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.
Kofi Annan has said that developed countries had failed to respond to the crisis until it reached their shores.
The former UN secretary general told the BBC's Newsnight:
I am bitterly disappointed by the response. I am disappointed in the international community for not moving faster. In this world we are in it together.
If the crisis had hit some other region it probably would have been handled very differently. In fact when you look at the evolution of the crisis, the international community really woke up when the disease got to America and Europe. And yet we should have known that in this interconnected world it was only a matter of time.
I point the finger of blame at the governments with capacity ... I think there's enough blame to go around. The African countries in the region could have done a bit more, they could have asked for help much faster and the international community could have organised ourselves in a much better way to offer assistance. We didn't need to take months to do what we are doing today.
Victoria Beckham was the new face on the United Nations stage in New York today.
The pop star turned UN goodwill ambassador talked about the plight of babies born with the HIV virus in South Africa.
ITV News' Rebecca Barry reports:
British actress Emma Watson called for more feminists in society and called upon men to promote gender equality in a speech at the weekend.Read the full story ›
A United Nation human rights official has said that the actual number of people killed in the conflict in eastern Ukraine "may be significantly higher" than previously reported.
Politicians should not act as "cheerleaders" for the intelligence and security services, according to former Liberal Democrat leader Lord Ashdown.
Writing in The Observer newspaper he said the Conservative Party had used the decision to raise the terror threat level "to tell us how frightened we should be and why this required a range of new powers for them to exercise".
He added that the new range of executive powers "could endanger our liberties."
He said: "Of course, in these circumstances, the police and the security services will lead the clamour for more powers. They are charged by us to maintain our safety."
He added: "But it is the job of politicians to act, not as cheerleaders for those demands, but as jealous protectors of our liberties who measure any demand for their reduction against necessity, supported by evidence."
Islamist militants have reinforced the siege against 40 Filipino peacekeepers trapped in Syria's Golan Heights, according to diplomatic sources at the United Nations.
More than 20 vehicles carrying armed fighters were seen approaching the region which has been demilitarized and monitored by the United Nations Disengagement Observer Force since Israel and Syria signed an armistice in 1974.
Earlier, nearly three dozen peacekeepers who were trapped by militants in the Golan Heights were successfully extracted, according to UN officials.