Despite the international set-to over Iraq and caustic reviews for the recent U.N. Summit on Sustainable Development in Johannesburg, there is still some good news on cooperation and the environment.

According to a report released this month by the U.N. Environment Program and the World Meteorological Organization, worldwide efforts that began more than a decade ago to protect the ozone layer appear to be bearing fruit. Though a massive "hole" still forms over Antarctica each year, scientists report that the chemicals that have been destroying the ozone layer are now "at or near peak," and could begin to dissipate slowly -- if nations stay the course.

This is very good news for several reasons. First, the chances of people contracting skin cancer from exposure to increased ultraviolet light (which is partially filtered out by the ozone layer before it reaches Earth) could begin to decrease, particularly for those living in the Southern Hemisphere where the risk is highest. Second, the potential for systemic harm to plants, animals and ecosystems as a result of increased exposure to UV rays should decline.

Last but not least, these findings confirm -- to the credit of the community of nations -- that when policymakers, scientists, industry and consumers cooperate, international environmental laws can be crafted to benefit humans and the Earth.

The laws responsible for bringing these ozone-busting chemicals under control are contained in The 1985 Vienna Convention for the Protection of the Ozone Layer, and its 1987 Montreal Protocol on Substances that Deplete the Ozone Layer.

But laws are only as strong as our desire to abide by them and to enforce them, and this legal regime and our ozone layer are still extremely vulnerable.

Even if, as this month's report suggests, levels of offending chemicals in the atmosphere have peaked, we still have a long way to go before they start to drop measurably and ozone levels begin returning to previous levels. In the meantime, there is very real danger that governments and industry, assuming the worst is over, will begin to shirk compliance with the law.

As the authors of the report warn, "Failure to comply with the Montreal Protocol would delay or could even prevent recovery of the ozone layer. For example, continued constant production of ozone-depleting substances at the 1999 amount would likely extend the recovery of the ozone layer well past the year 2100. The total atmospheric abundance of ozone-depleting gases will decline to pre-Antarctic ozone-hole amounts only with adherence to the Montreal Protocol's full provisions on production of ozone-depleting substances."

In other words, developing countries (which under the protocol are not yet required to completely stop production and consumption of ozone-depleting materials) must stay on course to a total ban, as developed nations have done.

The new report, titled "Executive Summary, Scientific Assessment of Ozone Depletion: 2002," reflects the consensus of 250 scientists from 37 countries who are members of the scientific panel of the Vienna Convention and the Montreal Protocol. The complete, final report is due out next year.

Laws, though, are only part of the equation. Perhaps the more interesting part is ozone itself, and some of the chemicals that have been destroying the ozone layer. Ozone is a wonderful example of nature's simple complexity. A form of oxygen comprising three oxygen atoms rather than the usual two, ozone is both essential for life and a toxic pollutant -- a veritable Jekyll and Hyde.

In the upper atmosphere, or stratosphere, ozone is spread thinly throughout a wide band between 15 and 50 km over our heads. This "ozone layer" surrounds the planet and protects Earth from the sun's ultraviolet-B and deadly UV-C radiation; its ozone is primarily the natural result of oxygen reacting with UV light. Incredibly, there is so little ozone in the stratosphere that if it were all compressed at ground level, it would be a thin film just 0.3 cm thick.

Meanwhile, below the stratosphere, in the troposphere (which extends from ground level to 6-10 km up), the same ozone takes on a more sinister role. This "bad" ozone, which is a component of the smog that hovers over our cities, is created when pollutants, such as those from trucks and cars, react with sunlight. Ozone causes irritation of the eyes, nose and throat, and is harmful to our lungs. It is a pungent chemical, and when levels are high enough, it can leave a steely taste in the mouth.

Of course it is tempting to suggest, as many of my students have over the years, that we simply pump all that offensive smog-bound ozone up to the depleted ozone layer and solve two problems at once. Though this idea has a certain logical symmetry, it is both impractical and unwise.

Unforeseeable complications have a way of undermining even the most well-intended human ventures -- much the way once "perfect" chemicals have been found to be destroying the ozone layer. Although there are a number of chemicals in the halogen family responsible for depleting stratospheric ozone, the primary culprits are chemicals called chlorofluorocarbons. CFCs were seen as a miraculous breakthrough when they were discovered early in the last century, because they are stable and chemically inert. Within decades they were being mass-produced and used worldwide in sprays, foams, refrigerators, air conditioners and fire extinguishers.

The problem is that CFCs are so stable they do not break down in the environment. They can remain chemically intact for decades, only breaking up when they reach the stratosphere and encounter intense UV radiation. Unfortunately, when CFC molecules do break down, they release chlorine atoms -- which attach to, and split apart, ozone molecules. Other halogens that deplete ozone to a lesser extent are halons, bromine and chlorinated solvents, as well as some of the substitutes now being used for CFCs.

However, despite these numerous threats, the UNEP/WMO report notes that if all nations comply with the Montreal Protocol as scheduled, "Antarctic ozone levels will be increasing by 2010 due to projected decreases of halogens in the stratosphere." Nevertheless, the report warns, "The ozone layer will remain particularly vulnerable during the next decade or so, even with full compliance."

In other words, we have begun to turn the corner, but too many uncertainties remain to be sure of rounding the bend safely.

As for the "hole," stratospheric ozone thins as much as 70 percent at its peak during springtime in the Southern Hemisphere, but there is never a total absence of ozone over Antarctica. Calling it a hole may be a literal exaggeration, but the term captured the world's imagination and brought about a decisive consensus and effective legal action.

It is worth recalling that, years ago, when the ozone layer was much more of a mystery, the CFC industry accused scientists of falsely claiming the sky was falling. Today we know that those scientists were right, despite gaps in their knowledge: Not a bad lesson to keep in mind whenever policymakers and industry demand scientific certainty before taking action on environmental threats.