Why doesn’t the earth get warmer after being irradiated by the sun for millions of years?

Answer

The Earth itself also radiates light and heat to the universe. A balance has been established between the energy that arrives at Earth from the sun, and how much goes back from Earth to the universe: either reflected directly, or first transformed into another form (e.g. light to heat and then radiated that heat; or light to trees, then to diesel, then burnt to heat and that heat to the universe).

This balance, of course, depends on the composition of where the light from the sun arrives and where the energy also returns to the universe – ie Earth’s surface and atmosphere. A problem with our current atmospheric change from CO . emissions₂ etc., or less trees, or less reflective ice, is that we move the balance. Namely, the Earth is more difficult to radiate heat to the universe, so it must be slightly warmer to radiate the amount of heat to the universe that compensates for the oncoming solar radiation.

Accurately describing this balance in function of all parameters is of course very difficult and therefore the work of climate researchers.