Climate change is happening, it is almost entirely man's fault and limiting its impacts may require reducing greenhouse gas emissions to zero this century, the UN's panel on climate science has said.
UN Secretary-General Ban Ki-moon, who was at the news conference, stressed that "if we act now, immediately and decisively we have the means to build better and more sustainable world.
He later said: "Inaction of climate action will cost much, much more. This climate action and economic growth are two sides of just one coin".
We must act on climate change now, the Energy Secretary has said, after the publication of a major report on climate change.
Ed Davey, said: "It sends a clear message that should be heard across the world - we must act on climate change now.
"It's now up to the politicians - we must safeguard the world for future generations by striking a new climate deal in Paris next year."
The scientific case for prioritising action on climate change was clearer than ever, the chairman of the Intergovernmental Panel on Climate Change (IPCC) told a conference, after a new UN report was published.
We have little time before the window of opportunity to stay within 2C of warming closes.
To keep a good chance of staying below 2C, and at manageable costs, our emissions should drop by 40% to 70% globally between 2010 and 2050, falling to zero or below by 2100.
We have that opportunity, and the choice is in our hands.
Power generated from fossil fuels should be phased out completely by 2100 a new report has warned.
The United Nations climate body published the final report of its latest assessment on the science of climate change, drawing together three studies published in the past year.
In the majority of low?concentration stabilization scenarios, the share of low?carbon electricity supply (comprising renewable energy (RE), nuclear and Carbon Capture Schemes (CCS), including BECCS) increases from the current share of approximately 30% to more than 80% by 2050, and fossil fuel power generation without CCS is phased out almost entirely by 2100.
The report comes as efforts build towards securing a new global treaty on climate change, which it is hoped can be agreed in Paris at the end of next year, and it said that international co-operation was "critical" for effective efforts to tackle the problem.
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Protesters took to the streets of New York again this evening, as world leaders prepare to fly in for a summit on climate change.
Organisers say this meeting - and next year's in Paris - will be different and insist promises made will be kept. ITV News' Science Correspondent Alok Jha reports:
Greenpeace UK has said world leaders should aim for more than "just another sticking-plaster deal" on climate change.
Executive director John Sauven said: "Never has a generation of world leaders stood a better chance of clinching a global climate deal.
"This time there's enough momentum to aim for something better than just another sticking-plaster deal with a short shelf life.
"David Cameron now has the opportunity to argue for a similar system that can drive a global countdown towards zero emissions - he should seize it with both hands."
A coalition of environmental groups and charities have set out what they want to see from a global climate deal.
The group, including Greenpeace, WWF, the RSPB and Christian Aid, said an international agreement should include ambitious plans by countries for taking action both before and after 2020.
It should also provide a clear legal framework for delivering and monitoring greenhouse gas emissions cuts.
An agreement, which backers want to see signed at the end of 2015, establish a framework with rolling commitments to reduce emissions and support efforts to adapt to a changing climate, they added.
David Cameron should push other leaders to follow the UK's lead in setting a series of targets to drive down climate emissions, environmental groups have urged.
A new international climate deal, which backers want to see signed at the end of 2015, should follow the example of the UK's Climate Change Act, which includes a series of five-year "carbon budgets" for reducing greenhouse gases over time, they said.
They stressed that the Prime Minister should use the momentum building ahead of a climate summit in New York later this month to push for a strong international deal in 2015.