The UN Climate Change Conference kicking off in Copenhagen has all the elements of a science-fiction thriller.
It is a global meeting intended to agree on a strategy to avert a threat to the entire planet. And the menace, if most scientists are right, is growing greater and nearer with each passing decade.
The UN calculates that if carbon emissions continue to rise at their current pace, and are allowed to double from their pre-industrial level, the world will face an average temperature rise of 3 degrees centigrade this century.
That would likely create more and more frequent extreme weather patterns, such as storms and drought. At the same time, the sea level would rise, flooding coastal areas and cities.
To prevent this grim scenario, world leaders and scientists are heading to Copenhagen for the 12-day meeting with a specific goal in mind.
They want to assure that the maximum temperature rise over the coming decades is no more than 2 degrees centigrade.
"We cannot negotiate against the laws of nature, against science," European Commission President Jose Manuel Barroso said last month in Beijing. "And what science tells us is that 2 degrees Celsius is indeed the maximum that we can accept in terms of an increase in global temperatures."
But if the danger of uncontrolled emissions is well understood, there is little agreement among individual countries of how much they must do to rein in the problem.
One major challenge at Copenhagen will be to get developed countries, like the United States, and developing countries, like China, to agree on substantial limits to their emissions over the coming decade.
"I believe that negotiators now have the clearest signal ever from world leaders to draft a solid set of proposals to implement rapid action," Reuters quoted UN climate chief Yvo de Boer as telling reporters the day before the two-week talks were to begin.
"Never in the 17 years of climate change negotiations have so many different nations made so many firm pledges together. Almost every day countries announce new targets or plans of action to cut emissions," he said.Biggest Emitters
The United States and China will be closely watched. Neither the United States nor China signed the world's previous accord to reduce global emissions -- the Kyoto Protocol -- which expires in 2012.
Between them, the two countries are now the world's largest polluters, emitting almost half of the world's carbon dioxide.
Beijing and Washington have announced their intention to set voluntary emission limits.
The U.S. government, for example, has promised that by 2020 it will cut 17 percent off the level of greenhouse gases it emitted in 2005.
But the legislation that would permit Washington to actually make such cuts is currently stalled in Congress and is unlikely to pass before Obama arrives in Copenhagen. That raises doubts about how much Washington can commit itself to during the summit.
China has promised what at first glance appear to be staggering cuts. Beijing says by 2020 it will emit 40 to 45 percent less than it did in 2005.
But Beijing's emissions calculation is based on a vaguely defined formula of 'carbon dioxide emitted for each unit of GDP.' Many experts say the formula would really allow China's emissions to continue growing substantially.Leading Participants
The difficulties in reaching a global deal in Copenhagen are underlined by the fact that at least 65 heads of state or government will be joining the summit which, nominally, is for environmental ministers and officials.
U.S. President Barack Obama will attend the end of the conference after changing earlier plans to drop-in shortly after the start. The White House announced on December 4 that the change comes as Obama has been encouraged by the "momentum" of preparatory efforts to get a meaningful summit deal.
Most of the other top leaders, from countries as diverse as Australia, Ethiopia, and Brazil are expected to visit during the last two days of the conference.
Also attending will be hundreds - possibly thousands -- of members of NGOs, interest groups and the media, either as participants or on the sidelines.
The run-up to the summit already has seen several high-profile efforts to focus public attention on the dangers of global warming.
On December 4, the Nepalese government held a cabinet meeting close to the base camp of Mount Everest, the world's highest mountain. Nepal fears the glaciers of the Himalayas could recede sharply with global warming, threatening the supply of water to much of the region.
"There is the need for the whole world to unite and mitigate the problem, and to attack the problem," Prime Minister Madhav Kumar Nepal told reporters, "and that is why we want to draw the attention of the world and [say], 'Look, global warming has this negative impact on the existence of mankind.'"Basic Problems
Still other major issues to watch are whether the Copenhagen meeting can agree on a single baseline for measuring emissions cuts and on what those reduction amounts should be.
Closely tied to those questions is yet another: How much are developed nations ready to financially help developing nations move to greener technologies.
The EU estimates that some $150 billion would have to be provided to developing nations by 2020 to make significant changes. That would be a huge new commitment for countries still reeling from the recent global economic recession.
All these questions guarantee that the Copenhagen summit will be closely watched.
But they also guarantee, as with any science fiction thriller, that there is no certainty of knowing how the story will end until the last page.