It is an international treaty, brokered by the United Nations, which binds developed nations to reduce their emissions of greenhouse gases.
Signed in 1997, the Kyoto protocol did not come into force until 2005. The protocol was an addition to the 1992 United Nations Framework Convention on Climate Change, which had pledged the world to try to avoid dangerous climate change but did not set out how this was to be achieved.
At Kyoto, developed nations agreed to cut their emissions of six greenhouse gases by an average of 5 per cent overall, compared with 1990 levels, in what was termed the first commitment period, which was to end in 2012.
Developing countries were not asked to take on any emissions targets.
Ex-Soviet bloc countries were also given targets, but in effect these had already been achieved – because the baseline was set for 1990, before the collapse of communist industry.
But, although the protocol was signed by the US, it was never ratified by the US Congress. The Clinton administration believed it could gather together the necessary support in Congress, but quickly realised that this would be impossible. The protocol was never voted upon.
As a result, the world’s biggest emitter of greenhouse gases and biggest economy was left out of the emissions-cutting agreement. When George W Bush took on the presidency in 2001, he rejected ratifying the treaty during his term of office.
This was one major factor ensuring that the protocol could not come into effect – for the agreement to be operational, countries making up at least 55 per cent of the world’s total greenhouse gas emissions had to ratify it. The European Union, Japan, Canada and other developed countries agreed, but the absence of the US made it hard for this to be reached.
Then, late in 2004, after holding out for seven years, the Russian government finally agreed to ratify the protocol, which meant the threshold had been reached and Kyoto could come into effect. Russia is thought to have been persuaded by a promise from the EU to support its bid for membership of the World Trade Organisation.
The move rescued the Kyoto protocol from the scrapheap of history. Developed countries were bound to begin cutting their emissions. But this task was now much harder than it had looked in 1997, because in the intervening years - as the protocol seemed unlikely ever to come into force - most countries had failed to take much action to curb their greenhouse gas output. And now they had only seven years to make the necessary cuts.
The European Union – which had fought hard to keep the Kyoto protocol alive – is on track to meet its targets, but most developed countries are far adrift of theirs.