A planet undergoing a runaway greenhouse effect

[Beli Orao]

How long did it take for the runaway greenhouse effect to strip Venus of its hydrogen as its oceans boiled? What temperature was required for the water vapor feedback to enter a "runaway" stage (I'm assuming something over 100 degrees)? Did Venus have a "cold trap" in the tropopause before the oceans started boiling away, and if so, how did it lose it?

I'm working on a conworld in which civilization is forced underground as runaway global warming slowly destroys the planet.

[Mashmakhan]

It is not certain whether Venus even had water or not. The planet has always been volcanically active, and as far as we know, has never had life on it. We know (or at least are pretty sure) that liquid water can't exist on Venus because of its proximity to the sun. No liquid water likely means no life. No life means no conversion of all that CO2 into Oxygen. CO2 is a greenhouse gas and a significant contributor to rising temperatures. If the CO2 has always been there, then so has the planet's greenhouse effect.

Given external temperature from your planet's star, I might give it anywhere from 100-200 years or so. Water will not help much here because if your planet had life then it will also be cold enough to have liquid water. In this case water would act more like a moderator of temperature than a contributor to a runaway greenhouse effect. You will still have a greenhouse effect, but at the very most, it will just mean your people will need to adapt to the surface conditions rather than hide from them.

Changing the planet's environment in other ways might make a difference, though. Earth has been through several greenhouse effects in the past and its surface wasn't destroyed then, making me think that you may need something extra that wasn't there before to create the kinds of conditions you are looking for.

[Beli Orao]

We know (or at least are pretty sure) that liquid water can't exist on Venus because of its proximity to the sun.

But has that always been the case? The solar luminosity of the Sun has increased over its lifetime, perhaps when L was lower H2O could exist on the surface without evaporating. AFAIK the scientific consensus (or something close to it) is that Venus originally had liquid water that evaporated, and the H2O was blasted with UV rays causing the H to escape to space and depriving Venus of water.

No H2O meant that chemical weathering no longer worked, and then degassing perpetually released CO2 into the atmosphere.

Earth has been through several greenhouse effects in the past and its surface wasn't destroyed then, making me think that you may need something extra that wasn't there before to create the kinds of conditions you are looking for.

Weathering and degassing have maintained a balance in pCO2 in the long run. I'm talking about a scenario where weathering fails like on Venus (though I could also have one where degassing fails like on Mars - but how do you make a planet geologically dead on a relatively short timescale?)

[bulbaquil]

We know (or at least are pretty sure) that liquid water can't exist on Venus because of its proximity to the sun.

But has that always been the case? The solar luminosity of the Sun has increased over its lifetime, perhaps when L was lower H2O could exist on the surface without evaporating. AFAIK the scientific consensus (or something close to it) is that Venus originally had liquid water that evaporated, and the H2O was blasted with UV rays causing the H to escape to space and depriving Venus of water.

No H2O meant that chemical weathering no longer worked, and then degassing perpetually released CO2 into the atmosphere.

But if such an event were to occur, the loss of H2O would cause more immediate effects to life than a resultant runaway greenhouse effect. They'd die of thirst long before boiling.

[Beli Orao]

But if such an event were to occur, the loss of H2O would cause more immediate effects to life than a resultant runaway greenhouse effect. They'd die of thirst long before boiling.

An advanced enough civilization that can store enough water underground (and create some kind of contained breathable environment) should be able to survive a pretty long time.

[Soap]

Eh. WOuld going underground really be the best way out? Wouldnt it be wiser to move to the poles, or have they already done that and now theyre living in the caves at hte poles? If the only place cool enough for humans to survive is underground at 90 degrees latitude I think the planet is pretty much cooked (no pun intended). Humans are pretty tough when it comes to temperature, and if the world is too hot for humans, it's probably too hot for just about everything else except reptiles and a few exotic lifeforms that are not good for food sourcces.

[Torco]

Eh. WOuld going underground really be the best way out?

Where else? I mean plenty of people would go to the poles or to very high mountains, but I imagine, as the thing gets hotter, a lot of people are gonna prefer to go undeground. I imagine a gradual process... first, people building basements. Then authorities building underground roads. then underground malls, and then people living in a thriving underground community below the ruins of the old city. Most likely over a bunch of generations.

Unless the RGW is like obscenely fast.

[LinguistCat]

Some scientists have speculated that humans could move to Venus (now) by living in balloons filled with Earth air, which would float in the Venusian atmosphere. I don't know if that could work out in this situation, but considering a runaway greenhouse effect and that by the time it's like Venus the surface temperature melts lead...

[Mashmakhan]

We know (or at least are pretty sure) that liquid water can't exist on Venus because of its proximity to the sun.

But has that always been the case?

I don't know. I wasn't talking about then, I was talking about now.

The solar luminosity of the Sun has increased over its lifetime, perhaps when L was lower H2O could exist on the surface without evaporating.

Maybe... Are we even sure Venus was around and fully formed by then? Not that it matters very much - what you are describing takes billions of years to happen, and it happens gradually. No human civilization would be around long enough to notice a difference in their sun's luminosity unless they were technologically advanced enough to move to a new planet by the time life on the planet in question became unbearable.

AFAIK the scientific consensus (or something close to it) is that Venus originally had liquid water that evaporated, and the H2O was blasted with UV rays causing the H to escape to space and depriving Venus of water.

I believe that may have been water vapour, not liquid water. To exist as a liquid in significant amounts long-term, water needs a temperature below 100°C. That is a far cry from the 460°C Mean that Venus has now. And then you need to keep enough of it on the surface long enough to use it for biological processes like drinking. As bulbaquil typed earlier, if there were people on your planet while this had been happening, it is more likely they would have died from dehydration before they could do anything about it.

No H2O meant that chemical weathering no longer worked, and then degassing perpetually released CO2 into the atmosphere.

Chemical weathering does not lead to a greater production of CO2. The CO2 was always there. On Earth, we have less volcanic activity and more means of using the CO2 up. Venus never had that, so it never had to make up for a lack of CO2 to begin with. What we are seeing on Venus is probably the same quantity of CO2 without the additional moderating effects of any water it might have had on its surface. If it even had water to begin with, and that has yet to be determined.

Earth has been through several greenhouse effects in the past and its surface wasn't destroyed then, making me think that you may need something extra that wasn't there before to create the kinds of conditions you are looking for.

Weathering and degassing have maintained a balance in pCO2 in the long run. I'm talking about a scenario where weathering fails like on Venus (though I could also have one where degassing fails like on Mars - but how do you make a planet geologically dead on a relatively short timescale?)

You won't get an instance on a planet where weathering fails if the planet was made to sustain liquid water in the long run. Earth is significantly colder than Venus is, so if there is water on it (and there is, obviously), then it can stay in a liquid state longer. The permanent existence of liquid water on Earth's surface is what is responsible for the chemical weathering. The existence of liquid water on Venus would have been a much more delicate occurance because of external solar temperatures so there was a greater margin for failure.

IMO if you wanted a natural planet-wide catastrophe on a shorter timescale, I would go with making the planet geologically dead. It would be more rare and maybe more unlikely, but it would be quicker.

[Soap]

Eh. WOuld going underground really be the best way out?

Where else? I mean plenty of people would go to the poles or to very high mountains, but I imagine, as the thing gets hotter, a lot of people are gonna prefer to go undeground. I imagine a gradual process... first, people building basements. Then authorities building underground roads. then underground malls, and then people living in a thriving underground community below the ruins of the old city. Most likely over a bunch of generations.

Unless the RGW is like obscenely fast.

Underground is not actually any colder than on the surface, though, since it's heated from above. The seasonal amplitude is less, but the overall termpature is still the same. So if the average summer temperature of someplace is 150F, and the winter temperature 100F, underground is still going to begin at 125F and get hotter as you go down. That, amnd they will have to go outside to get any food unless they can somehow grow everything they need with no light, and if they can, they probably have enough tech to build permanently air conditioned units above ground in which case they could at least have access to the surface. But as above, it would porbbably be best to just get out of the places where it's hot and go to the North Pole or someplace just as cold. I dont really think it makes a lot of sense to stay put.

[Mashmakhan]

To add to what Soap typed, apparently cave temperatures within the first few miles or so are dependent on the outside temperatures. Caves tend to moderate surface temperatures somewhat, so caves in higher latitudes will be cooler than caves in lower latitudes will be. Then, after a few miles, the planet's own thermal gradient takes over. It is still uncertain at what depth this occurs, and if this is also variable among different caves, but I have seen and heard instances where cavers have had to remove heavier articles of clothing because of the heat.

A few other things to keep in mind:

1. -
2. Air - Oxygen levels decrease the lower you go, to a point where oxygen deprivation can become an issue; at the same time, CO2 levels rise and replace the Oxygen. You will sooner experience an overdose in CO2 before Oxygen deprivation occurs, but lower Oxygen levels do occur so filtering out the CO2 alone will not help.
3. Sunlight - On Earth, UVB levels are just right for synthesis of vitamin D in most organisms; artificial UV lights pale (no pun intended) in comparison. You will want to account for a lack of natural Vitamin D synthesis.
4. Energy - Some form of energy will be needed that is compatible with the surrounding environment, as you will not be able to go outside.You will need enough energy to fuel the UV and visual lighting, not to mention a few other things that you might need.
5. Water - if you are certain that there will no longer be any water on your planet's surface (I am still skeptical, but...) then you will need some way in which to get more of it from from somewhere underground.

Finally, I wanted to point out that if the runaway greenhouse effect does raise CO2 on the surface, it will make the atmosphere denser. This will create more uniformity across the entire planet in such a way that the poles might not be such a good idea after all. Venus' surface temperatures are relatively uniform across the latitudes. Uranus' temperatures even more so.

IMO if such a scenario were to occur where all of the water on the planet's surface was depleted, I do not think any humans would be able to survive in the long run. That said, more likely the water would remain and you would just get warmer temperatures. In which case humans would probably just go on living on the surface of the planet, albeit in a slightly-to-drastically different manner than before.

[Torco]

Eh. WOuld going underground really be the best way out?

Where else? I mean plenty of people would go to the poles or to very high mountains, but I imagine, as the thing gets hotter, a lot of people are gonna prefer to go undeground. I imagine a gradual process... first, people building basements. Then authorities building underground roads. then underground malls, and then people living in a thriving underground community below the ruins of the old city. Most likely over a bunch of generations.

Unless the RGW is like obscenely fast.

Underground is not actually any colder than on the surface, though, since it's heated from above. The seasonal amplitude is less, but the overall termpature is still the same. So if the average summer temperature of someplace is 150F, and the winter temperature 100F, underground is still going to begin at 125F and get hotter as you go down. That, amnd they will have to go outside to get any food unless they can somehow grow everything they need with no light, and if they can, they probably have enough tech to build permanently air conditioned units above ground in which case they could at least have access to the surface. But as above, it would porbbably be best to just get out of the places where it's hot and go to the North Pole or someplace just as cold. I dont really think it makes a lot of sense to stay put.

Actually it is, at least in the summer, which is the problem in the early stages of the process in most places. Away from volcanic areas it gets nearer the yearly average as you go down, and perhaps more importantly, it gets easier to refrigerate. Underground water deposits might be the sole option on a planet with considerable warming, too.

But as above, it would porbbably be best to just get out of the places where it's hot and go to the North Pole or someplace just as cold. I dont really think it makes a lot of sense to stay put

under RGW the poles get nearly as horrible as the equator. Besides, think of it as a gradual process in which people are involved, as opposed to a linear thing protagonized by robots. Would you go live in the south pole? [the north one isn't going to be available under significant GW]. Leave behind your city, your job, your family? sure, some people would, but some of those with the resources are going to prefer to stay put and, I dunno, build gargantuan air conditioning.

[Beli Orao]

Chemical weathering does not lead to a greater production of CO2. The CO2 was always there. On Earth, we have less volcanic activity and more means of using the CO2 up. Venus never had that, so it never had to make up for a lack of CO2 to begin with. What we are seeing on Venus is probably the same quantity of CO2 without the additional moderating effects of any water it might have had on its surface. If it even had water to begin with, and that has yet to be determined.

Of course, hence my discussion of the balance between weathering and degassing forming a weathering "thermostat." The way I've learned it is that Venus originally had liquid oceans that balanced the amount of carbon in the solid earth and in the atmosphere, but as the temperature got hotter and hotter the water started boiling away, the weathering thermostat "broke," and most of the carbon made its way to the atmosphere. Granted, most of the literature I read has more to do with the Earth's climate rather than the history of Venus, but most of what I've read implies that the idea that runaway warming led to Venus's atmosphere is uncontroversial.

IMO if you wanted a natural planet-wide catastrophe on a shorter timescale, I would go with making the planet geologically dead. It would be more rare and maybe more unlikely, but it would be quicker.

Wouldn't it take a long time for the core to cool down? I'm not exactly sure how long it took for Mars to die, I assume that on Earth it would take longer. Maybe some kind of man-made disaster (with technology we don't have yet probably) could make it happen?

Underground water deposits might be the sole option on a planet with considerable warming, too.

This is what I was banking on. In an environment where the surface temperature exceeds 100 C throughout the year and all bodies of water are in the process of boiling away, water that is already underground would stay there and bolster any artificial water reserves the humans can bring down with them before it boils away.

[Mashmakhan]

The way I've learned it is that Venus originally had liquid oceans that balanced the amount of carbon in the solid earth and in the atmosphere, but as the temperature got hotter and hotter the water started boiling away, the weathering thermostat "broke," and most of the carbon made its way to the atmosphere. Granted, most of the literature I read has more to do with the Earth's climate rather than the history of Venus, but most of what I've read implies that the idea that runaway warming led to Venus's atmosphere is uncontroversial.

It isn't the runaway greenhouse effect that is controversial - that part is pretty much agreed upon. It is the idea of liquid water existing long-term on the planet's surface that is controversial. As far as I know, the existence of permanent bodies of water on Venus' surface in the past has still not yet been proven. What I am saying is that a planet with large bodies of liquid water permanently on its surface is less likely to lose it due to a RGE. Probably because - as you typed earlier - the planet's star wouldn't yet be strong enough to strip the Hydrogen away.

IMO if you wanted a natural planet-wide catastrophe on a shorter timescale, I would go with making the planet geologically dead. It would be more rare and maybe more unlikely, but it would be quicker.

Wouldn't it take a long time for the core to cool down? I'm not exactly sure how long it took for Mars to die, I assume that on Earth it would take longer. Maybe some kind of man-made disaster (with technology we don't have yet probably) could make it happen?

Yeah, it would. Frovably a few thousand years at the very least. I was thinking of it in comparison with that scenario you typed about with the star increasing its luminosity, which would give you the atmospheric characteristics you were looking for in your RGE. That would take a lot longer.

[HandsomeRob]

It might be interesting to add to the conversation that there's evidence that Venus' crust completely recycled itself around 500 million years ago. Now there's a planet-wide disaster scenario.

-Rob

[Beli Orao]

What I am saying is that a planet with large bodies of liquid water permanently on its surface is less likely to lose it due to a RGE. Probably because - as you typed earlier - the planet's star wouldn't yet be strong enough to strip the Hydrogen away.

Yeah, this has been the biggest obstacle in creating a realistic scenario for me. I want this planet to be Earth-like, with a similar orbit and a similar solar L. There's not enough carbon in fossil fuels on Earth to cause RGW on a cataclysmic scale, so I have to rule that out.

[Mashmakhan]

It might be interesting to add to the conversation that there's evidence that Venus' crust completely recycled itself around 500 million years ago. Now there's a planet-wide disaster scenario.

Yeah, I had read about this. Apparently it was why scientists used to think Venus was younger than Earth. Then they developed more accurate radio scans and started saying the reverse.

What I am saying is that a planet with large bodies of liquid water permanently on its surface is less likely to lose it due to a RGE. Probably because - as you typed earlier - the planet's star wouldn't yet be strong enough to strip the Hydrogen away.

Yeah, this has been the biggest obstacle in creating a realistic scenario for me. I want this planet to be Earth-like, with a similar orbit and a similar solar L. There's not enough carbon in fossil fuels on Earth to cause RGW on a cataclysmic scale, so I have to rule that out.

One thing you can do is make your planet's sun a different spectral type. If it becomes a white giant while your people are still living on the planet, then you can get the effects of outgassing that you were looking for along with the RGE. The oroblem for this, along with my earlier-noted skepticism toward their survival on such a planet, would be that not long after the RGE occurs, the sun would go out and then so would the planet. If it didn't burn to a crisp before then. The people on the planet might be able to avoid this if they evacuated the planet before then, but I am not sure whether you were looking for that or not.

Someone else on here who knows more might be able to give you a better answer so it won't hurt to be patient.

[Drydic]

Yeah I don't think a white giant would last long enough. They live for like 2 hours then KABLOOIE