## Motivation

First, I thought the global warming is inevitable since only defense from Earth’s perpective would be to reflect the incoming energy back to space and if that’s not possible, the Earth would just accumulate energy from the Sun (neglecting internal nuclear Earth’s sources of energy).

Thus the question is – what is a spherical black body temperature one astronomical unit away from the Sun?

## Calculation

The power received from Sun at 1 astronomical unit is

Feeding this into Wolfram Alpha (reducing incoming power by Earth albedo), we obtain the equilibrium temperature of Earth $T \approx -25 °C$.