"Climate change is rapidly emerging as one of the most serious threats that humanity has ever faced," Kenyan Vice President Moody Awori said at the start of the UN meeting, the first to be held in Africa, the continent most at risk.
The two-week Conference of Parties to the UN Framework Convention on Climate Change (UNFCCC) is expected to draw 6,000 experts, officials and lobbyists from 189 nations.
"The worst impacts of climate change can be prevented only if governments act now," said Steve Sawyer of the environmental watchdog Greenpeace.
Dire predictions
Experts warn global warming is having and will have the largest impact on Africa, where some 800 million people are increasingly threatened by unpredictable weather patterns, floods and droughts.
A weekend UN report warned that unless steps are taken to help African countries, some 480 million people on the continent may face water security problems by 2025 and 25 to 40 percent of species' habitats could be lost by 2085.
The meeting in Kenya is to discuss what steps to take after the UN Kyoto Protocol on curbing greenhouse gas emissions expires in 2012. It will seek to convince top polluters who have not ratified the treaty to do more to stop global warming.
As the conference got underway, environmental groups issued a string of bleak assessments about the global dangers posed by climate change, particularly for developing nations, many of them in Africa.
"Climate change is starting to hit home and we can clearly see the first impacts here in eastern Africa," environmental group WWF said. It warned of food and water shortages, malaria outbreaks and conflict over scarce resources.
Kyoto pressure
Industrialised countries whose leaders signed the Kyoto Protocol but whose parliaments have failed to ratify it will also be put under the spotlight, activists said.
These include the United States, the world's biggest single polluter, and Australia, which is the biggest polluter on a per capita basis and a major supplier of fossil fuels, such as coal, which cause global warming.
Kyoto has been hamstrung by the refusal of some major industrialised nations to take part and by the uncontrolled release of greenhouse gases from developing nations like India, China and Brazil.
Experts warn that the treaty in its present form will not even dent the problem.
The Nairobi summit will also address ways in which rapidly industrialising countries such as India, China and Brazil can minimise the environmental damage of their economic progress.
Despite being the world's second largest single polluter, China is still exempt from commitments to slash its greenhouse gas emissions because of its status as a developing country.
Kyoto came into effect in February 2005 with much fanfare. It was intended to commit industrialised countries to bringing their greenhouse gas emissions to an average of five percent below their 1990 level, by a deadline of 2012.
A report last week by former World Bank chief economist Sir Nicholas Stern said the costs of unrestrained global warming could be far worse than the measures needed to slow it down.
The Stern review estimates that worldwide inaction could cost the equivalent of between five and 20 percent of global gross domestic product every year, forever.