Transcript Climate Change

CLIMATE CHANGE
Mariam Elizbarashvili
Global Energy Balance
 Milankovich theory of climate change

The Climate
THE WEATHER
• What is the weather, what is the
climate?
•What is the weather like today?
 Weather
is the specific condition of the atmosphere at a
particular place and time and is measured in terms of
things such as wind, temperature, humidity, atmospheric
pressure, cloudiness, and precipitation. Weather describes
the short-term state of the atmosphere.
 Climate
is the average pattern of weather for a particular
region, usually taken over a 30-year time period. Climatic
elements can include precipitation, temperature, humidity,
wind velocity, fog, frost and other measures.

The Sun is the only significant source of energy for
Earth’s Atmosphere. Millions of Other stars radiate
energy, but they are too far away to affect Erath. Energy
is also released
from inside Earth, primarily as
radioactive decay of minerals, although not enough to
influence the atmosphere significantly. Thus, The Sun
supplies essentially all of the energy that drives most of
the atmospheric processes.
SUN PROPERTIES:
NOTES:
1. Energy source is nuclear fusion (hydrogen, mainly)
2. Luminosity increased about 30% over earth lifetime
3. Projected lifetime: ~ 11 billion years
4. Sun is about middle age

Global climate is determined by the radiation
balance of the planet. As solar energy reaches the
Earth’s surface, a fraction of it is absorbed and the
Earth’s surface warms up. The remaining fraction
is reflected immediately off the surface back into
the atmosphere and space. The surface of the Earth
(land and water) that has been warmed by the
radiation then emits energy back in the form of
heat into the atmosphere and toward space.

Since the Earth’s surface temperature is much
lower than that of the Sun, it emits radiation at
longer wavelengths and with energy levels much
lower than that from the Sun, in this case at
infrared (heat) wavelengths (not at visible
wavelengths like the Sun).

Earth's atmosphere is made up of gases, and these
gases allow some solar radiation to reach the
planet but also absorb some of the heat radiating
from the planet, trapping it and radiating it back
downward to the surface.

This cycle is called the greenhouse effect, because it is similar to the
warming process inside a glass-walled greenhouse. Earth's
atmosphere traps enough heat to keep the entire planet warm;
without it, the average temperature of the Earth's surface would be
much colder.

What is the average temperature on the Earth?
If there was no greenhouse effect, what would be the avarege Temperature?

If the Earth and the atmosphere did not emit radiation but only absorbed
radiation, the Earth and the atmosphere would continue to get hotter and
hotter until it would be uninhabitable. If more radiation were emitted than
absorbed, over time the Earth would get colder and colder. Neither of these
happens because the Earth is roughly in energy balance. At a particular time
and place, the energy emitted by the Earth might not balance the energy
absorbed by the Sun, but when averaged over the Earth’s entire surface for a
long time period, the input and output of energy are nearly in balance.
QUESTION 1: CALCULATE THE EMISSION TEMPERATURE OF VENUS.
the emission temperature is the temperature that the planet needs to
maintain energy balance.
ENERGY BALANCE OF THE EARTH





We will use the principle of planetary energy balance: assuming that
i) no energy is stored by the earth, and that
ii) the earth does not do work to its surroundings
then by the first law of thermodynamics, the net energy going into the
earth must be zero. It means that
Incoming solar radiation = outgoing planetary radiation
Incoming solar

Outgoing longwave radiative energy
FLUX, FLUX DENSITY, SOLAR CONSTANT




The Sun puts out a nearly constant flux of energy that we call the luminosity Lo 
3.9×1026 W
Assume radiative flux is spherically uniform .
flux density (Sd)  power per unit area. We assume that the flux density is uniform
over sphere, and write the flux density at any distance d from the sun as Sd.
Since space is effectively a vacuum and Energy is conserved, the amount of energy
passing outward through any sphere with the sun at its center should be equal to the
luminosity, or total energy flux from the sun. So: Lo = flux density x area of sphere
 Sd × 4 π d2

d


Solar constant (So)  flux density at
distance d  Lo / 4 π d2 =1367 W/m2
The mean earth-sun distance d  1.5
x 1011 m
So Flux density Sd is inversely
proportional to the square to
distance to the sun.
We assume that the Stefan-Boltzmann Law can be used
E = σ T4
 σ  5.67x10-8 Wm-2K-4

NOTES:
 1. E has units of power per unit area (W/m2)
 2. Temperature has to be in Kelvin in calculations
EMISSION TEMPERATURE
LET’S APPLY PLANETARY ENERGY BALANCE TO THE EARTH:
the emission temperature is the temperature that the earth needs to maintain
energy balance.
Incoming solar radiation = So(1-αp) π rp2
where rp is the radius of the earth
albedo (αp)  fraction of solar radiation reflected
Outgoing planetary radiation
 (radiation emitted) x (area of planet)  σT44 π rp2
Solar
radiation
is
essentially a parallel and
uniform beam for a
planetary body in the
solar system, because the
planets all have diameters
that are small compared
to their distance from the
sun. the amount of energy
incident on a planet is
equal to the solar constant
times the area that the
planet sweeps out of the
beam of parallel energy
flux.
IF YOU EQUATE THE INCOMING RADIATION TO OUTGOING RADIATION,
YOU CAN SOLVE FOR THE TEMPERATURE TO GET EQUATION
Te 
4
( So / 4)(1   p )


This is the emission temperature which is the temperature that the earth
needs to maintain energy balance.

1. It depends on the solar constant and albedo of earth.

2. Emission temperiture for earth ~ 255K (18 C)
QUESTION 1: CALCULATE THE EMISSION TEMPERATURE OF VENUS.
Te 
4
( S o / 4)1(1   p )

σ  5.67x10-8 Wm-2K-4
Solar constant (So)  flux density at distance d  Lo / 4 π d2
The emission temperature is much less than the observed global mean
surface temperature of 288 K = 15 C. To understand the difference we
need to consider the greenhouse effect.
GREENHOUSE EFFECT
Let’s now add a simplified ‘slab’ atmosphere to our planet with
the properties that the atmosphere:
 1. Lets sunlight through without absorbing or reflecting it
 2. Lets this “stab” absorbs all terrestrial radiation
 3. Has the same temperature everywhere
 We can then work out the energy balance for each of the layers
in this atmosphere-earth system - the top of the atmosphere
(TOA), in the atmosphere, and on the surface.
 Let Ts be the surface temperature, and TA the atmospheric
temperature, then


I. Top of Atmosphere (TOA) balance:

II. Atmospheric balance:

III. Surface balance:
Ts = 21/4Te
Since Te = 255K
(earth’s
emission
temperature), it follows
that Ts = 303K
or 30 degrees C!
The surface temperature is increased because the atmosphere does not inhibit the flow of
solar energy to the surface, but augments the solar heating of the surface with its own
downward emissin of longwave radiation, which in this case is equal to the solar heating.


There are three fundamental ways the Earth’s radiation
balance can change, thereby causing a climate change: (1)
changing the incoming solar radiation (e.g., by changes in
the Earth’s orbit or in the Sun itself), (2) changing the
fraction of solar radiation that is reflected (this fraction is
called the albedo – it can be changed, for example, by
changes in cloud cover, small particles called aerosols or
land cover), and (3) altering the longwave energy radiated
back to space (e.g., by changes in greenhouse gas
concentrations).
Milankovich Theory of climate change
Characteristics of earth’s orbit around the Sun
The earth’s orbit is slightly elliptical. Eccentricity is a
measure of how far earth’s orbit is from being circular.
Define eccentricity (e) by raphelion= (1 +e) rmean ; e ~ 0.0167 today
Mean distance: 1.496 x 1011m
Max (aphelion): 1.521 x 1011m
Min (perihelion): 1.471 x 1011m
Obliquity (tilt) is the angle between the
rotation axis of the planet and the plane of
the orbit around the sun
Currently 23.45 degrees; varies between 22.1and
24.5 degrees over a period of ~41,000 years.
This change affects the amount of solar
radiation received by the higher latitudes.
More tilt results in more solar radiation
being received at higher latitudes during the
summer, while less tilt results in less solar
radiation being received at higher latitudes
during the summer.
Earth axis “wobbles” like a spinning top, and so over time it points in
different directions relative to the stars in a 25 800  26 000 year cycle
called precession. Precession alters the timing of the seasons relative to
Earth’s position in its orbit around the Sun.
Hipparchos (190 BC – 120 BC), was a Greek
astronomer, geographer, and mathematician of
the Hellenistic period. He is considered the
founder of trigonometry but is most famous for
his incidental discovery of precession of the
equinoxes.
Question 2: Over the last 2000 years on what angle “rottated” the Earth
axis as a result of precession?


It is well known from astronomical calculations that periodic changes in
parameters of the orbit of the Earth around the Sun modify the seasonal
and latitudinal distribution of incoming solar radiation at the top of the
atmosphere (‘insolation’).
These cycles are known as Milankovich cycles, after Milutin
Milankovich. As these long-term cycles “overlap” there are
periods of time when significantly less radiation reaches Erath
surface (especially in the high latitudes), and periods of time
when there are greater or smaller seasonal contrasts. For
examle, during periods when there are smalller “seasonality” –
in other words, when there are smaller contrasts between
winter and summer – more snow can accumulate in high
latitudes due to greater snow fall from winters and less melting
will take place due to lower summer temperature.
THANK YOU FOR ATTENTION!
YOU CAN USE THE SAME EQUATION TO GET THE EMISSION TEMPERATURE
FOR OTHER PLANETS, BUT YOU’LL NEED TO CHANGE THE SOLAR CONSTANT
TO THE SOLAR FLUX APPROPRIATE FOR THAT PLANET, AS WELL AS THE
ALBEDO.
Te  4
( S o / 4)1(1   p )
σ  5.67x10-8 Wm-2K-4

Solar constant (So)  flux density at distance d  Lo / 4 π d2
THE DIFFERENCE BETWEEN THE ACTUAL SURFACE TEMP WITH THE EMISSION TEMP
REFLECTS THE INFLUENCE OF DIFFERENT PLANETARY ALBEDO AND GREENHOUSE EFFECTS.
Venus (actual temp ~730K)
Earth’s Te
~255K
Earth (actual temp ~288K)
Emission temp
Te (K)
Distance from Sun (x106 km)