A deal to slow climate change being thrashed out in Paris fails to map out steep enough cuts in carbon dioxide emissions to limit global warming to the target of at least "well below" 2 degrees Celsius (3.6 Fahrenheit), scientists said yesterday.

Negotiations on the draft agreement to curb greenhouse gas emissions, blamed for warming the planet and disrupting the climate, were extended by a day on Friday to Saturday to try to overcome stubborn divisions among the 195 countries taking part.

The draft text, released on Thursday and subject to revision, also proposes that emissions peak "as soon as possible", with rapid cuts thereafter towards achieving "greenhouse gas emissions neutrality in the second half of the century".

Neutrality refers to all greenhouse gases, not just carbon dioxide, and means net zero man-made emissions from all sectors.

Overall emissions would need to be reduced to as close to zero as possible and any remaining would have to be soaked up by forests and soils or buried underground by costly technology such as carbon capture and storage.

"This is wishful thinking. You might call it pie in the sky," Hans Joachim Schellnhuber, head of the Potsdam Institute for Climate Impact Research, told Reuters. He said emissions neutrality would have to be reached by 2050 to achieve the 1.5C goal, yet the text was too vague by talking about the second half ofthe century - up to 2099.

To meet a 2C limit, global emissions would have to peak by 2020 with net zero emissions of carbon dioxide by 2070, according to the UN panel of climate scientists.

Current national emissions cut plans put the planet on a far higher path, unless the world could abruptly shift to "negative emissions", such as soaking up greenhouse gases from nature after 2030 with new technologies, Schellnhubersaid.

So far, more than 180 nations have put forward plans to cut emissions but they put the world on a path to warming anywhere from 2.7Cto3.7C, according to studies.

Scientists also said the language was weaker than in previous drafts.

Meeting a 1.5C limit would require higher energy prices to spur investment in cleaner energy sources, bioenergy and carbon capture and storage (CCS) technology, which captures carbon dioxide and stores it underground. CCS technology is still small scale and very costly. There are currently 15 projects in operation worldwide.

The International Energy Agency has said that by 2040, four billion tonnes of carbon dioxide emissions must be captured to keep global warming at bay, which is 100 times more than the total CCS projects expected to be online in the next 18 months.