World leaders are gathering in Paris amid tight security for a critical UN climate meeting.

The 21st session of the Conference of the Parties, known as COP21, starts on November 30 and will try to craft a long-term deal to limit carbon emissions.

Observers say that the recent terror attacks in Paris will increase the chances of a new agreement.

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

The gathering of 147 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.

While many leaders including Presidents Barack 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.

Unlike at Copenhagen, the French organizers 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.

Delegates are in little doubt that the shadow cast over the city by the attacks will enhance the chances of agreement.

While the mood music around the event is very positive, there are still considerable differences between the parties.

One key problem is what form an agreement will take. The US for instance will not sign up to a legally binding deal as there would be little hope of getting it through a Senate dominated by Republicans.

“We’re looking for an agreement that has broad, really full participation,” said US lead negotiator Todd Stern at a news briefing earlier this week.

“We were quite convinced that an agreement that required actually legally binding targets would have many countries unable to participate.”

Many developing countries fundamentally disagree, as does the European Union.

“We must translate the momentum we have seen on the road to Paris into an ambitious, operational, legally binding agreement,” said EU commissioner Miguel Arias Canete, in a statement.

As well as the form there are also many issues with the content.

There are a wide range of views on what the long-term goal of the agreement should be.

While it will ostensibly come down to keeping temperatures from rising more than 2C above the pre-industrial level, how that will be represented in the text is the subject of much wrangling.

Some countries reject the very notion of 2C and say 1.5C must be the standard. Others want to talk about decarbonising the world by the middle or end of this century.

For major oil producers the very idea is anathema.

While the fact that more than 180 countries have put forward national plans to cut emissions is a major strength of this conference, there are still big questions marks about how to verify those commitments that will actually be carried out.