Paris: Earth has heated up by one degree Celsius (1.6 degrees Fahrenheit), Britain's weather office said today, as greenhouse gases hit record levels just weeks before a crucial climate summit in Paris.
Other reports forecast rising seas were set to swamp large swathes of New York and Shanghai, and that global warming would drive millions of people into poverty worldwide.
The slew of fresh planetary warnings came as ministers gathered in Paris to search for common ground on divisive issues ahead of the summit, which runs from November 30 to December 11.
French President Francois Hollande, meanwhile, called for the creation of an "environmental security council" to verify and enforce measures to be adopted at the summit.
"I hope that binding measures emerge from the agreement in Paris" to curb dangerous levels of global warming, he told a scientific gathering in the French capital.
But verification remains a problem, he said. "So the next step is to have an organisation in the form of an environmental security council."
If the planet heats up by four degrees Celsius -- double the targeted UN ceiling -- oceans will swallow land inhabited by more than 600 million people, said a study by Climate Central, a US-based research group.
Even a two-degree jump would submerge land currently occupied by 280 million people, it said.
At the same time, the World Meteorological Organization reported that the level of climate-altering gases in the air punched through the psychological barrier of 400 parts per million.
"Concentrations of greenhouse gases in the atmosphere are now reaching levels not seen on Earth for more than 800,000, maybe even one million years," WMO chief Michel Jarraud told reporters.
"This means we are now really in uncharted territory for the human race," he warned.