Earth’s Climate System (part 1)

• climate system• electromagenetic spectrum• Earth’s radiation budget• albedo• greenhouse effect

Earth’s climate system

• climate driven by “solar energy”

• climate operates to distribute solar energyacross surface

Can ask:

Q1. What is “solar energy”?

Q2. How does solar energy interact with planet?

What is solar energy?

-- electromagnetic radiation (light)

both a particle (photon) and wave

photons can have different energies (wavelengths)

high energies = shorter wavelengthslow energies = longer wavelengths

Electromagneticspectrum:

describes whatlight of differentwavelengthsis called

How does solar energy interact with planet?

-- some is stopped by atmosphere, some is not

-- depends on wavelength

atmosphere “transparent” for visible light,less so for other wavelengths

Consider the transparency of Earth’s atmosphere to light of

(1) short wavelengths

(2) medium wavelengths

(3) long wavelengths

Short wavelengths a.k.a. “shortwave radiation”

transparencyin Earth’satmosphere:

very low(shortwaves almost completely blocked)

If not blocked, life as we know it wouldn’t exist on Earth

-- cell damage!

UV & Ozone layer

Medium wavelengths a.k.a. “visible light”

transparencyin Earth’satmosphere:

mostly high (visible light mostly getsthrough, except when cloudy)

If not transparent, life as we know it wouldn’t exist on Earth

--too cold!

Long wavelengths a.k.a. “longwave radiation”

transparencyin Earth’satmosphere:

somewhat transparent (some longwaves get through, but not all)

If not somewhat transparent, life as we know it wouldn’t exist on Earth

-- too hot or cold! (Greenhouse Effect)

Radiation budget

• describes inflow & outflow of solar energy

• “budget” because energy is conserved

energy in = energy used for warming

+ energy radiated back to space

-- most of the “energy in” is visible light

-- energy radiated towards space is visible light & IR light

Energy used for warming:

-- absorbed by atmosphere-- absorbed by Earth’s surface

Energy radiated back to space:

-- reflected or scattered off ofclouds or surface

~70% ofincoming

~30% ofincoming

Energy in

If sun overhead,

get more photons

concentrated in a

smaller area...

more energy in

Earth’s spin axis is inclined, so we get seasons

Albedo --the brightness of a surface

can be quantified:

0% albedo 100% albedo

--darkest surface --brightest surface --all light absorbed --all light reflected none reflected none absorbed

Energy out is controlled by albedo

energy in = energy used for warming

+ energy radiated back to space

% energy radiated back to space = albedo

% energy used for warming = (100 - albedo)

the amount of light absorbed depends on the incident angle of sunlight

Low incident angle

High incidentangle

Albedo of water listed as 5-10%…these diagrams imply not always true

Energy radiated back to space:

-- reflected or scattered off ofclouds or surface

~30% ofincoming

If ~30% of incoming solar energy isreflected back to space, what does this say about the overall average albedo of Earth?

~30%overallaverage

Can havetemperature-albedofeedback

Can we havehavetemperature-albedofeedbackif the initialchange isclimatewarming?

warming

Can havetemperature-vegetation-albedofeedback

Energy used for warming:

-- absorbed by atmosphere-- absorbed by Earth’s surface

~70% ofincoming

Visible light that is absorbed does two things:

(1) it raises the temperature directly(2) it is converted into lower energy

(infrared) light

But recall that atmosphere is not completely transparent to IR light…

...this means that the IR light can’t be radiatedback to space easily

...so it becomes trapped

This leads to the Greenhouse Effect

Greenhouse Effect

Visible light from the sun passesthrough the atmosphere and warmsthe surface.

Heat radiated from the surface (infraredor IR light) travels back out into spacebut is absorbed or deflected back to thesurface by certain gas molecules.

Greenhouse Effect...

Trapped IR light warms theatmosphere, which warms the surface.

Temperature goes up gradually.

Greenhouseeffect.

Energy out

Main pointof thisis that ~95-96%of the IR lightradiated fromthe surfaceis trapped inthe atmosphere

--warming it &the Earth’ssurface

Confusing!X

Only some gases contribute to the Greenhouse Effect.

• N2 and O2 (the main constituents of our atmosphere) are not greenhouse gases.

• H2O, CFCs (chloroflourocarbons), CH4, CO2 are “greenhouse gases” and absorb IR light.

• SO2 is not a greenhouse gas. It combines with water to form H2SO4 (sulfuric acid) droplets. These droplets reflect incoming solar light back into space, resulting in planetary cooling.

Greenhouse gases.

• H2O is ubiquitous; we can’t control the amount of it in the atmosphere.

• CH4 is a fermentation product. Large amounts could be released from ocean (ice) deposits if the ocean warms.

• CFCs are largely man-made. International agreements in the 1970s limited their use, because of their harm to the ozone layer.

Greenhouse gases.

• CO2 is released by volcanoes.

• CO2 is produced by burning petroleum products

and by burning trees. This can be controlled.

Is the greenhouse effect all bad?

No!

It makes life as we know it possible on Earth.

Earth gets about 31o C of greenhouse warming.

Taverage = 15 oC now, with Greenhouse Effect.

Taverage = 15 - 31 oC = -16oC, without Greenhouse Effect.

Venus Earth Mars

avg. temp. 460 oC 15 oC -55 oC

greenhouse 285 oC 31 oC 5 oC warming

avg. temp. 175 oC -16 oC -60 oC with no

greenhouse

Climates on three planets

Too cold

Just right

Venus Earth Mars

greenhouse 285 oC 31 oC 5 oC warming

atmosphere 96% CO2 77% N2 95% CO2

composition 3.5% N2 21% O2 2.7% N2

0.037% CO2

0.01% H2O

Climates on three planets

OK with

10000 ppm = 1%so 370 ppm = 0.037%

CO2 abundance is rising in our atmosphere

CH4 abundance is also rising

1725 ppb = 1.725 ppm = 0.0001725%

Can havetemperature-H2O vaporfeedback

possibly alsowith CO2 andCH4