By Robert Hazen, George Mason University
Human activities can affect the global environment, often in ways that are difficult to predict. The law of unintended consequences states that we can’t change one part of a system without altering other parts, often in unexpected ways—sometimes in catastrophic ways. Modern human society has run smack into this law at the global level.
Short- and Long-term Problems
The most pressing unanswered questions about the environment relate to specific human actions and their consequences.
It is quite easy to spot many of the localized, short-term problems, like toxic chemical spills, nuclear accidents, oil spills etc. And these are cleaned up rather quickly. Then we have reports that link cancer deaths to all sorts of chemicals, artificial sweeteners, pesticides and poor diet. We hear about asbestos and radon, DDT, PCBs, CFCs, and dozens of other concerns and chemicals that compete for public attention. Governments are generally quite excellent at passing legislation to fix these localized, specific problems, whether they’re real or imagined.
However, as the problems become more and more gradual, less localized, and as the details of the cause and effect become fuzzier, solutions become more costly. It becomes inconvenient to try to fix these problems, and calls for action seem somewhat less compelling.
This is a transcript from the video series The Joy of Science. Watch it now, on Wondrium.
The Ozone Layer
A great, global environmental problem is the ozone hole. The ozone layer is not really a layer; it is just a region of the atmosphere about 35 kilometers up, a thick band in which there’s a slightly increased concentration of the molecule ozone.
Ozone is O3, three atoms of oxygen bound together, and it only comprises a small fraction of the percentage of the atmosphere at that level; most of the atmosphere is still nitrogen and oxygen; CO2 and water vapor are much more abundant minor components than ozone, but still, there’s an increased concentration of ozone in this region.
This trace amount of ozone absorbs the Sun’s harmful UVB radiation: the ultraviolet radiation that can cause sunburn, and can severely affect phytoplankton and plant growth. It’s harmful to life on the surface of Earth, and ozone, thus, provides a protective layer that shields us from this radiation.
CFCs and Ozone Hole
We have to go back to the 1950s to see where this ozone hole problem came from. In the 1950s, gases called chlorofluorocarbons, or CFCs, were first introduced and found to provide cheap, non-toxic materials for refrigerators. They were also used for aerosols; they were used for other everyday applications, for cleaning surfaces, and so forth.
Production of the gas freon and other CFCs quickly grew into a major chemical industry. There seemed to be many benefits, because these were nontoxic gases, and there didn’t seem to be a downside at all to CFCs. Doubts about the safety of CFCs were first raised in the 1970s, following a series of chemical experiments on ozone.
In 1970, Dutch chemist Paul Crutzen demonstrated that ozone can be depleted by chemical reactions with nitrogen oxides—that’s a common ingredient of smog. A few years later, Americans Mario Molina, of MIT, and F. Sherwood Rowland, of the University of California at Irvine, added to the concerns, because they showed that CFCs can cause rapid destruction of ozone through a series of simple chemical reactions.
Chlorine Atoms
What happens, high in the atmosphere, is that we have normally stable CFC molecules, but they’re fragmented by ultraviolet light, the very ultraviolet light that ozone absorbs. When they’re fragmented, it triggers the release of single, highly reactive chlorine atoms. Chlorine is one of the halogens in the next-to-last column of the periodic table—highly reactive gases.
If a chlorine atom comes in contact with ozone, the ozone’s going to be split into two pieces: an O2 molecule and a ClO molecule. This breaks down the ozone. Given enough chlorine, the ozone layer could be effectively hurt by destruction in this reaction.
Chemists were at first quite optimistic that the chlorine atoms would eventually become locked into chlorine nitrate—that’s the compound ClNO3, which seems to be fairly stable, and if we lock up the chlorine in that compound then we don’t have to worry about the ozone depletion.
Depletion of Ozone
However, these hopes were dashed in 1985, when a British scientific survey revealed the severe depletion of the ozone over the Antarctic. This depletion is called the ozone hole, but this is somewhat of a misnomer. It’s not really a hole; it’s just a region of the atmosphere where there’s much less ozone than we normally would expect to find.
Scientists now issue yearly reports on the size of the hole, the duration of the hole and the depth of the hole—depth means the percent depletion.
Ozone and Chlorine
Since 1985, the hole has gotten steadily larger, and deeper, and longer in duration. Each year, it seems, it gets worse; and indeed, in 1998 it was worse than at any time previous. At its maximum, more than 70 percent of the ozone may be destroyed in this ozone hole.
Crutzen and others quickly realized that stratospheric ice particles, which concentrate over Antarctica during the dark months of winter, provide a surface on which chlorine nitrate breaks down, so chlorine is no longer stabilized in the atmosphere by this chlorine nitrate molecule. Thus, the single chlorine atom continuously recycles and keeps breaking down more and more ozone; indeed, chlorine becomes a catalyst for the destruction of ozone. One chlorine atom, therefore, can destroy thousands of ozone molecules.
Common Questions about the Ozone Layer and the Problem of the Ozone Hole
The ozone layer is a region of the atmosphere about 35 kilometers up, a thick band in which there’s a slightly increased concentration of the molecule ozone. This ozone absorbs the Sun’s UVB radiation, which is harmful to life on the surface of Earth.
In the 1950s, gases called chlorofluorocarbons, or CFCs, were first introduced and found to provide cheap, non-toxic materials for refrigerators, aerosols and other everyday applications. Doubts about the safety of CFCs were first raised in the 1970s, following a series of chemical experiments on ozone. It was later found that CFCs can cause rapid destruction of ozone through a series of simple chemical reactions.
The ozone hole is a region of the atmosphere where there is much less ozone than we normally would expect to find; it is not really a hole.
