UN Secretary General Ban Ki-moon has warned that climate change poses as much of a danger to the world as war.
Ban Ki-moon urged the US to lead the way in tackling climate change
In his first address on the issue, Mr Ban said changes in the environment were likely to become a major driver of future war and conflicts.
He urged the US - the world's biggest producer of greenhouse gases - to take the lead in fighting global warming.
Mr Ban said he would focus on the issue in talks with leaders of the G8 group of industrialised nations in June.
The UN is also due to hold a conference on climate change in Bali in December.
UN environment officials have been urging Mr Ban to take up the issue, says the BBC's Laura Trevelyan in New York, arguing that global leadership is needed and that he could make an impact.
Speaking to schoolchildren at a UN conference in New York, Mr Ban said his generation had been "somewhat careless" with the planet but that he was hopeful that that was changing.
"The majority of the United Nations' work still focuses on preventing and ending conflict," he said.
"But the danger posed by war to all of humanity and to our planet is at least matched by the climate crisis and global warming."
Last month, a panel of scientists organised by the UN published a report showing that human activity was "very likely" to be causing climate change, and predicted rises in temperatures and sea levels.
Mr Ban warned that poor people living in Africa and small island states would suffer most from the effects of global warming, even though they were least responsible.
New measures must be agreed to control emissions after 2012
And in future years, the upheaval caused by environmental changes such as droughts and coastal flooding was likely to drive conflicts, he pointed out.
He said the world needed a more coherent system of international environmental governance in order to tackle global warming beyond the expiry of the Kyoto Protocol in 2012.
"I hope that the United States, while they have taken their role in innovative technologies as well as promoting cleaner energies, will also take the lead in this very important and urgent issue," he added.
The US, which produces a quarter of the world's greenhouse gas emissions, is not a signatory to Kyoto.