What is the primary source of energy for the Earth's climate system?

• A. The Sun
• B. The Moon
• C. Internal Earth heat
• D. Geothermal energy

Answer: (A)

The Sun powers the Earth's climate system. Solar radiation is the primary source of energy that heats the land, oceans, and atmosphere, driving evaporation, weather, and the global circulation that redistributes heat around the planet.

In numbers, the solar constant is about 1361 W/m^2 at the top of the atmosphere. When averaged over the whole planet, that becomes roughly 340 W/m^2, and after accounting for reflection by clouds and the surface (albedo) and re-radiation, about 240 W/m^2 is absorbed on average. This absorbed energy establishes the Earth’s energy budget and fuels the climate system.

Other sources are far smaller in comparison. The heat coming from Earth’s interior—geothermal energy—is tiny on a global scale, around 0.03 to 0.1 W/m^2. The Moon does not add meaningful energy to the climate system; it influences tides and some dynamics, but it’s not a significant energy input for climate on Earth.

So the Sun is the primary energy source driving Earth's climate.