
Now, look back at the graph of economic damage from the paper by Timothy Neal, Ben Newell, and Andy Pitman — they expect 18% economic damage at 2°C of warming. This, to me, sounds utterly ludicrous. Perhaps, if you’d look at any of the effects of climate change in isolation, 18% damage sounds reasonable, or even pessimistic, but if you take all of these effects into account and realize that their further effects on societies and economies interact and strengthen each other, then 18% just looks absurd. With hundreds of millions of refugees, deadly heatwaves, widespread famine, and a battery of plagues and “natural” disasters, economic damage will far, far exceed that.
Perhaps, when reading this, you say that I’m overly pessimistic, but peculiarly, when it comes to climate change the terms “pessimism” and “optimism” become somewhat meaningless. I started this article with the most “optimistic” scenario that isn’t “completely bonkers” (see above) and found that this leads to close to 4°C of average global warming, which would utterly devastate the planet and leave only a small human population (less than a billion) struggling for survival in a ravaged wasteland. (Provided that these 4°C do not pass tipping points and/or set of positive feedbacks that lead to a few more degrees and human extinction.) “Optimism”, thus, turns out to be extremely pessimist.
Now, if you say that what I write above about the effects of 2°C is pessimist, let me respond by saying that it is actually optimist. Increases and decreases in carbon emissions are correlated to economic growth, and economic collapse also comes with a very large reduction in emissions. Hence, if the effects of 2°C are sufficient to cause widespread societal and economic collapse, then this will very drastically curb carbon emissions. Instead of approaching 4°C, we might even stay under 3°C of average global warming. That will still be devastatingly bad, but not nearly as bad as 4°C. (The relation between warming and “badness” isn’t linear, but exponential.) Hence, “pessimism” is actually optimist (at least when compared to the “optimist” scenario.)
This isn’t the end of the story — or of this article, at least. The effects of 2°C will not suddenly appear out of the blue. All of the things I wrote about the effects of 2°C are already happening — they just aren’t that bad yet. But they will gradually get worse and worse, and sooner or later, people will realize where we are heading. Powerful people. People with money. People who can “do” things.
We humans are bad at solving problems, especially when solutions are difficult and/or costly. What we do when we face a problem is look for a cheap fix: something to deal with the symptoms. The problem of climate change won’t be “solved”, as any solution is unthinkable (at least, for the political and financial elite) — it would require abandoning capitalism and many other very drastic changes in the way we live.9 But when the problems caused by climate change are becoming severe and it becomes ever clearer that we’re heading for collapse, the rich and powerful will look for a cheap fix, for something to deal with the worst symptoms, while doing nothing about the deeper cause (i.e., our socio-economic system and its dependence on fossil fuels10).
For all we know now, there is such a cheap fix, namely solar radiation management (SRM). SRM works by reducing the amount of sunlight that reaches the Earth’s surface. The cheapest and easiest way to do this is to use airplanes to spread sulfur or sulfate in the stratosphere. It’s cheap and easy enough to be within the capabilities of many countries to implement on their own, but of course, the effects would be global. The intended effect is to reduce warming, but in addition to that intended effect, there could be quite a few side effects about which there is considerable uncertainty. It might lead to ozone depletion, for example, increasing skin cancer. It would probably increase acid rain, damaging forests and buildings. It might “bleach” the sky (i.e., make the sky look white(r) instead of blue). It might also have unexpected effects on atmospheric air circulations, leading to changing patterns in wind and precipitation. And it would probably lead to a loss in agricultural food production due to the reduction of sunlight, and this loss would mostly or completely cancel out the gains resulting from the cooling effect.11 There is a lot of uncertainty about the extent and nature of the side effects of SRM, but what is relatively certain is that the effects of SRM will be regionally unequal. Some regions will experience more of the cooling effect than others. Some regions will be more affected by changing precipitation patterns than others. Some will see net positive effects on agricultural production, while others see net negative effects. And so forth. It’s unlikely that the losers will be happy. It’s unlikely that they will just sit by and watch the airplanes (controlled by the winners) spread cooling chemicals in the sky. SRM might lead to terrorism or war. (But if the losers are poor, while the winners of SRM are rich, this wouldn’t be a serious threat to SRM. The rich winners would just use violence to protect their interests.)
Furthermore, if SRM suddenly stops, average global temperatures will very quickly jump to the temperature dictated by the composition of the atmosphere (and other relevant factors). Hence, a sudden stop of SRM (due to war, for example) would result in a fairly sudden increase of 2 or 3°C (or even more). Such a temperature shock would be even worse than the relatively gradual (albeit still insanely fast from a geological perspective) warming we’re causing now. It would destroy ecosystems and have many other calamitous effects.
Regardless of whether you think SRM is a bad idea or a good idea, we need to know urgently what its likely effects and side effects are going to be. I write “are going to be” and not “could be”, because it is exceedingly unlikely that SRM is not going to happen. It’s cheap. It’s easy. It fixes one symptom (namely, heat; it doesn’t do anything with regards to ocean acidification, for example, so the oceans will still die), while making it possible, at least for some people, to let the show go on. SRM might turn out to be one more way for the rich to screw (and kill) the poor, while protecting their own (financial and other) interests. That in itself might be a reason to oppose it, but that might also be a reason to believe it is going to happen, because if there is one thing history teaches us, it’s that if the rich and powerful need to screw and kill the poor to protect their interests, they will do without hesitation.
A few paragraphs back I wrote “this isn’t the end of the story”. What I meant is not just that it wasn’t the end of this article, but that SRM changes the two scenarios sketched above. To refresh your memory, one of those two scenarios was “optimistic” leading to the pessimistic conclusion of close to 4°C and a small human population struggling for survival in a devastated wasteland. The other scenario was “pessimistic”, because it assumes that 2°C will already be bad enough to set of a cascade of societal and economic collapse, resulting in less emissions and less warming, and making it eventually possible for a larger population to survive on a slightly less devastated planet Earth. Now a third scenario needs to be added, and this third scenario may very well be the most realistic scenario. In this third scenario, when the effects of climate change become more severe and when it becomes clear that we are heading for collapse, the rich and powerful will try to implement some cheap and easy fixes to protect their interests. Neo-fascism, techno-feudalism, or some other form of authoritarianism will probably be one aspect thereof, but what matters most in the present context is solar radiation management to artificially turn the Earth’s thermostat down a few notches. In this scenario, emissions keep rising, the ocean still dies, but the global average temperature is artificially kept under control. What exactly the other effects will be, is hard to say, but it is likely that the rich and powerful will be able to sufficiently insulate themselves from most of the negative side effects, while the positive effect (i.e., keeping the temperature down) makes it possible for them to maintain the systems on which their wealth and power depends. And that is exactly why SRM will happen.
Whether this is reason for optimism or pessimism is hard to say. Or to be more precise, I think that this is a very pessimistic prediction, but I’m not so sure whether any realistic alternative would be any less bad. If that’s the case indeed, then this “pessimistic” prediction might be relatively optimistic. As I wrote above, when it comes to climate change, the terms “pessimism” and “optimism” have become somewhat meaningless.
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Notes
- See also section 4 of Capitalism and Climate Collapse.
- “Reconsidering the macroeconomic damage of severe warming”, Environmental Research Letters 20 (2025): 044029. See figure 3 on page 6.
- Mark Lynas (2007), Six Degrees: Our Future on a Hotter Planet (Harper Collins).
- Mark Lynas (2020), Our Final Warning: Six Degrees of Climate Emergency (London: 4th Estate).
- Ibid., p. ix.
- Ibid., back cover of the first edition by 4th estate. (Different editions have different back covers.)
- The next four paragraphs are based on Lynas’s Our Final Warning (2020), the IPCC reports Climate Change and Land (2019) and Global Warming of 1.5°C (2018), and the sources mentioned in previous posts about heat and drought.
- Industrial agricultural corporations in rich countries could use air-conditioned agricultural machinery, of course, and keep livestock in air-conditioned buildings, but “solutions” like those will make food very expensive and are unavailable to the vast majority of the world’s farmers.
- I wrote about these changes in The Lesser Dystopia and (more recently) in Rent, Profit, and Degrowth..
- See: Capitalism and Climate Collapse.
- Jonathan Proctor, Solomon Hsiang, Jennifer Burney, Marshall Burke, & Wolfram Schlenker (2018). “Estimating Global Agricultural Effects of Geoengineering Using Volcanic Eruptions”, Nature, August 8.