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?
- The more luminous the Sun, the greater the flux.
- The greater the distance from the Sun, the lower the flux.
- More quantitatively, the flux increases in proportion to the
luminosity of the Sun and inversely proportional to the square of the distance
from the Sun.
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?
- Because energy is conserved (neither created nor destroyed), the energy
coming from the Sun would keep accumulating as heat in the Earth. The
Earth would need to get hotter and hotter.
So what is necessary for the Earth to maintain a constant temperature?
- The Earth also needs to cool down, losing the same energy to space
as it gains from the Sun, so that its temperature is in balance.
How does the Earth cool down?
- All material objects are continuously emitting radiation from
their surfaces. The higher their temperature, the more radiation they
emit. The Earth emits radiation to space.
- This radiation emitted by all objects, sometimes called thermal
radiation, has a characteristic wavelength that depends on the temperature.
At the temperature of the Earth surface, thermal radiation is at infrared
wavelengths, about 10 microns. The human eye cannot detect this radiation.
As the temperature of an object increases, the wavelength of the emitted
radiation shortens. If you heat a piece of iron or coal, it starts glowing
with a red color. The red color is the longest wavelength radiation our
eyes can see. As the temperature is further increased, the color would change
to yellow and then white (as the characteristic wavelength gets shorter).
The Sun is very hot and its light is the thermal radiation from its surface,
with a white color. An even hotter object than the Sun would become blue,
and emit also a lot of ultraviolet light (at even shorter wavelengths).
- If you look at a piece of iron that is cold, it looks black and
it seems not to emit radiation. However, it is still emitting, although
the amount of radiation is much less than when it is hot, and has a long
wavelength that your eye does not see. At room temperature objects emit
only infrared radiation.
- Similarly the Earth emits infrared radiation to space. If you
have an infrared camera in space, the Earth would look like an object
glowing in the darkness of space. This infrared glow comes both from the
day side and the night side. The glow increases with temperature, so it
would be brightest from the equator where the Earth is warmer, and dimmest
at the poles.
- This thermal emission is totally different than the reflected
light from the Sun, which is what the visual images of the Earth
from space show. Reflection has nothing to do with the temperature of an
object, it simply occurs because the incident light from the Sun does not
all get absorbed by the Earth, some of it is reflected back, the same way
the light from a bulb is reflected off the objects around it.
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.
- To be more precise, some energy is actually generated in Earth's
interior and continuously flows to the surface. This is because the interior
of the Earth is very hot, and its heat is maintained by long-lived radioactive
decays of uranium and other radioactive elements. The power flowing from
Earth's interior is much smaller (about 10000 times smaller) than the power
the Earth receives from the Sun. So the energy emitted by the Earth needs to
balance both the energy input from the Sun and from Earth's interior. But of
course, the sum of the two is just a tiny little bit larger than the
energy input from the Sun alone.
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 orbit of the Earth is not exacly circular, but an ellipse
with a small eccentricity. At its closest distance from the Sun, the
Earth is about 147 million kilometers, and at its furthest distance it
is about 152 million kilometers away. Can this be the reason for the
- When it is summer in North America, is it summer everywhere on
Earth? No, in the Southern hemisphere it is then Winter!
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 axis of rotation stays pointing in the same direction
as the Earth revolves around the Sun. When the North pole is tilted towards
the Sun, the Northern hemisphere gets a much larger amount of sunlight
than the Southern hemisphere, and it is summer in the North. Half a
revolution later, the North pole is tilted away from the Sun and the
South pole towards it. Then it is summer in the Southern hemisphere.
- Note: the summer in the Northern hemisphere is actually the season
after June 21 (the summer solstice). On June 21, the amount of sunlight
received is maximum in the North. During spring, the amount of sunlight
received in the North is just the same as in summer. But summer is
hotter than spring because it takes some time for the surface of the
Earth to warm up. The same thing happens in the Winter solstice: the minimum
insolation in the northern hemisphere occurs on December 21, but the
coldest days occur later around January, because the Earth surface takes
some time to cool down to its minimum temperature. This is an example of
the response time of the climate to a driving force (in this case, the
driving force is the amount of sunlight received in each hemisphere).
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
The maximum temperature in summer is reached some time after the
maximum insolation, on June 21, because of the response time of the climate.
Return to [
Main Page ]