03 Energy Balance

8/13/2019 03 Energy Balance

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Timescales

Geological time (big changes globally)

Glacial-interglacial cycles (really recent time)associated with shifting land masses and effects on

ocean circulation Human time (anthropocene)relationship to

geological forces

Rates of change

Timescales of movement of material and energy

Evolution versus extirpation versus extinction

Human-induced rates of change versus rates of naturalself-regulation


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The Earths Energy Balance

A budgeting exercise


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Over Time

Origin of earth (4.6 bybp)

Dating of extra-terrestrialmaterial

Hadean & Archaen (4.62.5 bybp)

unidirectional change inorganization ofmaterial/planetaryevolution; driven by energyfrom radioactive decay

Proterozoic &Phanerozoic (2.5 bybp to

present)

Solar energy more

important/recycling ofmaterials


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Major points

How do the various components of earth

system move energy and matter on the

earths surface?

Over long time scales the planet must be in

steady state (input = output)

In the present system, energy balance at theearths surface is driven by solar radiation


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Basic physics

Electromagnetic (EM) radiation if generally

propagated as a wave

But EM can often behave more like a

particle - photon


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c = ln

Fig. 3-2

Wave number (n) = 1/l(cycles/cm or cm-1)

(cm/cycle or cm)

Waves defined by their speed (c), wavelength l, and frequency n

Frequency and wavelength are inversely related

n= c/l


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Electromagnetic spectrum

E = hn = hc/l

At times, EM radiation behaves more like a particle - photon

High energy/high frequency/low Low energy/low frequency/high


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Flux

How energy (or any material) passes

through a unit surface area per unit time

Can think of energy as a particle

Units: some mass per unit area per unit time

mg/m2/hr (= mg m-2h-1)

mmol quanta/ m2

/hr


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Which passes through a larger unit area?

Which has the higher flux?


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Think about how that affects energy

flux with latitude on earth.


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Back to energy

Energy is expressed as Joules (J)measures heat,electricity and mechanical work

1 J = 0.239 calories

1 J = 2.7778 107 kilowatt hour

Power (the rate at which work is done or energy ismoved) is expressed in Watts (W)

1 W = 1 J/second

W/m2then is a unit of energy flux

=J/m2/s

Energy flux is important for global climate

Polar regions cooler due to lower energy flux(mass per unit area per unit time)


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Flux also depends on distance of an object or

observer from the object emitting the radiant

energy

Flux of solar energy

decreases with distance

from the sun.

Relationship is aninverse-square law

Double distance from the

sun and intensity of

radiation (or energy flux)decreases by a factor of

.


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Temperature

Measure of internal heat energy

Rate of motion of molecules in a substance

Faster movement = higher temperature


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Blackbody radiation

A body that emits radiation over all possiblefrequencies (remember inversely related to

wavelength) Wavelength distribution depends on the

temperature of the blackbody

The Planck function describes thewavelength distribution relative to radiationintensity


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Stefan-Boltzmann Law

F= sT4

Weins Law

The flux of radiation

emitted by a blackbody

reaches its peak value at

lmaxwhich depends inversely

On the bodys absolute temperature

The energy flux emitted by a blackbody

is related to the fourth power of thebodys absolute temperature


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UV Vis IR (heat)

Blackbody emission curve for sun and earthnote higher energy

shorter wavelength (higher frequency) emissions for the sun.

Energy flux and blackbody

radiation temp. are related.

Because it is hotter, sun emitsmore energy per unit area at all

wavelengths.

Changes in energy flux determine the

spectrum of the EM radiation emitted.

Shifts along the laxis.


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Energy output of the sun

Predicts a maximum of radiation emissionin the visible region (0.40.7 mm)

Large component of high energy UV(remember this is damaging)

Assume Earth is a blackbody radiator

The Earth System responds to both theamount of solar radiation and its EMspectrum


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Earths energy balance

If temperature is constant, the planet has to

be in radiative balance


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Fig. 3-19 Earths globally averaged atmospheric energy budget.

Energy input = energy output

Some incoming solar radiation is lost before reaching the earths surface (30%).

Some is returned to space as IR (70%)


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O3Plus CO2and H2O

Some incoming solar radiation is reflected by clouds

Some is absorbed in the atmosphere (ozone at low l; CO2and H2O at high l)


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UV Vis IR (heat)

O3 H2O, CO2

Fig. 3-8 Blackbody emission curves for the Sun and Earth.


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O3Plus CO2and H2O

Only 50% if incoming solar radiation gets through the atmosphere to the Earths

surface


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Blackbody radiation ofthe sun minus that lost

in the atmosphere.Solar spectrum clipped

at high and low endsEnergy loss shiftslmaxto lower wavelengthswhen energy is re-radiated by the Earth.Reradiated as IR

(heat).

Net effect of atmosphere on incoming and outgoing radiation

Shift in lmax

controlled

by Weins law

Assumes

Earth is a

blackbodyradiator.


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CO2andH2O

Outgoing IR also absorbed by greenhouse

gases.


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Greenhouse gas absorption

Major role in warming atmosphere

Traps heat

Leads to re-distribution of radiation before

it is re-radiated to space

Without greenhouse gases, planet would be

much colder (~ -20oC)


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Feedback loops

Feedback between lithosphere, hydrosphere,

atmosphere and biosphere interact to

maintain relatively constant, favorabletemperatures over most of geologic history


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Greenhouse effect

Planetary energy balance in the absence of agreenhouse

Simple model

Planet with no atmosphere and albedo that of Earth Energy emitted by Earth= energy absorbed by Earth

From distance of sun, Earth is a circle (projected area)

Energy absorbed = energy inputreflected light

Albedo is the average color of the planet Low albedo = dark color = absorbs heat (warms up)

High albedo = light color = high reflectance (cools down)

Assuming blackbody radiation, Stefan-Boltzman lawpredicts the temperature of the atmosphereless planet


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Energy absorbed=Energy inputReflected energy

=REarth2 SREarth

2 SA =REarth

2 S(1A)

Energy emitted= 4REarth2

sTe4

Energy emitted=Energy absorbed

sTe4=

S

4(1A)

Box Fig. 3-1

Where S is solar fluxA is earths albedo

We know how to

calculate area of a circle

Apply Stefan-Boltzman Law and simplify


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High albedo = high reflectivity

Low albedo = low reflectivity(light is absorbed)

Fig. 2-6


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=REarth2 SREarth

2 SA =REarth

2 S(1A)


sTe4


sTe4=

S

4(1A)

Box Fig. 3-1


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=REarth2 SREarth

2 SA =REarth

2 S(1A)


sTe4


sTe4=

S

4(1A)

Box Fig. 3-1


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Fig. 3-2 The greenhouse effect of a one-layer atmosphere.

sT4s= (S/4)*(1-A) + sT4e

sT4s= 2sT4e

sT4e= (S/4)*(1-A)

Provides 33o

C of surface warming

Treat atmosphere as black body

Energy balance


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Fig. 3-19 Earths globally averaged atmospheric energy budget.

03 Energy Balance

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