Greenhouse vs. ozone:
A greenhouse gas easily transmits sunlight, but absorbs/reflects longer-infrared light. (A greenhouse admits light and traps heat.) The earth's primary cooling mechanism is radiative: we shed heat as mid-to-long-wave IR. That's why clear nights are the colder ones;ever see frost on a cloudy morning? Our atmosphere is fairly transparent to IR radiation. Some gases though are colorless in sunlight but dark in the IR. Carbon dioxide is one. It's not awfully dark, but there's enough of it that it makes a difference in the thermal steady state of Earth's surface. The scary thing about CO2 is that there's a lot of it dissolved in the oceans. Its solubility drops as water temperature goes up (example: warm soda gets flat quickly). So the possibility exists (hard to test or model) that a big enough pulse of manmade CO2 could upset the oceans into releasing some of their own, making it warmer yet... in a "runaway greenhouse" scenario. To be fair, the reality or the critical CO2 load for such an event are not known. We don't know it will happen, or can be made to happen, but it's plausible. So scientists are advancing the runaway greenhouse as a cautionary hypothesis, a way of saying "We could be playing with a bomb here".
Other gases are smaller components of the atmosphere, but remarkably efficient at heat capture. Nitrous oxide (from soil nitrogen in fertilizer), methane (moo phwaaat), fluorocarbons (old aerosols and refrigerants, obsolete degreasers and electronic supplies) are representative. Tetrafluoromethane is of especial interest because it has a half-life in our atmosphere conservatively estimated at ten thousand years.
Ozone destroyers are halofluorocarbons and straight halocarbons. Freon (chlorofluorocarbons), Halon (bromofluorocarbons), and Carbon Tet are unreactive enough to get convectively mixed throughout the troposphere, then diffuse into the stratosphere, where the ozone layer resides. There the halogen catalytic cycle enunciated by the Master takes place. The more stable halogen sources act as "timed-release" halogen (chlorine, bromine) sources with half-lives averaging a century. That's why a total production ban on CFCs is no guarantee that the ozone hole will heal in our lifetimes. We need to wait it out. |
