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 Antarctic 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. ( BBC )