Negotiators on Monday launched a new round of UN climate talks to warnings that a newly breached threshold was a wakeup call to tackle surging carbon emissions.
The 12-day talks under the UN Framework Convention on Climate Change (UNFCCC) seek to breathe life into a quest to forge a pact on heat-trapping greenhouse gases.
Troubled by bickering, nit-picking and defence of national interests, the process coincides with the rise of carbon dioxide (CO2) to historic levels.
UNFCCC chief Christiana Figueres told delegates that US scientists in Hawaii last month had detected more than 400 parts per million (ppm) of CO2 in the atmosphere for the first time in human history.
"I do not need to remind you that we simply cannot afford not to deliver implementation results urgently," Figueres said.
After discussions among two technical bodies on Monday, the talks get down to tougher business on Tuesday.
They will try to flesh out commitments for the deal, expected to be signed in late 2015, and to beef up action in the years before the pact takes effect in 2020.
The negotiations lead up to the UNFCCC's annual minister-level talks, which this year will take place in Warsaw from November 11 to 22.
Political interest on tackling climate change at a global level peaked in the runup to the 2009 Copenhagen Summit.
But the low-level compromise that was brokered there, amid scenes of chaos and finger-pointing, has dashed expectations that the UN forum can do very much.
In the meantime, carbon emissions are reaching new record highs, driven by emerging countries that are devouring coal, oil and gas to power their economic growth.
The 195 UNFCCC parties have pledged to limit warming to two degrees Celsius (3.6 degrees Fahrenheit) compared to pre-industrial temperatures, when CO2 levels were 270 ppm.
But carbon concentrations are escalating at around two or three ppm each year, placing Earth on track for warming of as much as two degrees C (nine degrees F) by century's end, a figure that many scientists say would be catastrophic.
According to some opinions, emissions must peak within the next few years, and then decline sharply afterwards in order to reach safer levels.
"We have already seen real changes to rainfall patterns, to crop yields, increases in storm surges and droughts that are starving and parching vulnerable people across the globe. These impacts are the heralds of a planetary emergency," said Harjeet Singh with the group ActionAid.
In an interview in Manila with AFP, Greenpeace chief Kumi Naidoo said he saw "no reason to have even any sense of optimism" that momentum had improved since the last big UNFCCC meeting, held in Doha, Qatar, last December.
"The UN negotiating system is cumbersome, it is problematic, it is tedious. However, the UN, warts and all, is the best and only vehicle we have to ensure we can move forward as a united global family to address climate change."
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