Global average temperatures rose in the 20th century by 0.6C.
They are projected to rise by anything from 1.4C to 5.8C over the period 1990 to 2100.
A report this year by an international group chaired by the head of the UN's Intergovernmental Panel on Climate Change (IPPC) identified a
2C increase as the threshold beyond which "the risks to human societies and ecosystems grow significantly".
Such an increase is considered probable if atmospheric concentrations of carbon dioxide (CO2) exceed 400 parts per million (ppm).
Concentrations of CO2 now exceed 370ppm - their highest level for at least 420,000 years - and are still rising; in 1958 the level was 315ppm.
Last month, a joint statement by the science academies of 11 countries, including all the G8 nations, warned: "It is vital that all nations identify cost-effective steps that they can take now to contribute to substantial and long-term reduction in net global greenhouse gas emissions."
Since 1750, CO2 levels in the atmosphere have increased by about 35 per cent.
Nineteen of the 20 warmest years of the past 150 years have occurred since 1980. Nasa predicts that 2005 will be the warmest year since records began.
According to the IPPC, which used research from 2000 experts, "most of the warming observed over the past 50 years is attributable to human activity".
It is calculated that to halt global warming, greenhouse gas emissions would have to be cut by 60 per cent from 1990 levels. The 1995 Kyoto protocol aims to reduce them by 5.2 per cent by 2010, compared with 1990 levels.
The US, which produces 24 per cent of the world's carbon emissions, rejects Kyoto. Between 1990 and 2002, US CO2 emissions grew by 13 per cent.
Members of the G8 group of industrial nations account for 45 per cent of greenhouse gas emissions.
The US has resisted the use of the phrase 'Our world is warming' in a draft G8 statement on climate change.
- INDEPENDENT
