By Richard Black
BBC environment correspondent
The flags are flying, the bunting is out, the fireworks are aloft.
Kyoto celebrations took place around the world
In 34 countries around the world, environmental groups and environmentally minded politicians are welcoming in the Kyoto Protocol.
Even in Australia and the United States, the two countries whose withdrawal left the Protocol almost fatally wounded, supporters are partying like there is a tomorrow.
But how much difference will it make?
According to Jacqueline Karas, a climate change specialist in the Sustainable Development Programme at Chatham House, the Royal Institute for International Affairs in London, it is a day to celebrate.
"It's the first major international agreement to reduce greenhouse gases and tackle climate change," she told BBC News.
"But it was only intended to be a first step; and we need much more aggressive action if we are going to curb climate change."
The basic text of the Kyoto Protocol was agreed in December 1997; it grew from an earlier treaty, the United Nations Framework Convention on Climate Change, agreed at the Rio de Janeiro Earth Summit five years earlier.
Under the Protocol, developed nations agreed to restrict their production of six greenhouse gases.
Each country has its own target - some agreed to deep cuts, others are permitted increases; the net effect should be to reduce emissions from the developed world by 5.2% from 1990 levels by 2012.
But bringing the Protocol into force has been a fraught business.
A number of governments - Japan, Canada and Russia as well as the US and Australia - have argued that it would damage their economic interests.
As a result, there have been successive revisions to the mechanisms which governments could use to achieve their emissions targets.
Environmentalists hope the treaty will make a real difference
In 2001, the Yale University economist William Nordhaus, a former US Presidential advisor, published an analysis looking at what effect Kyoto would have when these revisions, and the US withdrawal, were taken into account.
"I estimate that emissions reductions will be about one and a half percent lower than a no-control scenario," he told BBC News at the time.
Achieving even that reduction depends on countries which have ratified the Kyoto treaty actually meeting their emission targets.
"The immediate impact of Kyoto coming into force is to put pressure on the European Union, Canada, Japan and the rest to deliver their promised emissions reductions," said Jacqueline Karas.
"Most have been long on words but short on action, and most are going to find it difficult to meet their targets."
Nevertheless Kyoto has been the only game in town.
"The Kyoto Accord entering into force is a key landmark on the long road to reducing emissions," Tony Juniper, director of Friends of the Earth UK, told BBC News.
"It is not sufficient, but it is necessary; now we have to move beyond the politics and back towards the science."
Though there are a few eminent scientists who find fault with theories of man-made global warming on strictly scientific grounds, there is no doubt about the consensus view: human production of greenhouse gases is changing the climate, and impacts will be dangerous.
The latest products of that consensus were on view two weeks ago at a conference organised by the UK Meteorological Office in Exeter.
"The early science on climate change was all computer modelling," Tom Burke, a visiting professor at Imperial College and University College in London, told BBC News.
"Now there is a second scientific wave of observations about what's happening - acidification of the oceans, melting of the west Antarctic ice sheet, and so on.
"What's becoming clear from the science is that the climate is more sensitive than we thought; and so the issue to focus on is not long-term measures, but what happens to concentrations of greenhouse gases in the shorter term."
This, then, is the challenge for negotiators as they clear away the champagne glasses and contemplate what comes after Kyoto.
The obvious route would be to try and bring the United States, the biggest single emitter of greenhouse gases, and its ally Australia back into international discussions.
Jacqueline Karas believes that blocs like the EU, now the prime political mover for action on climate change, may find that a Kyoto-2, or Child of Kyoto, involving large-scale routine sessions of detailed negotiations, is not the best way to proceed.
"One of the biggest questions is whether they should keep with the Kyoto process, or find other ways of bringing the United States and Australia back in," she told BBC News.
"It's simply unrealistic to expect the US to meet its original Kyoto commitments - it would be politically impossible, and probably practically impossible too.
"But there are a number of other possible approaches to involving them in international action; there is scope for mechanisms to develop new technologies and to disperse those technologies - and, as many individual US states now have greenhouse gas reduction programmes, to link states within the US to the European Union's emissions trading scheme."
Such plans of action, even if they were successful, might take too long, according to Tom Burke.
"We may be in a lot of trouble if the concentration of carbon dioxide in the atmosphere rises to 400 or 450 ppm (parts per million)," he told BBC News.
"And at current rates we'll be at 400 in 12 years' time."
Professor Burke believes that the European Union should be looking east, rather than west.
"There are something like 1,000 coal-fired power stations in some stage of construction or planning in the world - over half of them are in China," he said.
The main ceremony to mark entry into force was held in the ancient Japanese capital of Kyoto
"If they are built with conventional coal-burning technology, that would make it impossible to keep carbon dioxide at a safe level.
"China is going to burn that coal; so the important thing is investment to solve that problem, which we already know how to do technically."
Such action would, Professor Burke believes, cost $10-20bn dollars per year; but would give a window, a breathing-space, to explore other options.
Tony Juniper agrees that focussing on the big developing countries such as China is key.
"If I were Tony Blair, I would be putting considerable effort into the European Union Presidency which the UK holds later this year," he told BBC News.
"I would be leading the EU to build a strong position aimed at getting the big developing countries to come on board by the time of the next UN negotiating round.
"The Americans will inevitably be saying 'no deals, no targets' - we need to have developing countries onside by then."
But just as the bright day dawns and Europe seeks to pick up the reins of leadership, it finds its flagship programme, the Emissions Trading Scheme (ETS), in some disarray.
The European Commission is taking legal action against a number of member states, notably Italy, for failing to translate Directives on the ETS into national law.
Now the UK may be in trouble with the Commission too, after submitting a revised National Allocation Plan for carbon emissions - effectively increasing the amount of greenhouse gas it would permit companies to produce in the next three years.
"This ought to have been a good day for the UK, because Britain has done rather well," commented Tom Burke.
"Deputy Prime Minister John Prescott did very well in brokering the deal in Kyoto, and we are on course to meet our commitments in the Protocol.
"The Prime Minister has taken the issue up personally, and the UK set more ambitious targets than Kyoto required; and now we find ourselves mired in this unbelievably stupid dispute with the European Commission - it's unimaginable, and it is undermining the UK's whole strategy on climate change."
The ETS is intended to show that greenhouse gas production can be brought down without pain to business, by allowing markets to find the cheapest ways of reducing emissions and stimulate innovation.
As such, it could appeal to businesses around the world, not just in Europe.
But before it can spread and grow, it has to work, and deliver real reductions - as does the Protocol itself.