Lecture 9

Question 1

What is the global mean temperature?

Answer 1

15 degrees celcius

Question 2

Sources of power per unit area of the earth’s surface

Answer 2

Sun
Geothermal
Planetary
- basically just sun as others are negligible

Question 3

How does sun release energy?

Answer 3

Nuclear fusion

- proton-proton chain reactions

Question 4

How is the total power output of the sun estimated?

Answer 4

P = σ(T^4) * 4π(r^2)

σT^4 is Boltzmann law where σ is 5.67 x 10^-8 W/(m^2 K^4)
and is Boltzmann constant

T is temperature in kelvin (5780K) at suns surface
4πr^2 = surface area of sun —> radius of sun is 7x10^8m

= 3.89 x 10^26 W

Question 5

What is the solar constant?

Answer 5

S0 = The power per m^2 the earth receives from the sun

= P/4πr^2 = 1370 Wm^2
(r is the distance form the sun the earth is)

• not actually constant as varies with solar activity

Question 6

What is the distance between the sun and earth?

Answer 6

1 AU (astronomical unit)
roughly 1.5 x 10^11 m

Question 7

Why does the earth receive radiation from sun like a disc?

Answer 7

As only one side faces the sun
- so to get the power at earth P= S0 * πr^2 * (1-α)

• the pi r squared is from the disk
• the αis the albedo = 0.3 (30%)
• r is radius of earth

Question 8

What is blackbody radiation?

Answer 8

an object that absorbs all radiation incident upon it and perfectly re-radiates energy as a function of its temperature

E out = σT^4 * 4πr^2
where the 4 pi r squared as surface of sphere -> this is radiation if the earth was a black body

Question 9

What are the main causes of albedo on the earth?

x 2

Answer 9

Ice

Clouds

Question 10

If see the Earth as a blackbody object basically the temperature provided earth didnt have an atmosphere. What is this temperature?

Answer 10

255K (18 celcius)
T = fourth root (S0(1-α)/4σ)

This is 33 degrees lower than global mean temperature because the earth has an atmosphere and is a grey body radiator

Question 11

What is the measure of the magnitude of the Earth’s natural greenhouse effect?

Answer 11

33 degrees

- between actual temp and predicted blackbody temp of earth

Question 12

What is ε?

Answer 12

Emissivity = proportion of energy radiated from earth surface and atmosphere

UNITLESS

E(greybody) = εαT^4

1-ε is the proportion of radiation retained by earths atmosphere

ε=0.62 for earth

Question 13

Earths emits radiation in IR part of electromagnetic spectrum. What does this mean?

Answer 13

That the greenhouse gases are the atmospheric gases capable of absorbing IR

• CO2
• CH4
• nitrous oxide
• chlorofluorocarbons

Question 14

What is the CHZ?

Answer 14

circumstellar habitable zone

- where planetary surface can support liquid water

Question 15

How to deal with insolation with latitude?

Answer 15

• incoming solar radiation not evenly distributed in space or time
• lower latitudes receive more radiation
• so could use bands of 30 degrees latitude
• do energy budget for each but need to take into account the transfer of energy into and out of the band

Question 16

What does a zonal energy balance model demonstrate?

Answer 16

that the earth receives more radiation from sun than it loses through outgoing longwave radiation at lower latitudes

• at higher latitude the earth loses more energy than it receives
• this imbalance drives atmospheric and oceanic circulation
• heat surplus in tropics and net deficit in polar regions