Earth's thin blanket


When light strikes "stuff"

Three kinds of things might happen when light strikes something, depending on the wavelength of the light, and the material it hits:

  • Transmission, if the material is 'transparent',
  • Reflection (no loss of light energy to heat) or
  • Absorption: light energy is converted to heat.
    (Just like other high-quality kinds of energy getting converted to heat through friction.)
    P.S. the light doesn't make it through.

How do we know?

Glass (and carbon dioxide) are like this:

  • transparent if the light has short wavelength (in the visible region of the E-M spectrum),
  • not transparent to longer wavelengths (in the IR region).

How do we know?

How do we know?

...that water does not transmit microwave light?

(Think of what happens in a microwave oven as microwave 'light' shining on water).

Greenhouse effect

Here's a scrambled explanation of the 'greenhouse effect' in a parked car. Put these pieces in order...

  1. Visible light is absorbed by molecules in the seat and dashboard.
  2. Visible light from the sun is transmitted through the windshield.
  3. IR light hits the windshield but is not transmitted.
  4. Hot molecules 'glow' gently, giving off IR light.
  5. The molecules in the seat and dashboard get hotter.

In earth's atmosphere, substitute "carbon dioxide, water vapor, methane, and some other gases" for "windshield"!

Sunlight filtered by atmosphere

If you were designing a system to sense nearby objects by detecting the E-M waves bouncing off of them, and you could only use a small range of wavelengths--what wavelength of light should you use??

There are animals that are sensitive to light in each of these regions:

  • UV
  • visible
  • infrared

What's in air?

The Ozone layer

The SHIELD that protects us from ultra-violet (high energy) light from the sun...

90% of Earth's air mass is within 10 km of the surface of Earth.


Around 1900 - Refrigerators use toxic gases as coolants: methyl chloride, ammonia, sulfur dioxide.

1928 - GM chemists synthesize a chlorofluorocarbon (CFC), brand-named "Freon" used first in Frigidaire refrigerators: non-toxic, non-corrosive, chemically inert. Breathe it, and nothing happens to you!

1932 - Carrier uses Freon in its "Atmospheric Cabinet" - world's first self-contained home air-conditioner unit.

1945 - Freon used as the propellant in paints, hair sprays, health care products. Later, foam bubbling agent.


Refrigerators, air conditioners, and heat pumps all use a "heat engine in reverse" to transfer thermal energy out of something cold, and dump the thermal energy into a warmer location.

When a liquid evaporates, it absorbs energy from its surroundings.

Mechanical energy is used to re-compress the evaporated liquid on the 'hot' side.

Where did the CFC's go?

Molina Sherwood1974 - Mario Molina and Sherwood Rowland suggested that CFCs could be broken apart by high energy UV light, freeing chlorine, which can then do this...

`Cl + O_3 \rightarrow ClO + O_2`

A second reaction frees the Cl to destroy anew...

`ClO + ClO + [UVl i g h t] \rightarrow Cl+Cl+O_2`

1978 - Consumer boycott leads to U.S. ban on CFCs in spray propellants.

Antarctic measurements

English researchers at the Halley Bay research station in Antarctica had been measuring ozone since the late 1950s. They couldn't quite believe the drop after ~1975, checked and re-checked, and published in 1985.

They also noticed a new seasonal effect: Each spring (October), the ozone would suddenly drop precipitously, and then recover a few months later.

Effects of elevated UV radiation

Hazards of elevated UV radiation include

  • Phytoplankton (sunlight-absorbing plankton) are vulnerable to UV, and are at the bottom of food webs in the oceans near Antarctica.
  • Increased risk of skin cancer and cataracts in humans.
  • Genetic damage to plants and animals.

The smoking gun

1986 - Susan Solomon organized an expedition to measure ClO.She proposes that stratospheric ice breaks up enclosed CFCs, releasing Cl when heated by springtime sunlight.

Solomon data

Montreal Protocol

1987 - 24 nations signed the Montreal Protocol. As of Sept 2009, all 196 member states of the UN had signed on to the treaty to phase out ozone-destroying chemicals.

1995 - Nobel prize in Chemistry goes to Paul Crutzen, Mario Molina and Sherwood Rowland. The committee commends them for "contribut[ing] to our salvation from a global environmental problem that could have catastrophic consequences."

CFCs have an estimated 40 year lifetime in the atmosphere.


Image credits

Solar spectrum -, Windows to the Universe, Jason Swaby, IPCC