The Paris deal unites all the world's nations in a single agreement on tackling climate change for the first time in history.
Environmental experts today struck a note of caution on the path-breaking climate deal. They said that it may not be the panacea to the problem of reducing use of fossil fuels but is the "starting gun" for the race towards a low-carbon future.
While agreeing that it is a turning point, they said several difficulties will crop up in implementing the deal.
Once implemented, this would bring down greenhouse gas emissions to net zero within a few decades.
Christopher B Field, a leading American climate scientist, said, "I think this Paris outcome is going to change the world. We didn't solve the problem, but we laid the foundation."
Hans Joachim Schellnhuber, a pioneering environmental scientist, chairman of the German government's advisory committee on climate change and climate adviser to Pope Francis, was quoted by the New York Times as saying "this is a turning point in the human enterprise, where the great transformation towards sustainability begins."
The deal unites all the world's nations in a single agreement on tackling climate change for the first time in history.
Pledges thus far could see global temperatures rise by as much as 2.7 degrees Celsius, but the agreement lays out a roadmap for speeding up progress.
The key element of the deal is to keep global temperatures "well below" 2 degrees Celsius and an "endeavour to limit" them even more to 1.5 degrees Celsius.
WWF-UK Chief Executive David Nussbaum said, "Paris is just the starting gun for the race towards a low-carbon future."
Terming the climate deal as "weak and unambitious", Delhi-based Centre for Science and Environment (CSE) said it does not include any "meaningful" targets and has discharged developed nations from their historical responsibility.
"Developing countries have got words and promise of money while the developed countries have finally got rid of their historical responsibility of causing climate change.
They have no legally binding targets on finance or emissions cuts," CSE director general Sunita Narain said.
Unfazed by the euphoria over the deal, Greenpeace International said it is "only one step on a long road".
"Today the human race has joined in a common cause, but it's what happens after this conference that really matters.
The Paris Agreement is only one step on long a road, and there are parts of it that frustrate and disappoint me, but it is progress. This deal alone won't dig us out the hole we're in, but it makes the sides less steep," Greenpeace International executive director Kumi Naidoo said.
The Climate Group and its business and sub-national government partners from the US, Europe, China and India called today's climate agreement "a victory for science and vision which calls time on the fossil fuel age".
The Kyoto Protocol of 1997 set emission cutting targets for a handful of developed countries, but the US pulled out and others failed to comply.
The Paris pact aims to limit the amount of greenhouse gases emitted by human activity to the same levels that trees, soil and oceans can absorb naturally, beginning at some point between 2050 and 2100, experts said.
It also seeks to review each country's contribution to cutting emissions every five years so they scale up to the challenge.
The deal asks rich countries to help poorer nations by providing "climate finance" to adapt to climate change and switch to renewable energy.
Developing nations have been promised $100 billion a year by 2020 -- not as much as many countries would like.
The agreement requires rich nations to maintain a $100 billion a year funding pledge beyond 2020, and to use that figure as a "floor" for further support agreed by 2025.
For the first time, the accord lays out a longer-term plan for reaching a peak in greenhouse emissions "as soon as possible" and achieving a balance between output of man-made greenhouse gases and absorption -- by forests or the oceans -- "by the second half of this century".
"If agreed and implemented, this means bringing down greenhouse-gas emissions to net zero within a few decades. It is in line with the scientific evidence we presented," John Schellnhuber, Director of the Potsdam Institute for Climate Impact Research, said.
Dr Ilan Kelman of UCL, London, was quoted by BBC as saying the lack of time scales are "worrying".
"The starting point of $100 billion per year is helpful, but remains under 8 per cent of worldwide declared military spending each year," Mr Kelman said.
Only elements of the Paris pact will be legally binding.
The national pledges by countries to cut emissions are voluntary, and arguments over when to revisit the pledges -- with the aim of taking tougher action -- were a stumbling block in the talks.
The pact promises to make an assessment of progress in 2018, with further reviews every five years.
Prof John Shepherd of the National Oceanography Centre, University of Southampton, was quoted as saying the agreement includes some welcome aspirations but few people realise how difficult it will be to achieve the goals.
Scientists who closely monitored the talks here said it was not the agreement that humanity really needed. By itself, it will not save the planet.