> We estimate heavy losses, maybe near extinction

Do you mean human extinction or extinction of certain animal species? I can definitely see certain animal species dying out, I couldn't see a scenario where humans die out though. If you mean human extinction could you provide some further reading?

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.

Ray Dalio, the investing guru, says he’s been most surprised by events that hadn’t happened before in his lifetime, but have happened repeatedly if you look a century or two back in history. Global warming is not only new in our lifetimes but also in recent recorded history which makes it particularly hard to anticipate and plan for.

The "Medieval Warm Period" is well-documented, as is the "Roman Warm Period". Sadly, they and their significance as well as positive effects are being downplayed by activists lately.

Those climate events are worth studying but may have been regional instead of global in scope.

I'm not aware of any credible predictions that climate change might cause the near extinction of humanity.

The consequences of climate change might not be as bad as the consequences of preventing it. That's a pretty important thing to figure out because what if you end up creating a bigger calamity than you prevent?

What does "heavy losses" mean? If you don't quantify it, it means the same as "light losses". Is it worse than WWII? Worse than WWI? Worse than Communism? Worse than the Spanish Flu? Worse than AIDS? Worse than malaria? Worse than smoking? These things each killed many millions in the past century and somehow civilization has never been better!

So before worrying about preaching, first figure it out yourself. Otherwise you're nothing more than an amplifier for some other activist's arbitrary emotions.

You're missing my point. It is that you don't know what the consequences of climate change will be. Not even any kind of estimate at all. So how can you be sure it's worth the cost of preventing?

Even scientists hardly have any idea. I'm not saying they're wrong, I'm saying they don't know.

What if we really do get increased flooding, storms, etc. like every climate scientist says, and we cope just fine?

I'm not trying to kill any question. I'm trying to ask a question! What will the consequences of climate change be? It's a pretty important one. If you can only speak in emotional language like "calamity" then you don't have any idea at all.

The problem with this argument is that climate change presents an existential risk for our current global civilisation and ability to support the numbers of people currently living on earth. If climate change (along with the other types of environmental destruction we are currently reaping) is as bad as predicted (and scientists are actually fairly certain of the likely impacts. Read the IPCC reports and then remember that these are the outcome of a process designed to produce the most conservative estimates) then we face an enormous die off of the world's population as large swathes of the planet become essentially uninhabitable at current densities. The costs of mitigating climate change are, in comparison, tiny. A few percent of GDP if we don't replace current polluting industries with cleaner ones. Given this it makes sense to mitigate against climate change, even if it turns out that the predictions are overly pessimistic, because the risks are so high. It's similar to taking out fire insurance. Sure the chances of your house or business burning down are pretty low, but the impact on you if they do is so high that it makes sense to pay the relatively low cost.

Where in the IPCC reports do you find an existential risk to humanity? I’ve read most of AR5 and I haven’t come across it.

I’m worried about climate change, but activists like extinction rebellion are far from telling the truth, and the doomsaying is preventing action.

I don't claim an existential risk to humanity, I claim there's an existential risk to our current global civilization which manages to feed, cloth and shelter almost 8 billion people in largely tolerable conditions and which will need to find room for another 1.5 billion before global population peaks. If parts of the world where large numbers of people live become heat stressed to the extent that living in them is impossible without air conditioning and agriculture starts to become a lot less productive in currently fertile areas then the secondary effects of billions of people trying to find somewhere else to live in a short space of time are likely to be not good. I don't think we face extinction. I do think we face having to transition to a low carbon lifestyle. We can either do that in a peaceful way and maintain and even increase the standard of living for the majority of the human race or we can do nothing and face decades of major disruption that kills huge numbers of people.

Just thinking something doesn't make it true. Why can't they just use air conditioners like everyone else in hot climates does? They'll all be rich soon afterall, with poverty due to be effectively extinct by the end of the century.

Who are these billions of people? I haven't seen predictions of anywhere near a billion people needing to migrate.

You need to stick to the science here because there's so much misinformation spread by activists.

You're surely right that it's a risk to civilization but lots of things are and we still have to try. Falling birth rates are a risk, spread of Islam is a risk, meteor strike is a risk, nuclear war is a risk, pandemic is a risk, and importantly, suddenly stopping using fossil fuels is a risk! But these are just unquantified imaginary risks so we don't need to drop everything and solve them or we'd never get anywhere.

> enormous die off of the world's population as large swathes of the planet become essentially uninhabitable at current densities

Can you provide any more details to go on for that? I can't find such predictions in an IPCC report but they're huge so any extra specifics would help.

I can't imagine a place being essentially uninhabitable. Las Vegas and Israel are both full of people, for instance. Almost everywhere is uninhabitable without shelter.

I'm pretty sure nobody is predicting existential risk to civilization. You really need a reference for that.

Here we go

"2°C warming would imply [...] average GDP per capita would be reduced by 13% by 2100. By comparison, economic impacts at 1.5°C on GDP growth would be almost indistinguishable from current conditions (1°C)." https://350.org/wp-content/uploads/2018/10/The_Peoples_Dossi...

13% is like setting the global economy back 10 years by 2100. If we stop using oil, how much will it reduce GDP?

Other predictions are pretty tame, like a 60% increase in fires by 2100. So what? That's not as bad as HIV or cigarettes.

Just check what the representatives from island nations have to say and I think you will have a clearer idea. It is easy to disregard evidence when it stares at you.

https://en.wikipedia.org/wiki/Effects_of_climate_change_on_i...

No. Island nations aren't big enough to be a major global problem.