ESS15 Lecture 7The Greenhouse effect.

• First midterm is in one week.

• Open book, open notes.

• Covers material through end of Friday’s lecture

• Including today’s lecture (greenhouse effect)

• And Friday’s (energy balance of the Earth).

Housekeeping.

• Stay tuned for study guide, practice questions, and printable versions of lecture slides.

• Monday’s lecture will be a review.

• Don’t forget, I have office hours:

• Today 3:30-4pm, Monday 3-4pm.

Housekeeping.

Laboratory demonstration.

Review

• A: more, faster

• B: less, shorter

• C: more, at the same speed

• D: more, shorter

• E: less, at the same speed

i-clicker survey: Shorter wavelength radiation carries energy compared to longwave radiation and moves .

• Radiation travels as both waves and photons

• Waves do not require molecules to propagate

• Shorter waves have more energy than longer waves

• Waves interact with matter at similar scales as the waves

wavelengthspeed of light

frequency

wavenumber

Electromagnetic Radiation

Diagram of the solar radiation “budget”

30% reflected by clouds, air, dust, and surface 19% absorbed by the atmosphere (mostly clouds)

51% absorbed at the surface

• Peak emission at shorter waves at higher temperatures

Blackbody Radiation

SUN

EARTH

Stefan-Boltzmann Law: F = σT4

σ = 5.67 x 10-8 Wm-2K-4

• Hot objects emit much more energy than cold objects at all wavelengths

=

The power of math:

..solar constant,

…and the albedo

We can solve for a planet’s temperature!This is the “bare rock” model: A climate prediction from laws of

energy balance, black body radiation and geometry!

If we know…

Watts in = Watts out

What temperature does the “bare rock” model predict?

• Minus eighteen degrees Celsius. -18C !!!??

• If Earth were this cold it would:

• have solid frozen oceans and miles of ice.

• blindingly bright

• So something important must be missing from the model…

• The atmosphere! The Earth is not a bare rock.

• If it were it would be real cold here on the surface.

• But the atmosphere blankets our rocky surface. This makes a big difference to the temperature.

The Greenhouse Effect.

• Sun beams down 51 units of shortwave energy

• WOW: 96 more units of longwave energy are beamed down from atmospheric gases and clouds!

• Earth's surface loses these 147 units through convection, evaporation, and radiation emission (glowing)

Energy Balance with Atmosphere

• A: 1/3 B: 1/2 C: 2/3 D: 3/4

i-clicker exercise: of the energy beaming down to heat the surface of the Earth comes from air molecules.

Greenhouse Gases

https://www.youtube.com/watch?v=3JX-ioSmNW8&index=3&list=PL38EB9C0BC54A9EE2

National Academies: Greenhouse Gases: Climate Change

Greenhouse Gases have been known for over 150 years

JohnTyndall(1820-1893)

Laboratory apparatus in the era of steam-engines.

IRAbsorp;

on

Measuring how infrared radiation interacts with different types of gases.

“Without water vapor, the Earth’s surface would be held fast in the

iron grip of frost.”

How do Greenhouse Gases Absorb Energy?

http://changingclimates.colostate.edu/movies/scott_denning_796kbits.mov

• Gases are made of moving molecules separated by empty space

• Kinetic energy of molecular motion is proportional to temperature

• Gases don’t behave as blackbodies or even graybodies!

Atmospheric Gases

I-Clicker Survey

What is the most abundant gas in Earth’s atmosphere?

a. CH4 (Methane) b. N2 (Nitrogen)

c. O2 (Oxygen)

d. CO2 (Carbon Dioxide)

Experiments show both kinds of behavior:

• Oscillating fields with troughs and crests (waves)

• Individual packets of energy (particles)

Long Waves = small photons

Short Waves = BIG PHOTONS

Electromagne,cradia,onbehavesasbothwavesandpar,cles!

Waves and Photons

When radiation meets matter.

Energy is “Quantized”• When radiation interacts

with atoms and molecules, only certain “jumps” in energy are possible

• Electrons orbit at specific energy levels above an atomic nucleus

• Absorption of a photon of just the right energy can make them “jump up” to the next level

• Emission of a photon occurs when an electron “falls” down to a level below

Atomic Emission• Big jumps take lots of

energy, small jumps take less

• Big drops emit energetic photons (short wavelengths)

• Small drops emit less energetic photons (longer wavelengths)

Most electron transitions in gases absorb/emit visible or UV light

Molecules and Photons• Molecules are groups of atoms

that share electrons (bonds) • Molecular transitions involve

changes in vibration, rotation, bending, and stretching of chemical bonds

• Photons can interact with molecules to change states

• Transitions involve specific amounts of energy, so only certain wavelengths are active

Molecular transitions typically absorb and emit in thermal infrared

• Nearly all of the air is made of oxygen (O2) and nitrogen (N2) in which two atoms of the same element share electrons

Diatomic molecules can vibrate back and forth like balls on a

spring, but the ends are identical

Dancing molecules and heat rays

O O NN

• Infrared (heat) energy radiated up from the surface can be absorbed by these molecules, but not very well

Molecules that have many ways to wiggle are called “greenhouse”

molecules.

Dancing molecules and heat rays

• They have many more ways to vibrate and rotate, so they are very good at absorbing and emitting infrared (heat) radiation

O OC

• Carbon dioxide (CO2) and water vapor (H2O) are very different!

O

H H