Sir Chaos wrote:
But within what timeframe does this happen?
That depends on how fast the star is brightening, which depends mostly on its mass. 1% stellar brightening produces about ¼% increase in the black-body temperatures of planets, moons, and asteroids. For habitable ones with black-body temperatures about 270 K ¼% increase is about 0.7K. The Sun is brightening about 30% over 4.5 billion years, which is 1% per 150 million years. So temperature rises about 1K per 200 million years, until you hit runaway greenhouse, then it rises a couple of hundred degrees in centuries.
From the point when CO2 runs out, how long does it take before temperatures rise too high to support the planet´s dominant life forms? Thousands of years? Millions of years?
About a billion years for Earth, maybe? Longer for a late K star, much shorter for an early A.
And once you are at that point, how much more time does it take to get to the endpoint where the world is a desiccated, frozen post-garden?
That's not where that branch leads. It leads to a pressure-cooker.
Also it just occurred to me: We are currently producing excess CO2 in non-trivial amounts. Could a civilization about as advanced, or maybe a little more advanced, as ours potentially stave off the point where the CO2 runs out by intentionally producing large amounts of excess CO2? Not forever, but perhaps long enough to put some sort of Plan B (STL colony ship to wherever?) into action?
A civilisation could produce it CO2
, but it would harm, not help. The problem is that you reach the point where it would take a CO2
level below zero
to maintain an equable temperature, which is impossible, so temperature rises. Adding CO2
will only make it rise faster.