This is Melissa Clarke in Paris reporting for AM.  

MICHAEL BRISSENDEN: World leaders have spoken in dire terms about the challenge global warming poses at the start of a UN climate change conference in France.

Both the host, President Francois Hollande, and US President Barack Obama say it's crunch time when it comes to the negotiations on reducing greenhouse gas emissions.

Prime Minister Malcolm Turnbull has come to the talks with some announcements to show Australia is serious about tackling the issue.

He's signing Australia up to the second Kyoto Protocol and tipping more money into research and development for renewable energy.

Melissa Clarke reports from Paris.

MELISSA CLARKE: One of the most anticipated international conferences in recent times opened in modest fashion.

FRANCIS HOLLANDE: It gives me great pleasure to declare open the 21st session of the conference.

MELISSA CLARKE: But there was no shortage of bold pronouncements.

FRANCIS HOLLANDE (translated): Everything depends on us. Ladies and gentlemen, heads of state and governments, the hope of all humanity rests on your shoulders.

MELISSA CLARKE: Francois Hollande, the French President, welcomed the heads of state and appealed for more than just "good wishes" and "expressions of intent".

FRANCIS HOLLANDE (translated): We are going to decide in a few days the future for several decades. The greatest danger is not that we aim too high and that we miss. The greatest dangers is that we aim too low and that we hit it.

MELISSA CLARKE: US President Barack Obama noted the pressure and urged action, too.

BARACK OBAMA: And what should give us hope that this is a turning point, that this is the moment we finally determined we would save our planet, is the fact that our nations share a sense of urgency about this challenge and a growing realisation that it is within our power to do something about it.

MELISSA CLARKE: The positivity seemed to be infectious.

PRIME MINISTER: From Australia we come with confidence and optimism.

MELISSA CLARKE: Malcolm Turnbull delivered his leader's statement with Kevin Rudd looking on in the audience and he shifted Australia's climate change policy.

PRIME MINISTER: And today I announce that Australia will ratify the second commitment period of the Kyoto Protocol.

MELISSA CLARKE: It's a largely symbolic move, but one designed to show that Australia is serious about these negotiations, which will succeed Kyoto.

There's a bonus for the Government. As it will exceed its emissions reduction targets under the second Kyoto Protocol, it will be able to bank that to be counted in the next deal that's struck.

The Prime Minister also announced Australia's aid spending will increasingly focus on tackling climate change and a doubling of funding for research and development into renewable energy, with universities to get $200 million over five years.

PRIME MINISTER: Because we believe that we have as a global community, as humanity, the ability innovate imagine the technologies that will enable us to make these big cuts in green house gas emissions...

OPPOSITION LEADER: Well I think that it was, it's a pretty safe statement they've made. It's not really setting the world on fire, is it?

MELISSA CLARKE: Opposition Leader Bill Shorten isn't impressed by what he's seen from the Prime Minister in Paris.

OPPOSITION LEADER: Back in 2009 he said that he didn't want to lead a political party which wasn't serious about climate change. Now he's doing exactly that.

MELISSA CLARKE: Australia won't be signing onto a separate New Zealand-led initiative to eliminate fossil fuel subsidies.

Malcolm Turnbull says it's not out of a need to protect the politically sensitive diesel fuel rebate but rather a dispute with the technicalities.

PRIME MINISTER: The document that our very good friend John Key has prepared contains a reference in it to an IMF report, which frankly would be better that it were not there.

It's a gratuitous reference to an IMF report which goes much, much further than inefficient fuel subsides.

MELISSA CLARKE: The Prime Minister is trying to balance his desire to see global progress on climate change with his domestic political needs - a challenge every nation's leaders faces over the next two weeks of negotiations.

This is Melissa Clarke in Paris reporting for AM.