Earth Day: World leaders sign historic climate change agreement

World leaders gather in New York today to sign the Paris agreement on climate change, as more than a billion people around the world show their support for environmental protection in events to mark Earth Day.

UN secretary general Ban Ki-moon, French President Francois Hollande, Canadian Prime Minister Justin Trudeau and Italian premier Matteo Renzi will be joined by representatives from more than 160 nations to sign an agreement that will limit global warming to two degrees Celsius.

The United Nations has said this will be the biggest signing of an international treaty in history, surpassing the previous record of 119 signatures held by the opening day signing of the Law of the Sea treaty in 1994.

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The pact was agreed upon at the COP21 conference in Paris in December last year and comes into force only when it has been accepted or ratified by at least 55 countries accounting for least 55 per cent of total global greenhouse emissions.

"In theory, the Paris agreement could make a huge difference. In practice, everything depends on countries not just signing up to it, but acting on it," says ABC News.

Ban Ki-moon has said: "Paris was historic, but it's only the beginning," adding: "We must urgently accelerate our efforts to tackle climate change."

The importance of ratification by the US and China, the world's two biggest carbon emitters, cannot be overstated. With the agreement likely to be the "world's costliest-ever accord", no doubt "American presidential candidates will use the spectacle to make hay", says USA Today.

In line with President Obama, Democratic frontrunner Hillary Clinton has said the treaty is a "historic step forward" against "one of the greatest challenges" of our age. Her potential Republican rival for the White House, Donald Trump, has threatened to scupper the deal altogether, stating he is not a "great believer" in the theory of man-made climate change.