VALENCIA, Spain — The U.N.'s top climate official challenged world policymakers Monday to map out a path to curb climate change, charging that to ignore the urgency of global warming would be "nothing less than criminally irresponsible."

Yvo de Boer issued his warning at the opening of a weeklong conference that will complete a concise guide on the state of global warming and what can be done to stop the Earth from overheating. It is the fourth and last report issued this year by the Intergovernmental Panel on Climate Change, co-winner of this year's Nobel Peace Prize.

Environmentalists and authors of the report expected tense discussions on what to include and leave out of the document, which is a synthesis of thousands of scientific papers. A summary of about 25 pages will be negotiated line-by-line this week, then adopted by consensus.

The document to be issued Saturday sums up the scientific consensus on how rapidly the Earth is warming and the effects already observed; the impact it could have for billions of people; and what steps can be taken to keep the planet's temperature from rising to disastrous levels.

The IPCC already has established that the climate has begun to change because of the greenhouse gases emitted by humans, said de Boer, director of the U.N. Framework Convention on Climate Change.

Everyone will feel its effects, but global warming will hit the poorest countries hardest and will "threaten the very survival" of some people, he said.

"Failing to recognize the urgency of this message and act on it would be nothing less that criminally irresponsible" and a direct attack on the world's poorest people, De Boer said.

U.N. Secretary-General Ban Ki-moon is to attend the launch of the report, which will provide the factual underpinning for a crucial meeting next month in Bali, Indonesia.

That conference will begin exploring a new global strategy to curb greenhouse gas emissions after the 2012 expiration of the first phase of the Kyoto Protocol, the landmark agreement that assigned binding reduction targets to 36 countries.

De Boer, citing agreements reached earlier this year by European and other industrial countries, said political inertia seemed to be disappearing in the lead-up to Bali. But he cautioned that governments must come up with the political will to complete a post-2012 road map.

"It will not cost the earth to save the Earth," as little as 0.1 percent of the gross global product for 30 years, said Janos Pasztor, of the U.N. Environmental Program, a parent body of the IPCC.