Levels of carbon dioxide in the atmosphere have jumped abruptly, raising fears that global warming may be accelerating out of control.

Measurements by US government scientists show that concentrations of the gas, the main cause of the climate exchange, rose by a record amount over the past 12 months. It is the third successive year in which they have increased sharply, marking an unprecedented triennial surge.

Scientists are at a loss to explain why the rapid rise has taken place, but fear that it could show the first signs that global warming is feeding on itself, with rising temperatures causing increases in carbon dioxide, which then go on to drive the thermometer even higher. That would be a deeply alarming development, suggesting that this self-reinforcing heating could spiral upwards beyond the reach of any attempts to combat it.

The development comes as official figures show that Britain's emissions of the gas soared by three per cent last year, twice as fast as the year before. The increase - caused by rising energy use and by burning less gas and more coal in power stations - jeopardises the Government's target of reducing emissions by 19 per cent by 2010.

It also coincides with a new bid to break the log jam over the Kyoto treaty headed by Stephen Byers, the former transport secretary, who remains close to Tony Blair.

Mr Byers is co-chairing with US Republican Senator Olympia Snowe a new taskforce, run by the Institute of Public Policy Research and US and Australian think tanks, which is charged with devising proposals that could resolve the stalemate caused by President Bush's hostility to the treaty.

The carbon dioxide measurements have been taken from the 11,400ft summit of Hawaii's Mauna Loa, whose enormous dome makes it the most substantial mountain on earth, by scientists working for the US government's National Oceanic and Atmospheric Administration.

They have been taking the readings from the peak - effectively breathalysing the planet - for the past 46 years. It is an ideal site for the exercise, 2,000 miles from the nearest land and protected by freak climatic conditions from pollution from Hawaii, more than two miles below.

The latest measurements, taken a week ago, showed that carbon dioxide had reached about 379 parts per million (ppm), up from about 376ppm the year before, from 373ppm in 2002 and about 371ppm in 2001. These represent three of the four biggest increases on record (the other was in 1998), creating an unprecedented sequence. They add up to a 64 per cent rise over the average rate of growth over the past decade, of 1.8ppm a year.

The US scientists have yet to analyse the figures and stress that they could be just a remarkable blip. Professor Ralph Keeling - whose father Charles Keeling first set up the measurements from Mauna Loa - said:"We are moving into a warmer world".

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