UN chief asks nations to accelerate efforts to join Paris pact

In the ceremony, Ban noted that another 29 countries representing 16 per cent of global emissions are required to bring the Paris Agreement into force.

"I am hopeful and optimistic that we can do it before the end of this year and before my term as Secretary-General of the United Nations ends," Ban said.

So far, a total of 26 countries, accounting for 39 per cent per cent of global emissions have now officially joined the Paris Agreement, which will enter into force 30 days after at least 55 countries, accounting for 55 per cent of global greenhouse gas emissions, deposit their instruments of ratification or acceptance with the Secretary-General.

Ban particularly lauded Obama for his leadership on climate change and working with other nations to reach a deal to cut greenhouse gas emissions.

"While we were waiting, he asked me what I want to do after retirement. Well, Mr President, I am going to write some memoir: one of the many important, inspiring thing will be my inspiring moment of President Obama's leadership (in Copenhagen)," Ban said.

"Have you ever imagined the President of the United States, holding his own laptop, going around the Heads of State sitting in a small chair, proposing a draft and changing it on the spot? I would have never expected that the US President would do that and I was so much inspired. He inspired everybody," he said.

On September 21, the Secretary-General will host a high-level event at the UN to provide an opportunity for countries to deposit their legal instruments and to publicly commit to joining the Paris Agreement before the end of 2016.

The event will build on the "strong political momentum" from Paris for urgent global action on climate change by all countries, which is essential for achieving the Sustainable Development Goals and creating a more prosperous, equitable and liveable future for all people, he said.

"I urge all leaders, particularly G20 countries, to accelerate their domestic ratification processes so we can turn the aspirations of Paris into the transformative climate action the world so urgently needs," Ban said.