I was writing up a response to your initial guess of 10% by 2060, which was... Pretty dire. Past-the-Paris agreement, 2.5C to 3C warming dire.

20% by 2070-2090, on the other hand, is a lost cause. That would put atmospheric at 580 PPM by 2080. That's >4C of warming, if we don't hit any positive feedback loops.

I'm not particularly concerned about plastics. They can be synthesized from biomass.

Sorry, I got into a habit of submit then edit on HN because we used to get errors if you waited to long. I did not think we would hit 10% by 2060, but I hope we hit 40%.

US coal use is down to where it was around 1930. Petroleum is around 1960's usage levels. https://www.eia.gov/todayinenergy/detail.php?id=26912

Natrual gas has spiked, but it's realitvly speaking less of a global warming threat due to C(x) H(x) turning into H20 (x) + C02 (x) which produces less CO2 per heating vs just C into CO2 from coal.

So, while there are a huge range of predictions. What happens in the next 20 years is more important than what's going on 40 - 60 years from now.

Don't the inevitable methane leaks from natural gas create more of a GHG effect than the CO2 emitted from burning it? It's not just the environment disasters like in California, but everyday minor leaks.

Methane has a short half life in the atmosphere ~10 years ish and there is already quite a bit of it there. So, it's only really important when the amount released is increasing relative to the average released over the last 20 year. AKA if they cut release in 20 years then (ed: it's contribution to) temperature would drop fairly quickly. https://en.wikipedia.org/wiki/Atmospheric_methane

Note it does break down into more CO2 than the wieght of Methane because Oxygen molecules are heavy.

The real risk is if warming the arctic starts to release a lot of stored methane, which would be very bad.

They are not insignificant (right now, they're just a bit more than the methane from livestock) but the CO2 is emitted in much greater amounts.

In a few years the question will become "How to do geoengineering safely relative to the risks of warming?" The planet may be able to support 9-11 billion people with mostly renewable energy, but perhaps humanity will not get there fast enough to avert risk of hitting a positive feedback level that would commit humanity to far more massive and therefore far riskier geoengineering.

The planet is unable to sustainably support the current 7 billion people. The only way I see 9 billion fitting in here is if we switch to purely artificial stuff and eradicate all wildlife and their habitat.

The way I see it, the limit is at 1 billion people if we want to preserve current situation, and even less if we want to let the nature regenerate.

I don't dismiss the idea that we are past sustainable limits already. But the fact is that increased living standards have not caused commodities crunches yet. Also, stabilizing population is the first step toward a sustainable population level, no matter what the right number is for a world that thinks in terms of a "long now."

As far as planes go, an intriguing option is liquid hydrogen. Hydrogen has a large energy density by mass, which is why we use it in rockets. There have been proposals on how current airliners might be converted to run off of hydrogen in the near term[0] for short and medium distance travel. If we can get fuel cell electric propulsion to work at jetliner speeds there is even more potential, as fuel cells can be made more efficient than heat engines.

[0]http://www.fzt.haw-hamburg.de/pers/Scholz/dglr/hh/text_2004_...

I think it would be much easier to use methane. Just as carbon neutral if you make it yourself, and doesn't have as many exotic engineering requirements.

I didn't know that about fuel cells though, interesting.