Why? One idea is that Mr. Reagan himself had had skin cancer, and allowed a concern for public health to triumph over ideology. Eli Lehrer, the head of a Washington think tank called the R Street Institute and a longtime Reagan admirer, offered me a simpler theory: that the man truly loved nature. He was never happier than when riding horses and chopping wood. Perhaps the science of the ozone hole just spooked him. We know it spooked Margaret Thatcher, the British prime minister and Reagan ally, who had been a research chemist in her early life.

The Montreal Protocol on Substances That Deplete the Ozone Layer entered into force on Jan. 1, 1989, and in the years since, it has been used to phase out nearly 100 dangerous gases.

Now, the wheel of history often turns on chance, and here is one of the great coincidences of our time: Many of the substances that destroy the ozone layer also happen to be exceedingly powerful greenhouse gases.

If production had been allowed to continue, a batch of scientific studies show, the planet would most likely be warming a lot faster than it is. The latest of these studies came out only a few weeks ago. Led by Francisco Estrada of the Autonomous National University of Mexico, the paper suggests that the slowdown in global warming that has occurred over the past 15 years is a direct result, at least in part, of the success of the Montreal Protocol.

In fact, the evidence suggests the protocol has done far more to limit global warming than the better-known treaty adopted for that purpose, the Kyoto Protocol.

Could it do still more?

It turns out the gases phased out under the Montreal Protocol are being replaced by another set of chemicals, hydrofluorocarbons. They do not destroy the ozone layer, but they are potent at causing global warming.

Prodded by small island countries concerned about drowning on a warming planet, nations are considering an amendment to the Montreal Protocol that would phase out the worst chemicals in this group in favor of new ones that are safer for the climate.