We see changes in climate, we believe we humans are involved and we're projecting future climate changes much more significant over the next 100 years than the last 100 years

Dr Robert Watson, IPCC

They say world temperatures this century could rise by between 1.4 and 5.8 degrees Celsius.

Sea levels could also rise by tens of centimetres, threatening millions of people living in low-lying countries.

The Intergovernmental Panel on Climate Change (IPCC), which has been meeting in Shanghai, China, says an increasing body of observations gives a collective picture of a warming world. And it says the evidence is stronger than before for a human influence on the climate.

The head of the United Nations Environment Programme, Dr Klaus Toepfer, said: "The scientific consensus presented in this comprehensive report about human-induced climate change should sound alarm bells in every national capital and in every local community."

Dr Robert Watson, who heads the panel of scientists advising the United Nations, said there could be massive implications in terms of water shortages, drought, damage to agriculture and the increased spread of disease, with developing countries worst hit.

Hottest decade

He said: "There's no doubt the Earth's climate is changing. The decade of the 1990s was the hottest decade of the last century and the warming in this century is warmer than anything in the last 1,000 years in the Northern Hemisphere.

Dr Robert Watson: Human are affecting the climate

"We see changes in climate, we believe we humans are involved and we're projecting future climate changes much more significant over the next 100 years than the last 100 years."

In this third assessment report of its Working Group One on the science of climate change, the IPCC updates its 1995 Second Assessment Report (Sar). It says its confidence in the ability of models to project future climates has increased, with the greatest uncertainty still arising from the effects on climate of clouds.

The report notes: "The observed changes in climate over time have been documented extensively by a variety of techniques. Many of these trends are now established with high confidence; others are far less certain."

It gives details of several trends, for example:

• the global-average surface air temperature has increased since the mid-19th century
• in the last four decades, temperatures have risen in the lowest few kilometres of the atmosphere
• snow cover and ice extent have decreased
• global average sea level has risen, and ocean heat content has increased
• some important aspects of the global climate appear unchanged. No significant trends of Antarctic sea-ice extent are apparent over the last 30 years, and there are no clear long-term trends discernible in the intensity and frequency of tropical storms.

Global ice cover is diminishing

"The present CO2 concentration has not been exceeded during the past 420,000 years and likely not during the past 20 million years. The rate of increase is unprecedented during at least the past 20,000 years.

"Over two-thirds of the increase in atmospheric CO2 during the past 20 years is due to fossil fuel burning. The rest is due to land-use change, especially deforestation, and, to a lesser extent, cement production."

Methane concentrations have increased by a factor of 2.5 since 1750, and those of nitrous oxide by 16%.

The Sar concluded in 1995: "The balance of evidence suggests a discernible human influence on global climate."

This report says there is now stronger evidence for a human influence on global climate. It concludes: "It is likely that increasing concentrations of anthropogenic greenhouse gases have contributed substantially to the observed warming over the last 50 years."