Ministers and the UN chief fly into Lima this week to bolster negotiators in a final push for consensus on key elements of a world pact to curb potentially disastrous global warming.
With a week of talks gone, and five days left, parties remain deeply divided on key aspects of the deal they have committed to signing in Paris in December 2015, to take effect in 2020.
As Typhoon Hagupit pummeled the eastern Philippines over the weekend, negotiators were reminded of the "planetary emergency" looming.
"Essentially, if we continue as we are, we may drastically rewrite the relationship between humans and the planet, potentially, leading to the mass migration of perhaps hundreds of millions or billions of people," climate economist Nicholas Stern warned in a new report.
"History tells us this could result in long and sustained conflict. These are the stakes we are playing for."
The UN has set a target of curbing average global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels.
The goal must be met by deep cuts in soaring emissions of greenhouse gases -- requiring a costly shift from cheap and abundant fossil fuels to less polluting energy sources.
NGOs, activists and scientists observing the talks in Lima focused on sharing out the emissions budget said the pace was too slow.
"Unfortunately, the negotiators... seem to have forgotten that they are here to solve a planetary emergency," said Tasneem Essop of green group WWF.
Voltaire Alferez of the Philippines NGO Aksyon Klima added: "As we speak, people are paying for our leaders' lethargy with their lives and livelihoods."
The Lima talks have two main tasks: drafting a negotiating outline for the Paris deal, and agreeing on the format of carbon-curbing pledges that nations have committed to submit from the first quarter of next year.
But negotiators do not see eye to eye on some of the very basics: Will the pledges be legally binding on rich and poor nations alike? Must rich nations commit in writing to financial support for climate adaptation in the developing world? Will pledges be assessed for adequacy?
"None of this has been settled," French negotiator Laurence Tubiana told AFP, but added this was "normal" in climate diplomacy -- known for a poker-like approach with nations holding out until the very last minute.
After a free day, negotiators get back to work on Monday, hoping the arrival Tuesday of ministers and UN Secretary General Ban Ki-moon will provide a momentum boost.
- Red lines -
Ban, who hosted a leaders' summit in New York in September that yielded vows of renewed political commitment, will on Tuesday open the "high-level segment" of the talks and meet ministers separately.
Rich nations, including the United States, want the deal to focus on mitigation -- meaning emissions curbs -- but poor, developing and small island states at high risk of climate change-induced sea-level rise, demand guarantees of global support for adaptation to climate risk, and compensation for unavoidable loss and damage.
"From a developing country perspective... our red line is that the post-2020 agreement has to deal with adaptation," South African negotiator Judy Beaumont said.
"The impacts of climate change are already being felt, particularly in developing countries, and so therefore we already have to be building our capacity for adaptation."
On Friday, a UN report said developing nations may need as much as $500 billion per year by 2050 for adaptation.
Another sticking point is assessment of national pledges and their global impact on the two-degree Celsius goal, with emissions giant China emerging as a strong opponent last week.
But Beaumont said many other developing nations consider assessment a crucial part of the process.
"You want to make sure that what is put on the table... adds up and is adequate to meet the global temperature goal," she said.
If not, "then multilaterally we've got to take a decision on what we do about that."
Ministers are scheduled to meet Tuesday on the other divisive issue: climate finance for developing nations.
Scientists say the world is on target for four degrees Celsius, or more, with a resulting increase in extreme events like hurricanes and storms, sea-level rise, floods, droughts and desertification.