Why is a future increase in solar luminosity by 10% supposed to have disastrous consequences for life on earth when the last 30% did not?

Question

One remarkable thing about the climate history of earth is that despite the fact that solar luminosity was fully 30% lower in the beginning, temperatures were actually quite close to today. Since then temperatures have stayed in a relatively narrow range, allowing life to develop. This lets one conclude that there are dampening mechanisms in the earths climate system.

But predictions for the next few 100 millions of years are unequivocal that the about 10% increase of solar illumination will massively increase temperatures on earth, leading to annihilation of at least all higher life (For details, see for instance here: Solar_evolution, When will Earth lose its oceans?)

Now, it stands to reason that at some point the climate dampening mechanisms will be overwhelmed by additional solar irradiation. But there is no indication that the next 10% will be it, if at all the last 500 million years seem to indicate a trend to cooler temperatures:

What are the assumptions that these models are based on to come to their dire predictions?

Answer 1

In this regard you can think that the Earth system is like a house with heating but no Air conditioning. Therefore, it has a powerful mechanism to stay warm when the solar illumination is weak (greenhouse effect), but it has no such powerful mechanism to cool down if the solar illumination is too strong.

In our analogy, let's say you have used your heating to keep your house nice and warm at 22°C over winter, when outside temperatures were much colder than that. As the temperature outside goes up in the spring, you turn down the power of your heating, so to keep the house at 22°C. Now, a ~10% increase in solar power output would be equivalent to the point were the outside temperature surpasses 22°C. Therefore, even with the heating off, your house temperature will start to rise over 22°C for first time, an you can't do anything about it.

The heating mechanism of Earth the greenhouse effect, specially the one due to ${CO}_2$. But most important, is the thermostat that have kept Earth's temperature in the range of liquid water. This thermostat is based on the fact that an increase of temperature leads to and increase of silicate weathering, that in turn leads to a drop in atmospheric ${CO}_2$ concentrations, and therefore to a drop in temperature. This negative feedback counteracts any warming due to increases in solar irradiation.

However, that trick will work only until the point when there is no more ${CO}_2$ in the atmosphere. Then, any increase in solar irradiation will have a direct effect on temperature, and other amplifying feedback will kick in. In particular the greenhouse effect of water vapor: The higher the temperature, the most water vapour in the atmosphere. And water vapor is a very effective greenhouse gas, therefore, that will lead to further increases in temperature.

The following figure gives an idea of the different radiative forcings as calculated by the IPCC AR5:

However, those are relative to the year 1750. The total effect of green house gasses is about 155 W/m². That corresponds to a 45% of the total energy we receive from the Sun (340 W/m²). That would mean that if you take away all greenhouse gases you can counteract an increase of about 45% of solar power output. In other words, Earth can keep turning the heating down to keep the planet temperature stable as the Sun keeps increasing its power output. However, this calculation is very naive, because we can't remove all greenhouse gases (that would mean removing all the water too). The models you mention are more realistic in this sense and they suggest that the critical points is just a 10% of solar output increase, much closer than the 45% of my ultra-simplistic calculation above.

Said that, the references in that Wikipedia article don't seem to use models fully coupled with all the Earth's systems involved (carbon cycle, tectonics, methane, albedo feedback, etc.). Therefore, I would take that 10% value with a lot of caution. However, what is true is that at some point, if you increase solar power enough, the temperature controlling processes of our planet will be overwhelmed, and life on Earth's will face the bigger challenge ever for its subsistence.

Answer 2

Any star has a "goldilocks zone" https://en.wikipedia.org/wiki/Circumstellar_habitable_zone where the temperature is suitable for a planet to have Earthlike conditions. The extent of the zone - the edges where it's not too hot, not too cold, but just right - depends on the star's luminousity.

Now suppose that 4.5 billion years ago, the primeval Earth orbited somewhere close to the outer edge of the zone. (Which might be a factor in the "Snowball Earth"?) As the sun's luminousity increased, the zone moved outwards. If it continues to increase (as physics says it will), at some point the Earth's orbit will be inside the inner edge of the "just right" zone. Apparently this is calculated to be 10% over the current luminousity.

Answer 3

Those models seem fairly simple. The keyword there is the word "boil", I think. Consider the analogy with boiling water in a kettle. The water acts as a thermostat (of sorts) preventing the temperature from rising too much and ruining/burning the kettle. But once the water "runs out", i.e. it has all evaporated...

And when it comes to the Earth (or any planet), the extra vapor also acts a greenhouse gas, effectively turning up the heat. That much is made clear in the intro to the 2013 paper, which refers to the Simpson-Nakajima limit in that regard. Numerically estimating this limit is actually not that simple though.