According to Tony Blair, it is our biggest challenge; his chief scientific advisor believes it is a greater threat than international terrorism. Now, the first concerted effrot to tackle climate change has come into force. Nick Morrison reports.

JUST a few months ago it seemed dead in the water. Hailed as a landmark of international co-operation, and the best hope for halting global warming, the Kyoto Treaty was on the verge of collapse, the gruelling negotiations in Japan, and the deal brokered by Deputy Prime Minister John Prescott, seemed destined to be all for naught.

Now, the global plan to cut greenhouses gases is back, rejuvenated by Russia's belated decision to come on board. Yesterday, more than seven years after agreement was first reached, the Kyoto Protocol came into force.

But what does it involve, why has it been so beset with problems, and what difference will it make?

Q What is the Kyoto Protocol?

A It is an international agreement imposing limits on the emission of six gases, including carbon dioxide, blamed for global warming, the phenomenon of rising world temperatures, leading to melting ice caps, higher sea levels and ecological change. Tony Blair has said climate change is the biggest challenge facing the human race, while his chief scientific advisor, Sir David King, said it was a greater global threat than international terrorism.

The Kyoto Protocol was agreed in Japan in December 1997, and grew from an agreement on climate change reached at the Earth Summit in Rio de Janeiro in 1992. The protocol became a legally binding treaty at midnight New York time, 5am GMT, yesterday.

Q What are the targets?

A Signatories to the protocol have agreed to cut emissions of greenhouse gases by 5.2 per cent below 1990 levels to 2012. Each country has agreed its own target: the UK's is 12.5 per cent, the target across the EU as a whole is eight per cent and Japan's is six per cent. The US agreed to a seven per cent cut. Countries which had low emissions were permitted to increase them.

Meeting the targets will involve a combination of domestic and industrial change. Around 28 per cent of all carbon dioxide emissions in the UK come from the energy used to run our homes, and another 26 per cent comes from road vehicles.

Q Why has it taken so long to come into force?

A The provisions of the Kyoto Treaty were that it would only come into force when it had been ratified by at least 55 countries, and the signatories included countries which accounted for 55 per cent of total emissions of greenhouse gases. The first provision was met in 2002, and 141 countries have now signed up, but after the United States and Australia decided not to ratify the treaty, all eyes were on Russia.

Russia had been reluctant to ratify the treaty, fearing it would hamper its economic growth, but its parliament finally agreed to sign on November 18 last year, meaning the protocol could come into force 90 days later. The decision will have an economic cost for Russia, but in exchange it is hoping for stronger EU support for its application to join the World Trade Organisation.

Q Why did the US refuse to sign?

A The treaty was agreed by former President Clinton's administration, but one of President Bush's first actions when he took office in 2001 was to pull out. The President said the treaty would damage the US economy, and emissions should instead be reduced by voluntary action and harnessing new technologies, although cynics see the links between the President and the fossil fuel industry, particularly oil, as being more to blame. Australia also cited economic reasons behind its decision not to sign.

The US is responsible for about a quarter of the world's emissions of greenhouse gases and its decision not to ratify the treaty was a major blow to Kyoto. Mr Blair said yesterday the only way to tackle global warming was to get the US involved again.

The US also argued that Kyoto was "fatally flawed" because it did not require developing countries to reduce their gas emissions.

Q Why are developing countries exempt?

A The agreement acknowledges that cutting gas emissions, principally through reducing the use of fossil fuels, will hamper economic growth, and this will prove particularly disadvantageous for developing countries. If growth is slowed, fewer jobs will be created, and efforts to lift more people out of poverty will founder. The treaty also recognises that developing countries contribute least to global warming but are likely to suffer the most from the associated climate change, including more extreme weather and rising sea levels.

Some developing countries, including China and India, have still ratified the treaty, but instead of committing to specific targets, they have undertaken to monitor emission levels and develop initiatives to counter the effects of climate change.

Q Will the targets be met?

A The UK has already reached its 12.5 per cent target, although the Government last month admitted it would not meet its own 20 per cent aim by 2010. But the UK is one of only four EU countries on track to fulfil its obligations. Japan has already said it would be struggling to meet its target and Canada has increased its emissions by 20 per cent on 1990 levels.

Overall, industrialised countries have cut emissions by three per cent on 1990 levels, but this has largely been the result of the collapse in the economies of the former Soviet states, which hid a rise of eight per cent among the other signatories. Far from cutting emissions, industrialised countries are expected to be producing ten per cent more greenhouse gases by 2010.

Q What happens when countries miss their targets?

A Kyoto provides for countries expected to fall short of their obligations to make up the difference by emissions trading. This allows countries which are responsible for too much pollution to buy "credits" from countries which are not emitting as much greenhouse gas as they are allowed. The idea is the overall level of emissions will be the same, with "failing" countries hit with a financial penalty for missing their targets.

It is also possible for countries to gain credits for initiatives which offset the release of greenhouse gases, such as tree planting or soil conservation. In this way, they are allowed to exceed their target without any penalty.

Q What difference will it make?

A Some scientists say even if the overall Kyoto target is met, it will be just scratching the surface of the problem of global warming. Instead of the five per cent cut demanded by Kyoto, more like 60 per cent is needed if the worst consequences of global warming are to be avoided.

Sceptics have also claimed that the influence of human activity on the world's climate is negligible in any case. Although global warming is a recognised phenomenon - average surface temperatures have risen by 0.6 degrees Centigrade in the last 140 years and all of the ten warmest years on record have occurred since 1990 - they argue that this is part of the Earth's natural cycle, and it is pointless to isolate one gas as responsible.

But for Kyoto's supporters, tackling greenhouse gas emissions is the only way to avoid environmental catastrophe, and even if the protocol itself is largely toothless, its value is in providing a framework within which future negotiations can be conducted.

It may only be a small step, they argue, but the other road leads to disaster.