GPHY 314 Chapter 4: Simple Climate Model

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29 Dec 2020
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CHAPTER 4: A Simple Climate Model
First step to understanding climate to do an energy budget calculation, which requires calculating
energy in and energy out for the Earth
o First step calculating energy in to determine intensity of sunlight at Earth’s orbit
1360W/m2 the solar constant for the Earth, frequently represented in equations
by symbol S
Function of how far the planet is from the Sun
o Easiest way to quantitatively calculate solar energy falling on Earth to realize that if you
set up a screen behind the Earth, Earth would cast circular shadow, with radius equal to
radius of Earth
Amount of sunlight falling on Earth equal to amount that would have fallen into
the shadow area if Earth not there
Solar energy falling on Earth at rate of 1.8 x 1017W
If we could capture just 0.01% of solar energy falling on Earth, we could
satisfy all of the world’s current energy needs
Reflectivity of a planet called the albedo, frequently represented by symbol α
o Fraction of incident photons reflected back to space
Earth absorbs an average of 238W/m2 from the Sun, but that doesn’t mean every square meter of
Earth absorbs this amount
o Amount of solar energy varies widely across planet
Mid-latitudes receive less solar radiation per square meter than tropics, and polar
regions receive even less solar energy
o In addition to variations in incoming sunlight with latitude, the albedo of planet also
varies widely
Tropics are mainly open ocean, which is dark and has low albedo
Combined with large amount of solar energy per square meter, tropics
experience far more solar heating than anywhere else
High latitudes generally covered by snow and ice, so high albedo
Combined with small amount of solar energy received, means they
receive the least amount of solar energy, so coldest regions
Greenhouse Effect
Assumptions to understand impact of atmosphere on planets temperature:
o Earths atmosphere is transparent to visible photons emitted by Sun (which have
wavelengths from 0.3-0.8 microns) so these photons pass through the atmosphere and are
absorbed by the surface
o Atmosphere is opaque to infrared photons emitted by the surface (wavelengths longer
than 4 microns) so all photons are absorbed by atmosphere
o The atmosphere also behaves like a blackbody so it emits photons based on its
temperatureEmits equally both upward and downward
o Photons emitted by atmosphere in upward direction escape to space and carry energy
away from Earth, photons emitted downward are absorbed by surface
To calculate surface temperature, assume planet as whole, as well as surface and atmosphere
individually, must all be in energy balance where energy in equals energy out
Addition of an atmosphere that is opaque to infrared radiation has significantly warmed the
planets surface
o Conceptually occurs because surface of planet with atmosphere is heated not just by the
Sun but also by the atmosphere
Another way to think about greenhouse effect is that atmosphere warms the surface by making it
harder for the surface to lose energy to space
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