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Comment by blueflow | Hacker News Reader

Its not a "risk".

Water vapor (clouds) is a stonger greenhouse gas than carbon dioxide. We already got measurably higher temperatures, so we also have higher water evaporation, and from the last 5 years it looks like it happens every year.

So the runaway is already happening, until something stops it near hothouse conditions or hopefully earlier than that.

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Thank you for saying this. If you want to know the answer to what causes climate problems, you need to go back to the era of dinosaurs, where CO2 levels were multiple times higher than today. Trees could thrive because they could breathe in a lot of CO2. Dinosaurs got so big because there were plenty of food. How could dinosaurs happily live with such high CO2 levels? The key is that there were plenty of forests. Peter Wohlleben's book "The Power of Trees: How Ancient Forests Can Save Us if We Let Them" explains how forests naturally circulate water.

The climate system in those prehistoric times was in a different stable state. The world that we live in has different ecosystems that are well-adapted to the current stable state and we will likely face a mass-extinction event once the ecological scales tip over.

The problem is also the speed in which the CO2 levels are rising. Such a massive change in such a short geological time is very unusual.

Water vapor (clouds) also reflects sunlight. So it's complicated. We know the planet has had higher CO2 and higher temperatures in the past, and it did not "run away"

It doesn't matter if the top of the curve flattens out, if we can't survive outside of the bottom part that looks exponential.

It might not run away to infinity, but it may well run away in the sense that the rate of change could continue to increase even if humans stop contributing to it.

We can't say for sure that the current feedback loops will be identical to those that did or did not exist in the past. Differences in the initial state could result in different outcomes.

For example was there as much methane trapped in the arctics during the last time CO2 was high?

Does the rate of the increase of CO2 and temperature have an effect? Because it's currently getting hotter far faster (absurdly so) than any other period we have records for.

No citations to hand but Antarctica used to be temperate rainforest and many of the conditions of present day tropical rainforests and savanna could be found much farther from the equator.

The paleocene–eocene thermal maximum makes for interesting related reading. https://en.wikipedia.org/wiki/Paleocene%E2%80%93Eocene_therm...

> Water vapor (clouds) also reflects sunlight. So it's complicated. We know the planet has had higher CO2 and higher temperatures in the past, and it did not "run away"

Yes. But stars like ours burn brighter as they move through their lifetimes, and the Sun is a bit brighter now than it was back when we had higher CO2 levels. That's why a runaway GHG didn't happen back then, but is basically guaranteed to happen within a billion years.

So… the sun is hotter now than it was what… since the 1950s? The 1850s? The Jurassic period? What scale is do you need to make this claim reasonable?

Also, I see a lot of things presented as facts in your comment, you seem to have convinced yourself quite thoroughly.

10% every billion years. So negligible on our timescale, but it is significant if you talk about ancient climate conditions like this thread does.

complex life on earth is half a billion years old (cambrian explosion 550ma ago), so the sun shouldn't be that much of an factor.

True runaway (i.e. oceans boiling / Venus) cannot happen on Earth unless you significantly increase incoming radiation stream (or alternatively halve the planet's albedo).

The runaway effect is scary b/c at certain temperature (~400K) atmosphere consisting predominantly of water vapor looses its ability to radiate out more heat up until 1600K.

[0] https://www.nature.com/articles/ngeo1892 (see fig. 2b) (edit: the figure: https://imgur.com/a/ytoEXzd)

edit #2: I've measured some pixels and the starting runaway temp is closer to 315K / 42C, damn

Are you certain we can't become Venus? Venus itself has such a high albedo that it effectively receives less radiation than Earth does.

And that's just one of the many positive feedback loops.

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