What Science Actually Says About Climate Change's Effects

I read the most detailed report done on existential risks from climate change. Here is a brief summary of the likely effects of climate change.

Around half of Americans think that climate change will “destroy the planet.” But will it? I think the answer is no, and I made the case for this negative answer in a recent post titled What The Most Detailed Report Ever Compiled On Existential Risks From Climate Change Found. As you may be able to guess from the title if you are either particularly clever that post summarized the most detailed report to date on existential risks from climate change. The report concluded the risk of human extinction from climate change was well below one in a thousand.

But unfortunately, that post was well over 6,000 words. This means it is too long to be read by either the sitting president or anyone on TikTok (two demographics I’m not optimistic on reaching, on account of my articles containing words). For this reason, I thought I’d write a more concise summary of the true existential risks from climate change. For more detail and the sources for the claims made in this article, look to that other article. This one is just the briefer summary. In other words, this is a summary of a summary of a summary of existential risks from climate change!

So how serious are the risks? How likely is it that climate change ends the world? Not very likely, though it will be somewhat serious, in light of the following facts.

First of all, we’re in for about 2.7 C of warming (relative to the pre-industrial baseline—about 1.6 C over current temperatures). Scenarios with more extreme warming—e.g. 6 degrees (in that cursed way of measuring temperature known as celsius)—have probabilities below 1%. That means that if you trust prediction markets, the odds of 6 or more degrees of warming are about three times lower than the odds of Jesus returning in 2025 (which is surprising, because I’d guess that the second coming could easily lead to more than six degrees of warming).

Why such low odds of extreme warming? The main reason is that the world has taken serious action to address climate change. Costs of renewables have fallen. Western nations and China have both agreed to cut emissions. The chart below shows the decline in solar costs over time:

Intuitively, it’s hard to see how the world getting 3 degrees warmer would be at all likely to spell doom. Humans are not typically within three degrees of melting. Most people are not the wicked with of the west.

Second, there have been past periods of warming including the Miocene, Mid-Pliocene, the last interglacial, and the transition into the Holocene from the last glacial. These have generally been times of abundance and diversity, not collapse. During these times, CO2 levels were often much higher than they are today. So while there will be some serious short-term transition costs, this is unlikely to kill everyone. It hasn’t before. There aren’t actually critical feedback loops within the range of reasonable warming that will turn the Earth into Venus.

Third, the net impact of climate change on agriculture is likely to be real but not catastrophic. Food production has been increasing dramatically for centuries and will likely continue increasing. Climate change might decrease food production by a few percent at the high end, but even with climate change, food production per person will probably be much higher in the future than it is today.

Fourth, climate change is unlikely to collapse the global ecosystem. Species aren’t going extinct quickly enough for there to be a mass extinction event in the near future. Of the species going extinct, most are on isolated islands and thus unlikely to seriously imperil human civilization. As the report notes “The rate of continental species extinctions is close to one estimate of the ‘natural’ background rate.” Furthermore, even if many species died off, this probably wouldn’t trigger some massive catastrophic ecosystem collapse. There have been many examples of lots of species going extinct without any major ecosystem meltdown.

Lastly, even if the global ecosystem collapsed, this wouldn’t destroy human civilization. Other than perhaps reducing crop yields a bit if pollinators died off, the death of most animals on Earth wouldn’t seriously jeopardize human civilization.

Fifth, heat stress and sea level rise will likely be bad but not catastrophic. Extreme heat will probably kill a few hundred thousand or at the high end a few million people annually. Sea level rise will displace a few hundred thousand people, mostly causing them to move around in their own country. However, it will not trigger mass population collapse.

Sixth, while there are various tipping points, whereby warming will trigger more warming, these are likely to be small and non-catastrophic. The total warming caused by tipping points is small compared to the first-order effects of warming. In addition, the only really serious tipping points—cloud feedbacks and a system of Ocean currents called AMOC—are unlikely to be triggered in a major way.

Seventh, while climate change will probably hurt economic growth a bit by causing displacement, destruction of agriculture, and less favorable weather conditions for labor, this won’t be catastrophic. Economic estimates tend to guess GDP will be about 5% lower given climate change than it would have been otherwise. Even despite this small drop, however, the GDP in a century will probably be many times higher than it is today. By 2100, experts estimate GDP is likely to rise between 200% and 2000%.

Eighth, climate change will trigger a non-zero but non-existential level of migration. At most, climate change could increase migration by about 10%. Most migrants will likely relocate within their own country. Climate change might counterintuitively decrease the number of migrants, by making coasts, to which many people relocate, more hazardous. Maybe a few hundred thousand people will relocate, but the world will not end.

Ninth, climate wars are unlikely to be very serious. Wars don’t actually kill very many people compared to other global threats. Smoking, for instance, kills around a hundred times more people than wars most years. Climate change will have a very small effect on wars. At the high end, climate change will cause roughly an extra 40,000 war deaths by 2100. It’s unlikely to affect great power wars, because any extra wars will mostly be in very hot areas that do not contain great powers. The only nuclear-armed nations it might make slightly likelier to go to war are India and Pakistan. Still, the effect is probably quite small. In particular, water scarcity makes conflict less likely, because countries tend to cooperate over water rather than fight over it.

These are the likely effects of climate change. Millions of extra people are likely to die. Hundreds of thousands will be displaced. But the world is very unlikely to end. Climate change is a problem maybe on the level of tuberculosis, which kills about a million people a year. It will cause millions of people to die, but it will not trigger the kind of existential catastrophe that many fear.

Climate change’s likely impacts have been distorted by people on both sides of the political spectrum. Many on the left predict impending global doom. Many on the right deny that it is real. Both of these approaches are simplistic and wrong. Climate change is not a good reason to refrain from having children, nor is it the top global issue. Instead, it is one issue among many: important, real, but not likely to immanentize the eschaton.