United Nations climate talks have opened in Poland with pleas for urgent action to fight global warming despite the world economic slowdown, and a warning that inaction could mean water shortages for half the world by 2050.
The talks in the western Polish city of Poznan are the half-way point in a two-year push to agree a climate pact at the end of 2009 to succeed the Kyoto Protocol which expires in 2012.
Yvo de Boer, head of the UN framework convention on climate change, said the world had to step up work to reach a deal by next year.
"The clock is ticking, work now has to move into a higher gear," he said.
Anders Fogh Rasmussen, the Danish prime minister who will host the 2009 meeting in Copenhagen, said: "The financial crisis should not prevent the commitment to other urgent issues like climate change."
Rajendra Pachauri, head of the UN intergovernmental panel on climate change, said that many people had still not woken up to the risks of what could be "irreversible change" if the world failed to act.
He said the number of people living in river valleys with water stress could rise from more than 1.1 billion in 1995 to more than 4.3 billion in 2050, or "almost the majority of humanity".
Pachauri also warned that more and more species of animals and plants were at risk of extinction.
The talks in Poland, which will run to December 12, include 10,600 delegates from 186 nations.
Donald Tusk, Poland's prime minister, said: "Our work on the natural environment should be timeless... irrespective of the economic situation.
"We must understand, and let this idea be a landmark of this conference, that financial crises have happened in the past and will happen in the future."
Barack Obama, the United States president-elect, won praise at the opening ceremony of the talks for setting "ambitious" goals for fighting climate change.
Rasmussen said: "I am delighted to see that Obama is planning ambitious climate and energy policies as part of the solution to the economic slowdown."
Obama plans to cut US emissions of greenhouse gases to 1990 levels by 2020.
The emissions, mainly from burning fossil fuels in factories, power plants and cars, are about 14 per cent above 1990 levels.
De Boer described Obama's policies as "ambitious".
In Europe, the economic slowdown has exposed doubts about the costs of an European Union goal of cutting greenhouse gas emissions by 20 pe rcent below 1990 levels by 2020.
Poland, which gets 93 per cent of its electricity from coal, and Italy are leading a drive for concessions in a package meant to be agreed at a December 11-12 summit of EU leaders in Brussels.