By Richard Black
Environment correspondent, BBC News website, Valencia
The IPCC says more heat waves are very likely in the future
United Nations Secretary General Ban Ki-moon has challenged governments to act on the findings of a major new report on climate change.
Launching the latest report from the Intergovernmental Panel on Climate Change, he said real and affordable ways to deal with the problem existed.
The IPCC states that climate change is "unequivocal" and may bring "abrupt and irreversible" impacts.
Mr Ban urged politicians to respond at a UN climate change conference in Bali.
"Today the world's scientists have spoken clearly and with one voice," he said. "In Bali I expect the world's policymakers to do the same."
Mr Ban arrived at the IPCC meeting in Valencia from a fact-finding trip to Antarctica and South America.
"I come to you humbled after seeing some of the most precious treasures of our planet threatened by humanity's own hand," he said.
"All humanity must assume responsibility for these treasures."
The IPCC report synthesises the three aspects of climate change that it has already pronounced on earlier in the year, on the science, the likely impacts, and options for dealing with the problem.
Among the top-line conclusions are that climate change is "unequivocal", that humankind's emissions of greenhouse gases are more than 90% likely to be the main cause, and that impacts can be reduced at reasonable cost.
One declaration that reportedly caused heated discussion during the week-long talks here states that climate change may bring "abrupt and irreversible" impacts.
Such impacts could include the fast melting of glaciers and species extinctions.
"Approximately 20-30% of species assessed so far are likely to be at increased risk of extinction if increases in global average temperature exceed 1.5-2.5C (relative to the 1980-1999 average)," the summary concludes.
Other potential impacts highlighted in the text include:
- between 75m and 250m people are projected to have scarcer fresh water supplies than at present
- yields from rain-fed agriculture could be halved
- food security is likely to be further compromised in Africa
- there will be widespread impacts on coral reefs
The panel's chairman, Rajendra Pachauri, highlighted the need to deal with impacts which are coming whether or not global emissions are curbed.
Even if levels of CO2 in the atmosphere stayed where they are now, he said, research showed sea levels would rise by between 0.4 and 1.4 metres simply because sea water would continue warming up, which makes it expand.
"This is a very important finding, likely to bring major changes to coastlines and inundating low-lying areas, with a great effect in river deltas and low-lying islands," he said.
"If you add to this the melting of some of the ice bodies on Earth, this gives a picture of the kinds of issue we are likely to face."
Probable temperature rise between 1.8C and 4C
Possible temperature rise between 1.1C and 6.4C
Sea level most likely to rise by 28-43cm
Arctic summer sea ice disappears in second half of century
Increase in heat waves very likely
Increase in tropical storm intensity likely
This is the IPCC's fourth major assessment of global climate change since its formation nearly 20 years ago.
During the course of its existence, it has become more certain that modern-day climate change is real and principally due to human activities; it has also become firmer about the scale of the impacts.
"If you look at the overall picture of impacts, both those occurring now and those projected for the future, they appear to be both larger and appearing earlier than we thought [in our 2001 report]," Martin Parry, co-chair of the impacts working group, told BBC News.
"Some of the changes that we previously projected for around 2020 or 2030 are occurring now, such as the Arctic melt and shifts in the locations of various species."
There are indications that projected increases in droughts are also happening earlier than expected, he said, though that was less certain.
The IPCC considered about 29,000 pieces of real-world evidence in compiling this report, as well as the projections of computer models.
These include observations showing that dry areas of the world such as the Sahel and southern Africa are receiving less rainfall, while it has increased in northern Europe and parts of the Americas.
The panel suggests societies need to adapt to future impacts, as well as curbing emissions.
Without extra measures, carbon dioxide emissions will continue to rise; they are already growing faster than a decade ago, partly because of increasing use of coal.
The IPCC's economic analyses say that trend can be reversed at reasonable cost. Indeed, it says, there is "much evidence that mitigation actions can result in near-term co-benefits (e.g. improved health due to reduced air pollution)" that may offset costs.
The panel's scientists say the reversal needs to come within a decade or so if the worst effects of global warming are to be avoided.
The findings will feed into the Bali talks on the UN climate convention and the Kyoto Protocol which open on 3 December.