Nearly 150 world leaders are expected to descend on Paris for the start of the U.N. climate change summit that starts Monday in the French capital with the aim of reaching a landmark global deal on limiting greenhouse gas emissions.

The leaders of the United States, China and India — the world’s top three carbon-emitting countries — are among those scheduled to attend the opening day of the event, known as COP21.

Canadian Prime Minister Justin Trudeau arrived in the French capital Saturday night for the COP21 climate conference. Before leaving Malta, where Trudeau had taken part in his first Commonwealth leaders’ summit this week, the prime minister said he’s “very, very optimistic” that a new global agreement on fighting climate change is achievable.

“Leaders are massively expressing they are even more enthused and committed to attending the climate change conference in Paris because of the opportunity it offers to also stand in solidarity with Paris and with the people of France,” he said. Trudeau says Canada will do its part.

French President Hollande told the 53 countries represented in Malta that mankind has a moral duty to fight climate change as much as terrorism.

India’s Modi took a pass on the Malta summit, although his country the most populous in the Commonwealth. It’s also seen as a significant impediment to a global climate pact, given India’s refusal to rein in its greenhouse gas emissions. India, already the world’s third-largest emitter, has said its emissions could triple over the next 15 years as it expands coal-fired electricity generation to power a developing economy.

Around 40,000 people are expected to participate in the event, which runs until 11 December.

The gathering of 150 heads of state and government is set to be far bigger than the 115 or so who came to Copenhagen in 2009, the last time the world came close to agreeing a long-term deal on climate change. Unlike at Copenhagen, the French organisers are bringing the leaders in at the start of the conference rather than waiting for them to come in at the end, a tactic which failed spectacularly in the Danish capital.

While many leaders including Presidents Obama and Xi Jinping were always set to attend this conference, the recent violent attacks in Paris have encouraged others to come in an expression of solidarity with the French people.

The Conference is being held amid heavy security after the deadly terrorist attacks that struck Paris two weeks ago. French authorities have clamped down on public demonstrations in the aftermath of the attacks, blocking environmental campaigners’ plans for a big march on Sunday in Paris to highlight the climate change issue.

What is COP21?

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.

Security for the summit

  • Following the attacks, border controls temporarily imposed. They were set to begin on 30 November anyway
  • Almost 1,000 people considered security risks refused entry since 13 November
  • 8,000 police and gendarmes to carry out border checks
  • 2,800 of them at the conference venue at Le Bourget north of Paris
  • 120,000 police and troops already mobilised across France since the attacks
  • Sale and transport of flammable materials such as gas cylinders, domestic solvents, barbeque firelighters and firecrackers banned until 13 December
  • Some major roads to be closed for two days