Space/Science — HabitableZone

Space/Science » in reply to re: 5% less insolation is still a big deal

Sun's core is where the action is
Adam — (archived)

Hi gb

The Sun's output can be reasonably precisely computed from the solar "constant" and the Earth-Sun distance at that point. To four places it's quoted as 3.847E+26 J/s. It was lower in the past due to the core of the Sun being a little bit cooler. As helium "ash" builds up, the core contracts a little in response to the lowered average fusion output per unit volume. The contraction increases the density and thus the fusion rate a smidge bringing things to a new equilibrium - a slightly hotter core with a higher fusion rate.

Of course Earth's orbital evolution causes what's received from the Sun to vary a bit, thus we get Milankovitch cycles and all that. Antarctica and a largely land surrounded Arctic sea means the planet is susceptible to glaciation cycles controlled by the astronomical cycles too. But the Sun drives the whole show in the long term. Reducing its input by 5% results in freezing conditions unless the greenhouse compensates - which it did half an aeon ago. Increasing the Sun's input by 5% would probably cause the carbon dioxide levels to crash, strangling most of the land plants and driving the biosphere into the remnant oceans. Phytoplankton gets by on much lower CO2 levels and so will survive.

At least that's what the models tell us. The reality will, no doubt, be far stranger...

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