Lesson 7: Why Weather Matters
Greenhouse Gasses
A Swedish scientist named Svante Arrhenius first coined the term "greenhouse gasses" in the late 1800s. He used the phrase to describe how carbon dioxide keeps the entire planet warmer than it would be otherwise. Today, greenhouse gasses get a bad rap, but Arrhenius and other scientists weren't thinking about politics and doomsday scenarios in 1896.
Carbon dioxide gets the most attention in the media, but other so-called greenhouse gasses include water vapor, methane, chlorofluorocarbons, and nitrous oxide.
Fast fact: All molecules of greenhouse gasses consist of three or more atoms. (The oxygen we breathe only has two. Ozone, however, has three. See Weather: How it Works and Why it Matters, Chapter 11.)
Greenhouse gasses are actually beneficial. Water vapor and carbon dioxide in the atmosphere help keep the global thermostat at a comfortable level. Without these gasses, the world would plunge into a deep freeze. This is because the Earth is heated by two sources: the Sun and the atmosphere.
Earth's average temperature is currently 59 degrees Fahrenheit.
Without greenhouse gasses, the planet's average temperature would plummet to about zero degrees Fahrenheit. Earth would turn into a barren world, not too different in appearance from the interior of Antarctica today.
Global Cooling?
Scientists speculate that the troposphere -- the lowest layer of the atmosphere -- is heating up, and that worries us because that's where we live. Meanwhile, higher layers of the atmosphere may be victims of radiative cooling. Greenhouse gasses trap heat in the troposphere, which is close to the ground. So the warmer the lower atmosphere gets, the colder the upper atmosphere gets. A colder upper atmosphere may increase the rate of ozone depletion.
The Greenhouse Effect -- Too Much of a Good Thing?
Calling this phenomenon the "Greenhouse Effect" is a bit of misnomer. An actual greenhouse inhibits convection, the vertical transfer of warm air; but that's not quite how greenhouse gasses work.
Earth is actually heated twice: once by the Sun, directly, and then again by the atmosphere due to the greenhouse effect. As heat rises off the Earth's surface, a small percentage of it is absorbed by gasses, rather than escaping into space. Carbon dioxide and water vapor both absorb heat well. Later, when they emit heat, some of that energy is radiated back toward the surface. Without the heat radiated from the atmosphere, Florida at ground level would soon resemble the South Pole.
Why All the Fuss?
Greenhouse gasses keep the planet comfortably warm and hospitable. Anxieties about Global warming are fears of too much of a good thing. It's possible to take the greenhouse effect too far, and the atmosphere of our sister planet, Venus, is a picture portrait of what can go wrong. Due to the presence of an exceptionally thick layer of greenhouse gases, the average temperature of Venus' atmosphere is nearly 900 degrees Fahrenheit.
Rising sea levels are one of the problems anticipated if the worst global warming scenarios come to pass. That's bad news for islands and coastal regions. During the last ice age, the average sea level was much lower than today due to all the water locked up in the form of glaciers. The location of New York City, presently along the coast, was more than a hundred miles inland. When all that glacial ice started to melt, sea levels started to rise.
Interesting facts about sea level rise:
Sea levels around the world rose by several inches during the 20th Century.
If all the ice in Greenland melted, ocean levels would rise by more than 20 feet.
Melting the entire Antarctic ice cap would increase sea level by more than 200 feet.
Thermal Expansion
If you heat something up, it expands. A hot-air balloon inflates and starts to rise as the air it contains is heated. The same principle applies to ocean water. Any rise in ocean levels due to global warming wouldn't come just as a result of melting ice. The thermal expansion of water would also contribute to the ocean's encroachment on land
The Global Warming Debate
Data suggests that Earth's average temperature increased by about 1 degree Fahrenheit during the last century. That doesn't sound like much. If you walked outside on an autumn day and the temperature suddenly jumped from 59 to 60, you'd scarcely notice the difference. Taken on a global scale, however, even a small bump in the thermostat has enormous impact.
Tweaking the global thermostat just a few degrees could start a new ice age -- or cook the world with global warming. In the Pleistocene at the height of the last ice age, the average worldwide temperature dipped by about 10 degrees Fahrenheit, as indicated by a study of ice core samples from Antarctica and Greenland. A change of equal magnitude in the opposite direction is what many meteorologists now fear.
Global warming promises to have substantial effects on existing weather patterns. Heat is energy. Heat up the atmosphere, and weather systems become more energized. Rainstorms, blizzards, thunderstorms, and hurricanes will all pack a little bit more of a punch.
The Debate
Are humans to blame for global warming or is it part of a natural cycle -- or both? That's a debate that can't be settled in one short lesson. However, it's clear that carbon dioxide levels in the atmosphere are on the increase. Given the potential consequences, it's worth taking seriously. Further research will tell us what is -- or isn't -- at stake.
Carbon dioxide in the atmosphere has increased from less than 290 parts per million in the 1800s to nearly 350 parts per million today.
Fossil fuels, forest fires, and volcanoes all emit carbon dioxide. Once a carbon dioxide molecule is emitted into the atmosphere, it can remain there for 50 to 200 years.
