World leaders from nearly 150 nations gathered in Paris on Monday (Nov 30) to launch one of the most important environmental gatherings in a generation.

They were greeted at the 12-day summit by French President Francois Hollande and United Nations Secretary General Ban Ki Moon, AFP reported.

Negotiators have vowed to forge an ambitious deal to honour the 130 people killed in the Nov 13 bombing and shooting attacks that shook the French capital. They will hold a minute of silence to remember the victims when the event officially opens at 11am (6pm Singapore).

"The fate of humanity is at stake in this conference. After the attacks in France, we have to deal with the urgent priorities and respond to the terrorist challenge but also act for the long term," Hollande said.

At stake is a deal to step up quickly the global fight against climate change, with the world at greater risk than before from more extreme weather and rising sea levels as the planet heats up. The leaders are in Paris to ensure negotiators reach a deal and to show solidarity with the French people after the recent terror attacks on their capital.

Over the next two weeks, delegates from nearly 200 nations will negotiate the final elements of the new global pact. If reached, the deal, more than two decades in the making, would commit rich and poor nations to cutting greenhouse gas emissions. Just as importantly, a strong pact will give investors a powerful signal to escalate investment in cleaner sources of energy.

But risks remain, with disputes over cash flow to poorer nations to help them tackle climate-related issues among the hot-button issues that could bog down the talks.

In the run-up to Paris, the scientific evidence of climate change has grown ever stronger, with 2015 set to be the hottest year on record, the UN World Meteorological Organisation said last Wednesday (Nov 25).

"The evidence of global warming and climate change is in front of us. It is today," Mr Marco Lambertini, director-general of global conservation group WWF International, said at a recent media briefing.

The United Nations and the French government are hoping to prevent a repeat of the 2009 Copenhagen climate conference, which failed to agree on a global deal because of deep mistrust between wealthy and poorer countries.

Since then, the UN has worked to rebuild trust and win agreement for a new pact that commits all nations to emissions cuts based on what they feel they can achieve.

Ahead of Paris, more than 170 nations submitted to the UN their individual climate action plans, called INDCs, which will form the foundation of the new pact. Paris is meant to launch a new process with the idea being national action plans and climate finance for poorer nations can be strengthened over time.

The talks' ultimate aim is to slow the pace of global warming caused by the release of greenhouse gases, such as carbon dioxide, from burning coal, oil and gas as well as from deforestation and agriculture.

Scientists say the world needs to stay within a warming of 2 deg C to stand a fair chance of avoiding damaging weather extremes and rising sea levels that would eventually swamp coastal megacities from Manila to Shanghai to New York.

The current pledges commit the world to a 2.7 deg C rise, the UN has calculated, so deeper cuts will be needed, and soon. In Paris, negotiators are expected to agree on a system that conducts regular reviews of national pledges and ensures transparency, such as regular reporting, measurement and verification of actions to ensure no backsliding.

"Paris is different from the other climate meetings. It has the potential, if all goes to plan, to launch a new, more positive era for the negotiations," said Adjunct Professor Howard Bamsey of the Climate Change Institute at the Australian National University in Canberra.

"You can make a credible claim that this is a pathway to the 2 deg C objective," he said of the planned system of review and ratcheting up.

Plenty of risks remain for the talks, particularly in finance.

"There will be an expectation on developed economies to step up and show how they are scaling up their finances through time," said Mr Erwin Jackson, deputy chief executive officer of The Climate Institute in Sydney.

Some worry that the Paris agreement will be weak because it will rely on voluntary climate actions by each nation, without punitive sanctions if those targets are not met.

There is also concern Paris will leave too much work for later negotiations, particularly on finance and transparency of emissions cuts.

"The concern is that countries are unable to resolve these issues and a weak agreement emerges, leaving substantive work to be done in future meetings," said Dr Ho Juay Choy, Adjunct Professor at the Energy Studies Institute, National University of Singapore.

For now, an agreement does seem likely after the intense diplomatic efforts by the French and the UN. Leaders are also likely to show solidarity and support for France in the wake of the terror attacks.

"Countries might feel a sense of responsibility to get a successful outcome," said Ms Melissa Low, a research associate at the NUS's Energy Studies Institute.