"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."
The world's leading climatologists say global warming is happening faster than previously predicted.
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.
"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 range for globally-averaged surface air temperature increase by 2100 ranges from about 1.4 degrees Celsius to 5.8 degrees, an increase the report notes that "would be without precedent during the last 10,000 years". The projected sea level rise by 2100 is between 0.09 and 0.88 metres.
But the report does say that there are still many gaps in information and understanding. One priority, it says, is to "arrest the decline of observational networks in many parts of the world".
The report says that emissions of greenhouse gases continue to warm the Earth's surface, and that emissions of some types of aerosols help to cool it. It is clear that both are caused by human activities, although the report notes that natural factors, such as changes in solar output or volcanic eruptions, can also have an effect.
Carbon build-up
It estimates the warming caused by changes in solar energy since 1950 at about one-fifth of that attributable to carbon dioxide (CO2), and concludes that "natural agents have contributed small amounts" to warming over the last century.
The report quantifies the build-up of CO2 in the atmosphere. The concentration now is one-third more than in 1750, it says.
"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."