Would it be possible for humans to be alive to witness the beginning of a runaway greenhouse effect on Earth?

Question

We often hear about how when Earth reaches more than 1.1x insolation (or 10% more sunlight than it receives today), the Komabayashi–Ingersoll limit will be reached and water vapour will begin to accumulate in the atmosphere (in some models, quicker than it can photodissociate) until the Earth becomes a steam cooker. The temperature threshold for this to occur is estimated to be ~47°C on average (for low albedo) and ~67°C on average (if albedo increases), today, Earth’s average temperature is 15°C.

Are there any possible scenarios in which humans (modern human analogue) could still be around as a runaway greenhouse (just the ‘event’ itself) is beginning to take hold? What would these humans be able to witness?

Answer 1

Pre-Runaway Greenhouse

It’s highly unlikely that humans could survive the extreme conditions leading up to or during a runaway greenhouse transition (not an event really). The rapid buildup of water vapor in the atmosphere would drastically amplify the greenhouse effect, pushing temperatures well beyond what humans can endure.

To put this in perspective, the event you're talking about would start at global temperatures 32ºC to 52ºC higher than today's levels. Current climate change projections estimate a temperature rise of 1.5ºC to 4.5ºC by the end of the 21st century, and even that small increase would already force major changes in where and how humans live. Human survival under extreme heat is limited by factors like air temperature, humidity, and wind speed, with a wet-bulb temperature (WBT) of around 31ºC being the realistic upper limit.

In a scenario with a 3ºC global temperature increase (see Figure below), some areas could experience more than 30 days each year with WBTs over 30ºC, making normal activities impossible without cooling systems. As society would struggle to function under these conditions, mass migration to cooler areas would likely occur. Just for a feeling: The dark dot represents the city of Kolkatta, which is projected to experience about 50 days above our survival threshold. That is, we're talking about 1 day per week!

Your scenario, with a 32ºC to 52ºC increase, would make the situation far worse. It’s hard to predict exactly how red the map would look, but it’s safe to say tolerable climate zones would be scarce. The climate system is complex, so we can’t just extrapolate from current data.

Runaway Greenhouse Onset

If we entertain the idea—sci-fi style—that humanity somehow makes it to the brink of a runaway greenhouse effect (the point of no return), then the study by Chaverot et al. (2023) is worth checking out. They break down the runaway effect into three phases:

1. Evaporation Phase: The atmosphere gets loaded with water vapor.
2. Dry Transition Phase: After all the oceans evaporate, surface temperatures skyrocket.
3. Post-Runaway State: The planet settles into a hot, stable state, similar to Venus today.

They even give some timeframes: the evaporation phase would take over a hundred years to raise temperatures by about 80°C. But in the dry phase, temperatures could jump by around 100°C in just a few decades.

Given these numbers, it’s pretty clear that we wouldn't survive past the first phase. Since all life on Earth, including humans, depends on liquid water, it’s almost certain that no one would last long enough to see the second phase.

Answer 2

Would it be possible for humans to be alive to witness the beginning of a runaway greenhouse effect on Earth? It depends on what you call a human.

The Sun is predicted to become 10% more luminous than it is now in about 1 billion years. That is 1,000 million years. For comparison,

• The first species which can be assigned with certainty to the genus Homo, H. erectus, appeared 2 million years ago.

• The non-avian dinosaurs went extinct about 66 million years ago.

• The first four-limbed vertebrates which walked on land appeared about 400 million years ago.

• The first jawed verbrates appeared about 445 million years ago.

• The famous Cambrian explosion of diversification of complex life, which marked the emergence of most extant animal phyla, occurred about 539 million years ago. This event marks the beginning of the geological time scale we learned in high school: Cambrian–Ordovician–Silurian–Devonian–Carboniferous–Permian, Triassic–Jurassic–Cretaceous, Paleogene–Neogene–Quaternary.

  And it is still only about half the time until the sun becomes 10% more luminous.

• The first multi-cellular animals appeared maybe 665 million years ago.

On the average, the lifespan of a mammalian species is about 1 million years, with a maximum lifespan of maybe 10 million years... One billion years is a very long time.

Answer 3

Increasing Energy Budget

To have a running-greenhouse-effect you do not necessarily need much water vapor in the atmosphere. This is more of a secondary effect.

What you need to kick-off such cataclysmic events is a positive-energy-feedback-loop, meaning more energy is going into the planet's energy/temperature budget than can go out by dissipating heat into space again. And because of that (or at least across several other levels) even more energy is added which also cannot escape and so on.

Positive feedback loops are very unlikely to occur in nature, because of thermo-dynamics and entropy. Everything tries to reach some kind of equilibrium or balanced energy state. Nothing strives to set free or produce more and more energy. At some point the violent, energy dissipating processes are out of fuel and then other, counter-acting processes, take over.

Atmosphere

If the atmosphere is of a certain consistency to serve as a "glass lid" this can be beneficial of course (meaning, the atmosphere allows more energy in than out). While water vapor is a green house gas, it also has the property of reflecting much of the sunlight in a high layer of the atmosphere outwards again. The "glass lid" becomes opaque so to say, which normalizes the conditions on the planet again.

Sun

It is very unlikely that a running-greenhouse-effect will happen just by increasing luminosity of the sun from one instant to the next. There are so many effects counter-acting a heating up of the planet that there will be many states of a new equilibrium being formed over long periods of time before earth eventually loses all its water (talking billions of years, not 10 human generations or so).

Earth

Now, to have that positive-feedback-loop, I would not so much look towards the sun, but more inside earth. If you think of volcanic activity this is a strong source for adding to the planet's temperature budget. Problem is, it will most likely also eject many other particles into the atmosphere, so a kind of "fallout winter" scenario will take place, not a heating up to be a running "steam cooker" all the time.

Scenario

One event I can think of, would be volcanic activity below water line. Not too deep, because you want water vapor to escape into the atmosphere. But deep enough, so that secondary particles are more likely to stay bound to water than also being ejected into the air. Given the activity is taking place on a large surface area (pacific fire belt) and the process not being very violent/explosive, this might start adding up decent amounts of water vapor in the atmosphere.

Let's further assume, that this heats up conditions on the planet enough to melt methane deposits in the permafrost regions (siberian traps). Which will emit high amounts of greenhouse gases, further increasing the temperature on earth. This will lead to increased melting of the polar ice, causing a decline of the rising temperatures again. Because of the missing ice, even less sunlight is reflected, increasing the amount of energy earth has to deal with. But also, non-water-covered areas will be smaller. Because of higher temperatures, the atmosphere is able to take in a lot more water vapor, which means vapor will dissipate around the globe before building up again. And so on.

I still doubt, that this results in a "running-away"-effect, meaning in a self-sustaining and self-induced-increasing of the effect. There is some kind of limited fuel involved on each level. There has to be even more going on to be devastating for all the oceanic water.

The in-between steps are most likely a death sentence/threat to all life on earth, of course.

Long story short

Humankind might encounter catastrophes which have a short-term "boiling" effect, but a running-away-effect is implausible to a high degree. Having this effect would imply substantial changes in the state of the solar-system (e.g. dying sun, production of high amounts of greenhouse emissions on earth, losing of the atmosphere or shielding magnetic fields, etc.) and therefore solar-system-scale time-frames.

Another scenario is hinted at in this question about consequences of falling debris of the moon that was turned into a planetary ring. Hitting the ocean, this might also produce high amounts of water vapor.

Answer 4

Dip into sci-fi a bit, and humans might cause it

According to the question, the threshold would be around 10% more sunlight than today.

Apparently, this might happen naturally in a billion years...

But humans use energy for all kinds of stuff. Production, comfort, warfare... if we had a way to get energy from off-planet and use it on-planet, we would.

So if you want to describe such a scenario, handwave a few dozen to hundred years of development and start with something like

In the year XXXX, humanity has spread to the other planets of the solar system. But it is not united, states still compete and war just as they have done for millenia - sure, the names and flags changed, but human nature did not. With increased space infrastructure came cheaper manufacturing for anything that would stay up there - so moving energy production off-planet was a natural development.

First a satellite beamed power from vast orbital solar arrays to a relatively small antenna on earth - then when that proved cheaper and more politically convenient than finding places to build more fission plants against the complaints of protestors, more followed quickly. Cheap energy led to more energy use for industry and luxury, which increased the demand for cheap energy in turn.

Today, space-based power plants deliver 11% as much power as the sun naturally does to earth, power that would have missed our planet narrowly without them. Scientists warned us that heating up the planet would have consequences, but nobody wanted to let their enemies have cheap space power while going without themselves.

What would humans witness

I can only speculate without much grounding in science, but "it gets ever hotter" is right in the name of the effect, and the question itself mentions "water vapor accumulates in the atmosphere" - so the outside would get ever hotter, weather patterns would be wildly different but probably still vary by local geography...

Biodiversity

Any animal or plant not adapted or able to adapt to the higher temperatures will die out, or live only in zoos. Possibly space zoos, for the rich.

No more polar bears or penguins (obviously), the seas home to jellyfish and algae blooms instead of fish, corals are a thing for aquariums (and maybe europa, the jupiter moon) instead of earth's oceans.

Forget about temperate forests, and be happy if you have a savannah instead of an actual desert to look at from your arcology window.

As it goes on...

Well, "runaway" is in the name of the effect. That means it'll get worse.

You'll need ever more of that nifty space energy to power your air conditioning, as it labors to keep you in comfortable temperatures against ever hotter outsides.

One by one, the plants and animals that were able to adapt to temperature X will die out as temperature X+1 is reached.

Earth will go from "our cradle, where we came from, and where all the plants and animals that accompanied us through our development still live" to "a hellhole, not quite venus (no acid rains) but definitely worse than mars". Its importance will wane, kept up only by pride and religious zealots (the "holy land" is still there, after all).