Earth has heated up by one degree Celsius (1.6 degrees Fahrenheit), Britain's weather office said Monday, as greenhouse gases hit record levels just weeks before a crucial climate summit in Paris.
Other reports warned 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.
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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 1 million years," WMOchief Michel Jarraud told reporters.
"This means we are now really in uncharted territory for the human race," he warned.
‘Life on Earth threatened' Britain's Met Office announced Monday that local mean surface temperatures were set to reach 1 C above pre-industrial levels for the first time - half-way to the 2 C threshold which scientists say humanity must not cross.
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Environment and energy ministers in Paris are groping for convergence on issues dividing the 195 nations negotiating a climate rescue pact which must be inked at the upcoming summit.
A World Bank study Sunday said there could be "more than 100 million additional people in poverty by 2030″ unless action is taken to stem climate change.
French Foreign Minister Laurent Fabius, hosting ministers and climate envoys from 70 countries to prepare for the summit over which he will preside, issued his own dire warning.
"It is life on our planet itself which is at stake," he said at the start of talks Sunday.
"There is absolute urgency" to achieve the 2 C goal, said Fabius. Anything higher would have "catastrophic consequences" - including wars over dwindling water and other resources.
The summit will be opened by some 100 heads of state and government, among them US President Barack Obama, China's Xi Jinping, Narendra Modi of India, and Russia's Vladimir Putin.
It is meant to produce the first truly universal pact to rein in climate change by curbing emissions from burning fossil fuel. It is also tasked with helping poor countries adapt to climate impacts already in the pipeline.
Ministers are the ones who will ink the deal at the end.
But first, they must find political compromise to feed into a blueprint for the deal compiled by rank-and-file diplomats over years of tough negotiations in the UN climate forum.
Wrangling over finance, fairness For now, the draft remains little more than a laundry list of often directly opposing national options for dealing with the challenge at hand.
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Developing countries insist rich ones should lead the way in slashing emissions as they have polluted for longer.
They also want assurances of finance to make the shift from cheap and abundant fossil fuel to greener energy sources, and to shore up defences against climate change-induced superstorms, drought, flood and sea-level rise.
But industrialized countries point the finger at emerging giants such as China andIndia spewing carbon dioxide as they burn coal to power expanding populations and economies.
As the bickering continues, the UN last week issued a fresh warning that national carbon-curbing pledges submitted to date set the stage for warming of about 3 C, or more.
Last month, scientists said the first nine months of 2015 had been the hottest on record worldwide.