Throughout the Nineties, the irresistible force of the global- warming supertanker has been heading for collision with two twin, immovable objects - ordinary, everyday politics and economics.
The big bang happens next week, when ministers, diplomats and thousands of journalists, lobbyists and experts converge in the ancient Japanese city of Kyoto for 10 days to negotiate a United Nations climate protection treaty.
Stopping global warming has been the flagship campaign of environmentalists everywhere. Any politician wanting to be seen as green had to make speeches calling for action.
But the most important, obvious action to take involves cutting the world's remorselessly rising consumption of fossil fuels, still the lifeblood of Western economies and seemingly indispensable for the development of poorer nations. Such cuts necessitate big changes in energy taxation, transport, home heating and industry.
World leaders might be more likely to reach agreement, or at least to have more focussed negotiations, if they knew exactly how, and how fast, climates would change and sea levels rise.
Unfortunately, such certainties will not be available for at least a decade. Global warming came up the scientists' radar screens as a real threat in the mid-Nineties. An international research effort since then has confirmed that climate almost certainly will change. Indeed, as we add more and more carbon dioxide and other gases to the atmosphere, it seems to have started already.
The climatologists have been able to rule out some of the most frightening earlier predictions, at least for the next century. But most of them agree that climate and sea level will change quickly enough to pose real threats to people, their farming, forestry and cities - and that these will start to hit home sometime in the next 50 years.
If we start cutting our rising use of coal, oil and gas, and deal with some other greenhouse gas sources such as burning forests, we can begin to turn the supertanker of climate change around. At issue is not halting the changes, but slowing them down to a safer rate. Is there the will to do this?
There certainly was not at the Rio Earth Summit five years ago, when the UN Framework Convention on Climate Change was signed by world leaders. Back then, the developed countries grudgingly volunteered to stabilise their emissions of carbon dioxide - the most important of the greenhouse gases - at the 1990 level by 2000. Having caused the bulk of the pollution to date, this seemed only fair. Even so, most of the rich nations will fail to keep their promise by a wide margin.
Then, in Berlin in 1995, the developed countries set themselves a deadline for agreeing on a legally binding protocol, or treaty, for restraining their emissions early in the next century. No more volunteering; it was to be a matter of international law. The deadline was set for December 1997, in Kyoto. But despite having two and a half years in which to reach agreement, deep divisions remain between countries.
At one extreme, Australia's proposal for restraint is to let its emissions rise by 18 per cent by 2010. At the other, the European Union is holding out for a 15 per cent cut by then. The United States, the biggest emitter of all, advocates that all developed countries must get their emissions back down to the 1990 level by 2010.
There is another huge complication. The US and Australia say that in order for them to give concrete undertakings, large developing countries like China, whose emissions are rising rapidly, must also agree to some kind of restraint.
Some kind of deal will probably emerge by 10 December, when the conference ends. But it seems unlikely to make much difference to the course of the global-warming supertanker.