I agree with you, but something like a 30% reduction in human population is an extremely traumatic event for all the remaining human population.

Even if it is very good for the environment.

Why would the human population shrink 30%?

That depends on whether or not economies will collapse across the globe.

If they do, or the migration pressure throws countries into war, then all bets are off. You could plausibly raise that estimate to 60-70% (entire urban population + some margin). Cities aren't self-sufficient wrt. food. Many countries aren't either. So if, for some reason, national transport and power infrastructure goes down for good, you can scratch city dwellers off the map.

It’s only taken the last 25 years to add 30% to the human population, and the rapid increases of the last 50 years owe a lot to reduced infant mortality and people living longer.

So a rapid decline might come from a reversal of those trends.

It is just an example.

Some people mention an extinction level event.

I mention a number that is horribly catastrophic, just to show that even that huge number is very far from an extinction level event.

Might not be the most likely scenario, but a runaway green house effect is certainly possible.

If a positive temperature feedback loop happened to the earth’s atmosphere we could definitely end up like Venus, where no living thing can survive.

And as engineers we understand that positive feedback loops cause changes in exponential progression. It can be “almost fine” for a while and then suddenly “there’s nothing we can do” even in human timescales.

And it could happen fast enough that humans don’t develop tech robust enough to survive other planets unsupported, so game over for humanity, better luck next time.

I personally don’t think thats going to happen, but to be honest I wouldn’t want to risk it.

Earth survived way higher co2 levels and we don’t emit nearly enough sulfides. There is essentially zero chance of Venus event happening. I’m more inclined that climate change can cause catastrophic war than runaway to ultra high temperatures.

Past CO2 levels aren't relevant because over time things like solar irradiance change as well. The amount of CO2 needed to maintain a certain global temperature shrinks over long time frames (hundreds of millions of years) as the sun gets hotter and hotter until it finally dies.

Except sun hasn’t become meaningful hotter in the past hundred mil years. Also, we would need lots of sulfer dioxides to get emited, which isn’t happening.

Will earth suffer over the next billion years and lose atmosphere? Sure, but that’s not what’s happening now.

> but a runaway green house effect is certainly possible

Could you go into detail about how this could happen? My understanding is that this would require lots of greenhouse gasses getting pumped into the atmosphere somehow (Something like all the forests in the world burning down). I remember hearing one theory regarding methane calthrate [1] causing this effect, however I haven't read any studies going into the specifics of how large this effect could be.

[1]: https://en.wikipedia.org/wiki/Methane_clathrate#Methane_clat...

It doesn’t take much. A small disruption in the intricate supply chain of essential food is all it takes. We may not go extinct, but we will be set back a few thousand years of development.

PS: Current humans probably won’t survive without internet for a month. May be a few will survive without going crazy and killing each other

> A small disruption in the intricate supply chain of essential food is all it takes.

During the start of COVID the supply chain of food got disrupted quite a bit. Large restaurant suppliers no longer had customers because the restaurants were closed and grocery stores didn't have enough supply to keep up with customers. However, I didn't even have to dig into my emergency rice stash at all, and I haven't heard of anyone else actually going hungry.

Do you think a larger disruption might cause a cascading effect, or how exactly would that play out? The COVID example appears to show the opposite: our food supply chain is rather robust.

You are correct that the “end of world” predicted in initial months didn’t play out... yet. However, I feel even this is mostly luck. We got lucky with something that can be reasonably controlled with masks and hand washing.

Next pandemic will have countries hoarding masks and essentials even more early.

All that said, I was still talking about non-pandemic events.

The pandemic example was just an argument that our food supply system is robust in the face of some unexpected events. What do you think could happen during non-pandemic events which could topple it?

Antagonistic relations. During COVID-19 pandemic, most nations took the "we're all in it together" stance (despite being mostly busy handling things domestically). Climate change will create lots of opportunities for war - migration pressure, resource shortage pressure, new lands being suddenly available and competed for, etc.

If not direct human extinction, it could certainly mean complete collapse of advanced civilization.

Even if humans manage to survive for millions of years into the future, we've already burned all the easily accessible fossil fuels. What we're burning now takes a lot of technology to extract. It's not immediately obvious that we'll ever be able to become a space faring civilization without cheap fossil fuels to bootstrap us there. Eventually, we'll go extinct by some asteroid impact or supervolcano.

We should probably assume it's now or never for intelligence to spread off this rock.

“We gotta go to space!” is an impractical and frustrating response to climate change. Getting the human population off this planet and finding somewhere else for us to live is orders of mangnitude harder than fixing things on the planet for which we’re already perfectly adapted.

Getting to space is absolutely NOT a solution to climate change. We have to solve climate change SO THAT we can continue on our path to become space faring. If we can't manage to survive on an Earth decimated by climate change, then we obviously have no chance in space or any other planet in this solar system.

We 100% will go extinct eventually if we don't expand beyond Earth. And my point is that if climate change destroys our civilization now and we get flung back into a new dark age, I don't think it's likely that we'll ever manage to spread beyond Earth.

Climate change is an existential threat to our species because it threatens our ability to become space faring.

I think the idea is that if we burn our fossil fuels now then if something wipes out 99% of all humans there won't be another industrial revolution after we have rebuilt civilization and therefore no space exploration ever.

If I'm understanding you correctly, your argument is that if we don't become a multi-planetary species, extinction is assured. And we don't stand a chance of going multi-planetary if we don't take advantage of cheap fossil fuels within the next several years.

Why do think we need to leverage fossil fuels to build rockets? My impression was that solar and wind are already out-competing coal in terms of electricity production, and are close to beating natural gas too.

I also don't think fossil fuels are required to build rockets. Hydrogen can easily be generated from electrolysis (although a bit more expensive than from methane), Methane can be produced from plants, and while I haven't heard it tried yet I imagine a fuel similar to RP-1 could also be synthesized from plants, similar to how we make ethanol.

Do you think we won't be able to switch to renewables in time, or is my analysis on how effective renewables are off?

My reading of the comment you were responding to is that they were saying "We needed to leverage fossil fuels to get to the state of development where we can build rockets, and if we didn't have that fuel in the ground, we'd be stuck pre-industrial. Future societies won't have that fuel, so we're the only chance (for millions of years) of becoming multiplanetary"

Exactly this. Before the oil age, we were already rapidly decimating forests for fuel. In a way, the discover of fossil fuels averted that ecological disaster and there are more trees now (in America at least) than 100 years ago [1].

Solar and wind energy generation are great, but don't have a high ERoEI in early development. Oil that's gushing out of the ground has an insanely high ERoEI and we used that to advance our technology very quickly.

Also remember that we probably have only a few hundred million years left before the sun expands and heats the planet to the point were complex life can't exist. There will be no liquid water in 1 billion years.

This is all conjecture. It certainly could be possible that future civilizations can become space faring by a different path than we took, but that's a terrible gamble to make if you think intelligence in the universe is worth preserving and expanding.

1. https://www.treehugger.com/more-trees-than-there-were-years-...

Ah so if your civilization ends but humans ad a spicies survive, the next civilization won't have it so easy. That makes sense, thanks for clarifying.