How Earth’s Climate Changes Naturally (and Why Things Are Different Now) | Quanta Magazine

Faint Young Sun
Magnitude: No net temperature effect
Time frame: Constant
Though the sun’s brightness fluctuates on shorter timescales, it brightens overall by 0.009% per million years, and it has brightened by 48% since the birth of the solar system 4.5 billion years ago.
Scientists reason that the faintness of the young sun should have meant that Earth remained frozen solid for the first half of its existence. But, paradoxically, geologists have found 3.4-billion-year-old rocks that formed in wave-agitated water. Earth’s unexpectedly warm early climate is probably explained by some combination of less land erosion, clearer skies, a shorter day and a peculiar atmospheric composition before Earth had an oxygen-rich atmosphere.
Clement conditions in the second half of Earth’s existence, despite a brightening sun, do not create a paradox: Earth’s weathering thermostat counteracts the effects of the extra sunlight, stabilizing Earth’s temperature (see next section).