Why Does Earth's Temperature Stay Stable?
On a sunny day you feel warmth from the Sun, yet Earth's average temperature stays remarkably constant over long periods. It doesn't heat up endlessly, nor does it freeze completely at night. This stability comes from energy balance: the rate at which Earth absorbs energy from the Sun equals the rate at which it radiates energy back into space.
When this balance is disrupted , for example, by increasing concentrations of greenhouse gases , Earth's average temperature changes. Understanding the physics behind this balance is the foundation of climate science.
This subtopic covers:
- How much solar energy reaches Earth (the solar constant)
- How reflectivity (albedo) affects how much energy is absorbed
- How Earth radiates energy back to space (Stefan-Boltzmann law)
- How the greenhouse effect raises Earth's temperature above its simple equilibrium value
The Solar Constant
The Sun radiates energy as an almost perfect black body at a surface temperature of approximately 5,778 K, with a total power output of W. This energy spreads outward in all directions, so by the time it reaches Earth (at a distance m), it is spread over a sphere of surface area .
Solar Constant (S): The intensity of solar radiation incident on a surface perpendicular to the Sun's rays at the top of Earth's atmosphere. It is calculated as:
and has a value of approximately 1,400 W m⁻².
Substituting values:
The solar constant is not truly constant , it varies slightly with the Sun's activity cycle and with Earth's slightly elliptical orbit. For IB purposes, use W m⁻² unless the question specifies otherwise.
Picture a 1 m² solar panel held perpendicular to the Sun in space. It receives roughly 1,400 W , enough to power about 14 standard 100 W light bulbs continuously.