Question

The percentage of the incoming solar radiation reflected back to space by the earth is:

• A. 10
• B. 20
• C. 30
• D. 40

Hint:

Albedo values range from 0 (perfect absorber) to 1 (perfect reflector).
Fresh snow has an extremely high albedo ($\approx 0.8$ to $0.9$), while ocean surfaces have a very low albedo ($\approx 0.06$). The global average is about 0.3 (or 30%).

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
The Earth's planetary albedo is the fraction of incoming solar shortwave radiation that is reflected directly back into space by the atmosphere, clouds, and the Earth's surface, without being absorbed.
It is a crucial variable in planetary energy balance models and global warming calculations.

Step 2: Detailed Explanation:
Let us look at the breakdown of the Earth's average energy budget.
Out of the $100\%$ incoming solar radiation reaching the top of the atmosphere:
1. Approximately $23\%$ is reflected back to space by clouds and atmospheric aerosols.
2. Approximately $7\%$ is reflected by bright surfaces on the Earth (such as snow, ice, and deserts).
Combining these values:
\[ \text{Total Reflected Radiation (Albedo)} \approx 23\% + 7\% = 30\% \] The remaining $70\%$ is absorbed by the atmosphere ($23\%$) and the Earth's surface ($47\%$).
This absorbed energy warms the planet and is eventually re-emitted as longwave infrared radiation.
Therefore, the average planetary albedo of the Earth is approximately $30\%$.

Step 3: Final Answer:
The percentage of incoming solar radiation reflected back to space is $30\%$.