Questions: Solar Radiation and Earth's Energy Balance

Question 1

Earth's global average albedo is approximately 0.30. Which statement correctly describes what happens to incoming solar radiation?

• A. 30% is absorbed by the surface; 70% is reflected to space
• B. 70% is absorbed by the Earth system; 30% is reflected to space
• C. 30% heats the atmosphere directly; 70% reaches the surface
• D. 70% is absorbed by greenhouse gases in the atmosphere

Answer: (B)

Albedo is the fraction of incoming radiation that is reflected. An albedo of 0.30 means 30% is reflected back to space and 1 − 0.30 = 70% is absorbed by the Earth system (surface plus atmosphere combined). This absorbed fraction is what ultimately must be balanced by outgoing longwave radiation to maintain energy equilibrium.

Question 2

Earth is warmer in the Northern Hemisphere summer than in winter because Earth is closer to the Sun during summer.

• T. True
• F. False

Answer: False

This is one of the most pervasive misconceptions in Earth science. Seasons are caused by the tilt of Earth's rotational axis (~23.5°). When the Northern Hemisphere tilts toward the Sun, solar radiation strikes at a more direct angle and days are longer — both effects concentrate more energy per unit area. In fact, Earth is slightly farther from the Sun during Northern Hemisphere summer (at aphelion in early July) than in winter.

Question 3

Earth's effective radiating temperature is about 255 K, but the observed global average surface temperature is about 288 K. What explains the 33 K difference?

Model answer: The 33 K difference is due to the natural greenhouse effect. The atmosphere absorbs outgoing longwave radiation emitted by the surface and re-emits some of it back downward, supplementing the surface energy budget. As a result, the surface must reach a higher temperature than 255 K to ultimately radiate enough energy to space to balance incoming solar absorption.

The effective radiating temperature (255 K) is what Earth's temperature would be with no atmosphere — calculated by setting absorbed solar power equal to emitted blackbody power. The actual surface is warmer because the atmosphere acts as an insulating layer that intercepts and recycles outgoing radiation. This natural greenhouse effect is essential for life; without it, Earth's surface would be frozen.