The Ozone Hole
Ozone at ground level is pollution, but ozone in the stratosphere plays a vital role in protecting plant and animal life from harmful ultraviolet rays. Remember from Lesson 1 that temperatures in the stratosphere increase with altitude because the ozone layer is absorbing ultraviolet rays that would otherwise penetrate to the Earth.
From July through October -- that's winter and early spring in the Southern Hemisphere -- chlorofluorocarbons accumulate in icy clouds in the stratosphere. When the sun finally rises in the south polar spring, ultraviolet radiation breaks down these CFCs and starts a chain reaction in which chlorine destroys ozone. That's when the ozone "hole" reaches its greatest extent. The ozone hole was larger than all of Europe in 1995.
The ozone-destroying chain reaction is most effective at very low temperatures, such as those found in the stratosphere over the South Pole. Temperatures over the North Pole are a few degrees warmer. Therefore, ozone depletion over the Northern Hemisphere in March and April isn't quite as extreme.
Increasing temperatures in the troposphere may result in decreasing temperatures in the stratosphere, where the ozone layer is located. That could speed up the process of ozone depletion.
Chlorine breaks down ozone into "regular" oxygen. Ozone blocks ultraviolet rays, but two-atom oxygen -- the kind we breathe -- does not.
Plankton is at the base of the food chain and is highly susceptible to ultraviolet radiation.
In humans, ultraviolet radiation can cause melanoma and other forms of skin cancer.
More Information
For more about the ozone layer, see Weather: How it Works and Why it Matters, pages 162 to 165. Additional information and perspectives about Antarctica and the South Pole can be found in Tying Down the Wind, Chapter 19 ("Chasing the Sun from Pole to Pole") and Chapter 20 ("Antarctica
Climates: Past, Present, and Future
Earth's climate has been both much warmer and much colder in past eons. The pendulum swings both ways.
At certain times during the reign of the dinosaurs, Earth's average temperature was more than 10 degrees warmer than it is now, and the entire planet was virtually ice-free.
By contrast, about 18,000 years ago, glaciers covered New Jersey. A mile of ice sat atop the present-day location of Chicago.
Sea level at the peak of the last ice age was 400 feet lower than today, due to enormous quantities of water locked up as glacial ice.
Global temperatures fell by 1 to 2 degrees Fahrenheit in the years (between about) 1300 and 1850, a period known in history books as "The Little Ice Age."
Again, 1 or 2 degrees doesn't sound like a lot, but heating (or cooling) the entire atmosphere by a couple degrees requires an enormous amount of energy. Think of it this way: if your tea-kettle is completely full, it takes a lot longer -- and requires more energy -- to bring the water to a boil than it does when the kettle is only a quarter full.
Ocean currents -- such as the warm Gulf Stream or the cool Labrador Current -- also affect regional climates. Viking settlers had rich harvests in Greenland in the 1200s, but by the late 1300s Greenland had become an icy wasteland.