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.
Pasztor said this week's report, synthesizing the three scientific reports released earlier this year, will be the one document that the thousands of delegates at Bali "will be packing in their suitcases and carrying in their back pockets."
The top IPCC leaders will be in Oslo accepting the Nobel prize on Dec. 10, just when the Bali meeting reaches its stride with the final ministerial-level meetings. But panel chairman Rajendra Pachauri said technology will enable them to stay in touch.
Pachauri called this week's meeting a "watershed" that will issue the final product of years of work by 2,500 contributing researchers and hundreds of authors who reviewed the science and organized the data.
The report will be the first to include a brief chapter on "robust findings and key uncertainties," in which the authors pick out what they believe are the most relevant certainties and doubts about climate change.
"We summarize which kind of things we are very confident in and what is much less certain. That can be quite a complex discussion," said Bert Metz, one of about 40 authors. Some delegations want to stress certain points that others would prefer to avoid, he said.
Among the uncertainties cited in an early draft obtained by The Associated Press: the lack of data from key areas of the world, conflicting studies on the effects of cloud cover and carbon soaked up by oceans, and projections on how planners in developing countries will factor climate change into their decisions.
The IPCC has already been criticized for the selectivity and language of the policy summaries, which have been softened on several points because of objections by countries including the United States, China and some big oil-producing nations such as Saudi Arabia.
On Monday, WWF International, one of several environmental groups invited to observe the process, said "governments cut vital facts and important information" during the negotiations.
Without naming them, the WWF accused governments of "politically inspired trimming" of facts from the summaries, which it said diluted the urgency to make deep cuts in emissions of carbon dioxide and other greenhouse gases.
Scientists say the full reports on which the summaries are based, each comprising more than 1,000 pages, remain valid, and that their own presence during the discussions ensures the scientific integrity of the summaries.
De Boer said getting governments to sign off on the summaries is a critical element of the IPCC's value.
"Because those reports are adopted by governments, there is no government that can now stand up and say, 'I don't accept what's in the IPCC report.' That means that you have a common scientific base," he said in an interview Friday.