# Explain why the Earth's surface temperature remains approximately constant over time despite the continuous absorption of solar radiation.

> IB DP Physics Higher Level (2023 syllabus) — B.2 Greenhouse effect · Explain · 4 marks

> The Earth receives electromagnetic radiation from the Sun predominantly in the ultraviolet and visible regions of the spectrum. The Earth's surface and atmosphere also emit radiation, and certain atmospheric gases absorb and re-emit some of this outgoing radiation.

## Mark scheme (4 marks)

1. The Earth's surface absorbs incoming short-wavelength (UV/visible) solar radiation and re-emits longer-wavelength (infrared) radiation.
2. Greenhouse gases (e.g. CO₂, H₂O, CH₄) in the atmosphere absorb outgoing infrared radiation and re-emit it in all directions, including back towards the Earth's surface.
3. A dynamic equilibrium/energy balance is established such that the total power radiated to space equals the total power absorbed from the Sun.
4. The surface temperature stabilises at a value high enough that the total rate of emission (proportional to T⁴ by the Stefan–Boltzmann law) matches the total rate of energy input, maintaining a constant average temperature.

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