We've saved the ozone. What next?

This summer's sun-worshippers, constantly advised to keep their clothes on, may have little good to say about the state of the ozone layer. But exactly 10 years after British scientists discovered a hole in this protective screen, there is finally something to celebrate. The hole, it is true, is getting wider and deeper every spring over the Antarctic. But in the early part of the next century, it should shrink. One man-made threat, at least, is now scheduled for elimination.

This international success story shows that when governments are convinced of a scientific case, they are capable of taking dramatic action. Once the Antarctic hole was recognised as a fact, the world's nations were remarkably quick in agreeing to phase out production of ozone-depleting chlorofluorocarbons (CFCs).

So can we now feel reassured that the international community will act effectively when environmental needs demand?

Unfortunately not. The truth is that we take action on the environment only when faced with clear and observable disaster, whose causes are unmistakeable. The atom bombing of Hiroshima and Nagasaki, for example, helped to persuade many nations to sign the nuclear non-proliferation treaty. Likewise, the 1976 Flixborough chemical factory explosion, followed by accidents in Seveso and Bhopal, led to strict controls on the production of hazardous chemicals.

A similar process took place over the ozone layer. Effective moves to protect it were only instigated when the hole was identified. As early as 1972 two American scientists published evidence that CFCs could break down ozone in the upper atmosphere. Yet little concrete was done globally until after the discovery in 1985 of the hole.

The trouble is that catastrophes are not always preceded by dramatic early warnings. Indeed one of the greatest potential environmental threats may be developing before our very eyes with no single shattering event to shake us out of our complacency: namely global warming. Of course its characteristics are very different from ozone depletion. We are still unsure of the cause, of whether man-made emissions are exacerbating the natural greenhouse effect. There is no scientific consensus. And, so far, global warming has produced no disasters, just worrying trends. So international meetings, such as the Rio summit on climate change, have addressed the problem but avoided draconian measures.

Even if we were sure that we were the cause, the solutions would not be obvious, easy or cheap. On the contrary, they would be mind-boggling. When CFCs were banned, safer alternatives were readily available. By contrast, finding an alternative to fossil fuels would be difficult. Entire economies would be affected by change, producing massive resistance.

Yet if we do not act until global warming produces observable disaster, it may be too late. Greenhouse gases can stay in the atmosphere for hundreds of years. And historical examination of the world's ice cores indicates that temperature changes can occur suddenly with extreme and unpredictable consequences. It could, therefore, be an error to delay action until the scientific evidence is clear and unambiguous. The world's success in dealing with CFCs does not mean that other major environmental problems can be solved in a similar way.