Former Prime Minister Tony Blair has called for a "global environment revolution" to tackle climate change.
Mr Blair is on a visit to Japan to discuss global greenhouse gas targets.
In a speech to a meeting of G8 ministers building on the 2005 Gleneagles summit, he stressed the need for a "global deal".
He suggested it should be led by the UN and that failure to act on climate change "would be deeply and unforgivably irresponsible".
During his visit, organised by Climate Group, Mr Blair is due to meet climate change experts from China, Japan, Europe and the US.
He is attempting to guide attempts to secure a deal involving China and the US to slash emissions by 50% by 2050, on the first part of a trip that will also take him to China and India.
He said: "Unfortunately the source of the emissions is irrelevant. It is the fact and amount of them that matters.
"The UN machinery is valiantly striving to put this deal together. The UN and the UN alone is the right forum to reach the global agreement.
"What I found, whilst still in office as prime minister, was that countries had their own environmental policy. They talked to other nations of course, but there was no centre where it was brought together."
He also said that he could "see no way of tackling climate change without a renaissance of nuclear power".
Mr Blair, who stood down as prime minister last year, is also a peace envoy to the Middle East Climate Group for the "quartet" of the EU, Russia, the US and UN.
He also works as an advisor to investment bank JP Morgan and insurer Zurich.
Last week it was announced he would run a seminar on faith and globalisation at Yale University in the US.
In February Mr Blair said he would work to attract investment to Rwanda, as the central African country rebuilds its economy following the genocide of the mid-1990s.