To fresh warnings and appeals to seize the political momentum, UN talks opened in Lima Monday tasked with drawing the outlines of a 2015 deal to roll back climate change.

Gathering 195 states, the 12-day meeting also has to agree on the pact's heart -- a format for nations to make pledges to reduce Earth-warming carbon pollution.

These national commitments would form the cornerstone of an unprecedented accord to be sealed in Paris in December 2015 and take effect by 2020.

"2014 is threatening to be the hottest year in history and emissions continue to rise. We need to act urgently," Christiana Figueres, head of the UN Framework Convention on Climate Change (UNFCCC), told delegates.

"We need to put on the table the draft of a new universal climate agreement," she said, urging country negotiators to "make history".

UN nations have vowed to limit global warming to two degrees Celsius (3.6 degrees Fahrenheit) over pre-Industrial Revolution levels.

Scientists say Earth is on course for roughly twice this amount by the end of the century -- a recipe for worse droughts, floods, storms and rising seas.

They warn that scant time is left to reduce heat-trapping emissions to safer levels.

"The more we disrupt our climate, the more we risk severe, pervasive and irreversible impacts," said Rajendra Pachauri, head of the UN's panel of climate scientists.

"But fortunately we have the means to limit climate change, to build a more prosperous, sustainable future."

The Earth League, an alliance of climate scientists, weighed in with an appeal for urgency.

"Without collaborative action now, our shared Earth system may not be able to sustainably support a large proportion of humanity in the coming decades," it warned.

Reaching the 2 C target is a political headache, requiring nations to crack down on energy inefficiency and switch from cheap but polluting fossil fuels to cleaner sources.

Negotiations have been bedevilled for years by rifts between rich and poor over who should shoulder the burden -- a row complicated by the rise of developing giants such as China, India, Indonesia and Brazil which are now massive carbon emitters.

Peruvian Environment Minister and conference chairman Manuel Pulgar-Vidal said it was time "to build bridges."

"We want this conference to create the kind of trust, the kind of opportunity and the kind of determination that we need to achieve the concrete agreement that the world needs."

About 10,000 delegates, activists, journalists and backroom staff have been accredited for the conference, with some 40,000 police providing security.

- Out of the doldrums -

Since September, top-level interest has hauled the climate issue out of the doldrums where it had lingered after a near-fiasco at a summit in Copenhagen in 2009.

At a summit in New York, UN Secretary General Ban Ki-moon coaxed world leaders into renewing vows to fight the scourge.

Since then, the three biggest emitters -- China, the United States and Europe -- have sketched plans for contributing to the carbon cleanup.

But the Lima talks must clear several hurdles.

Countries must hammer out a workable negotiating text for next year -- a draft that will likely still have big gaps on issues like the final accord's status under international law, and how pledges should be policed.

Negotiators must also agree on a clear and transparent way by which countries next year will report national pledges to reduce climate-damaging greenhouse gases.

Without this foundation of trust, the voluntary approach that became the UNFCCC's strategy after Copenhagen could founder.

"A strong rules base is essential for ensuring that parties can trust each others' commitments," said European Union negotiator Elina Bardram.

The EU, she said, also insisted that the 2015 accord include regular reviews to monitor headway to the 2 C goal.

"This type of process would allow parties to respond to the latest science and consider increasing ambition in the light of new technological developments," she told a press conference.

The world's most climate-vulnerable countries -- small island states and impoverished African countries -- are lobbying for the UNFCCC to uphold a tougher target of 1.5 C -- an issue which comes up for review in 2015.

Another big issue is funding for poor countries, which will be hardest hit by climate change but are least to blame for it.

Developing countries want rich economies to show in Lima exactly how they intend to honour promises to muster up to $100 billion (80 billion euros) in climate finance per year from 2020.

By last week, nearly $10 billion in startup capital had been promised for the Green Climate Fund (GCF), the main vehicle for channelling the money.