Freshman Seminar:
Introduction to the Science of Climate Change
Prof. Jordi Miralda-Escudé, F 9:30

Lecture 1: What determines the temperature of the Earth?

The Energy Flux from the Sun.

Earth is heated by the Sun. The rate of heating depends on the flux of radiation energy that reaches the Earth from the Sun.

What is flux? Flux is the amount of energy that reaches the Earth per unit of area and per unit of time, or power reaching per unit of area.

What does the flux from the Sun depend on?

How does the Earth cool down?

If the energy coming from the Sun into the Earth was the only exchange of energy that the Earth had with outer space, what would happen to the Earth?

So what is necessary for the Earth to maintain a constant temperature?

How does the Earth cool down?

Earth's balance of energy

In order for the temperature of the Earth to be at equilibrium, there needs to be a balance between the energy coming in from the Sun and the energy getting out as infrared emission to space . If more energy is coming in as sunlight, the Earth will heat. If more energy is getting out as infrared emission, the Earth will cool.

The Orbit of the Earth, the Inclination of its Rotation Axis, and the reason for the seasons.

The Earth spins about its own axis, and it also revolves around the Sun. What causes the seasons?

The axis of rotation of the Earth is tilted, relative to the plane of its orbit around the Sun. The tilt angle is 23.5 degrees. This causes the seasons.


  • The Earth is heated by the flux of energy it receives from the Sun, and cools down by emitting infrared radiation to space.
  • The rates of incoming and outgoing energy have to be in balance if the Earth temperature is to remain constant.
  • The eccentricity of Earth's orbit causes the distance to the Sun to slightly change (being largest in July and shortest in January). The seasons, however, are caused by the tilt of the axis of rotation of the Earth.
  • The maximum temperature in summer is reached some time after the maximum insolation, on June 21, because of the response time of the climate.
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