The correct option is Roughly 35 units.
Explanation
The Earth's heat budget refers to the balance between incoming solar radiation (insolation) and outgoing terrestrial radiation. To maintain a constant average temperature, the Earth must radiate the same amount of energy back into space as it receives. A significant portion of incoming radiation is reflected back to space without heating the Earth-atmosphere system; this reflected fraction is known as the albedo of the Earth.
Detailed Analysis
Assuming the total incoming solar radiation at the top of the atmosphere is 100 units, the distribution is as follows:
- Reflected back to space (Albedo): Approximately 35 units are reflected back before effectively heating the Earth. This comprises:
- 27 units reflected by clouds.
- 6 units scattered by dust and small particles in the atmosphere.
- 2 units reflected by the Earth's surface (primarily snow and ice fields).
- Absorbed by the Atmosphere: Roughly 14 units are absorbed by gases, water vapour, and ozone.
- Absorbed by the Earth's Surface: The remaining 51 units reach and are absorbed by the Earth's surface (land and oceans).
Therefore, the roughly 35 units that are reflected back constitute the Earth's albedo and do not participate in heating the planetary system directly.
Key Takeaway: The Earth's albedo is approximately 35% (or 35 units out of 100), representing the total solar radiation reflected back to space by clouds, the atmosphere, and the surface.