Opposition leader Bill Shorten's decision to float a possible 45 per cent cut in Australia’s carbon emissions by 2030 is a "mad" thought bubble, according to Coalition frontbencher Christopher Pyne.

Mr Shorten unveiled the plan today along with a pledge to work towards carbon neutrality — meaning all emissions would need to be offset — by 2050.

"The Climate Change Authority recommended a baseline emissions reduction of 45 per cent by 2030, on 2005 levels," Mr Shorten told the Lowy Institute.

"Today I announce Labor will use the Climate Change Authority's recommendation of a 45 per cent reduction as the basis for our consultations with industry, employers, unions and the community."

Labor will announce a final policy next year.

Mr Shorten is also promising a review of his party's long-term climate change goals every five years.

Industry Minister Christopher Pyne claimed such a target would cause a jump in electricity prices.

"It is a mad policy," Mr Pyne told Channel Nine.

"Bill Shorten's policy, his thought bubble, 45 per cent reduction, would require them to introduce or reintroduce a carbon tax at double the rate of the carbon tax before.

"He wants to smash household budgets and smash the economy."

Environment Minister Greg Hunt said almost halving emissions by 2030 would come at huge cost.

"That target, which comes from a Climate Change Authority report, which was accompanied by a Treasury assessment, comes with an over $600 billion cost," he told PM.

Labor rejects the figure, but public policy expert Warwick McKibbin said it was a "reasonable" estimate.

But Mr Shorten denied the policy was "mad".

"You know what madness is? Knowing what's happening and refusing to do something about it," he said.

"Madness is when you know the temperatures are rising, that CO2 pollution, greenhouse heat-trapping gases enfolding our planet, increasing the temperature of our planet.

"Madness is knowing the science and refusing to act."

Turnbull optimistic global agreement will be reached in Paris

Prime Minister Malcolm Turnbull said he is "optimistic" about the chances of a global agreement being reached at next week's climate change talks in Paris.

Global leaders are set to descend on the French capital to try to achieve a legally binding agreement aimed at keeping global warming below 2 degrees Celsius.

Mr Turnbull will promote Australia's policy to cut emissions by 26 to 28 per cent on 2005 levels by 2030.

"I am optimistic ... and I notice that the French president is very optimistic," he told 7:30.

Similar meetings have failed to result in agreement but Mr Turnbull hopes Paris will be different.

French president Francois Hollande will make a guest appearance at a special session on climate change at the Commonwealth Heads of Government meeting in Malta in the lead up to the Paris talks.