No one thought of it that way at the time, but a landmark agreement reached two decades ago may be one of the greenest events to have occurred in modern history. This month marks the 20th anniversary of the Montreal Protocol.

When the protocol was negotiated in 1987, 24 countries signed it. Today, more than 190 countries have ratified the protocol, making it one of the most successful treaties in terms of global cooperation, Mack McFarland, chief atmospheric scientist for DuPont, says. His report on the protocol begins on Page 36.

To review, the protocol established an aggressive timetable to reduce and eventually eliminate ozone-depleting substances, including chlorofluorocarbons (CFCs) and hydrochlorofluorocarbons (HCFCs). Scientists determined that CFCs and HCFCs, long used in HVACR systems, were major contributors to the depletion of the ozone layer.

In the early 1980s, the depleting ozone appeared as a "hole" over Antarctica every September and October. Ozone levels have fallen by more than 60 percent during the worst years, while smaller, yet still significant, stratospheric decreases have been seen in other, more populated regions, including the United States. Thus, ozone depletion is global in scope.

Restoring the ozone to its proper level is a serious health issue. The stratospheric ozone layer is the earth’s main shield against harmful ultraviolet (UV) radiation from the sun. Depletion of the ozone layer can result in increased UV radiation reaching the earth’s surface.

Increased UV radiation heightens the incidence of human skin cancer, cataracts, and weakened human immune systems. Cases of melanoma, the most serious form of skin cancer, almost have doubled in the United States over the last two decades, with at least 32,000 new cases and 6,900 melanoma-related deaths estimated in 1994 alone, according to the U.S. Environmental Protection Agency.

The good news, according to McFarland, is that progress during the past 20 years has been rapid, and the actions taken because of the protocol have led to significant reductions in the current and future risk of ozone depletion and climate change. While it is a fragile process that requires ongoing vigilance and wise decisions, it still is cause for celebration. Also, it serves as proof that cooperation on a global scale is possible and can achieve great things.