A critical UN conference aimed at agreeing a new global approach to climate change is set to open in Paris.
Negotiators from 195 countries will try to reach a deal within two weeks. Leaders from 147 nations will address the meeting, known as COP21, on Monday.
But the world’s poorest countries say they fear being “left behind” in the push for a new treaty.
The French government will officially take over the running of the talks during Monday’s opening ceremony.
Police have locked down the conference centre in Le Bourget, closing roads amid strict security for the leaders’ visit.
Presidents and prime ministers will address the gathering amid a growing sense of optimism that an agreement can be secured.
“It will be the turning point, which is what the world requires,” said French Foreign Minister Laurent Fabius at the weekend. Mr Fabius will chair the conference until it reaches a conclusion.
UN climate conference 30 Nov – 11 Dec 2015
COP 21 – the 21st session of the Conference of the Parties – will see more than 190 nations gather in Paris to discuss a possible new global agreement on climate change, aimed at reducing greenhouse gas emissions to avoid the threat of dangerous warming due to human activities.
COP21 live: The latest updates from Paris
Explained: What is climate change?
In video: Why does the Paris conference matter?
Analysis: From BBC environment correspondent Matt McGrath
More: BBC News special report (or follow “UN Climate Change Conference” tag in the BBC News app)
Boost for clean energy
The leaders, who will only stay at the meeting for one day, are likely to make a number of significant announcements to step up the fight against rising temperatures.
Details emerged on Monday of new plans designed to boost investment in clean technology. France and India will announce a global alliance that aims to bring together 100 solar-rich countries in tropical regions to rapidly expand the availability of electricity from the Sun.
An initiative involving at least 19 governments and private individuals including Bill Gates and Facebook founder Mark Zuckerberg will see up to $20bn (£13bn) of public and private funds poured into the development of low cost, clean energy projects, every year from 2020.
Mr Gates is expected to put up $1bn of his own money while governments including the US, China, Saudi Arabia, India, Brazil and the UK are expected to promise to double their spending on low carbon technology over the next five years. The US is expected to increase its spending on clean technology research to $10bn per year.
The private finance is said to be contingent on the public funds becoming available.
Meanwhile a number of European nations, working with the World Bank, announced a $500m fund designed to help developing countries cut their carbon emissions. Germany, Norway, Sweden, and Switzerland are backing what’s termed the Transformative Carbon Asset Facility.
This will measure and pay for emission cuts in areas like renewable energy, transport, energy efficiency, solid waste management, and low carbon cities. According to the World Bank it could make payments for carbon cuts to countries that remove fossil fuel subsidies.
‘Catastrophic’ temperature rise
But among the warm words and good intentions, there are growing concerns among the very poorest countries that their interests might be sacrificed in the clamour for compromise.
Most of the discussions here will revolve around a new deal that would limit global warming to 2C.
Assessments of the more than 180 national plans that have been submitted by countries suggest that if they were implemented the world would see a rise of nearer to 3C.
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However, the 48 members of the least developed countries (LDC) group at these talks say that for them, anything more than 1.5 degrees would be catastrophic.
“For the LDCs, economic development, regional food security, ecosystems, and the very survival of their populations and livelihoods are at risk if talks aim only for a 2C world,” said Giza Gaspar Martins from Angola.
“The heads of state will be in Paris to set the tone for the negotiations. We renew our call for an ambitious, robust and binding climate deal that does not leave behind the most vulnerable among us.”
While the arrival of the leaders will give a significant boost to the conference, the practical difficulties of securing a deal have not gone away.
At present the negotiating text runs to more than 50 dense pages, filled with brackets, indicating disagreement.
Teams of negotiators began the work on Sunday conscious of the fact that so many issues remain unresolved.
The hope is that, by the end of this week, a new draft agreement will be ready for environment ministers to haggle over during the second half of the conference.
One of the biggest disagreements is said to be over what is termed “differentiation”.
The US and other wealthy countries object to the fact that in these negotiations, a country is determined to be developed or developing based on its wealth when this body was formed back in 1992.
They argue that any new deal must accurately reflect the current position, meaning that a larger number of countries would have to share the burden of cutting carbon.
Follow Matt on Twitter @mattmcgrathbbc