Former United Nations Secretary-General Ban Kim-moon said Thursday the 2015 signing of the Paris agreement on climate change was the proudest moment of his 10-year tenure, defending the often-questioned UN's role in solving complex global problems.

"I fought tirelessly to ensure that climate change stays at the top of the leaders' agenda and launched various initiatives to fight climate change on the ground. And finally the Paris Agreement was adopted in December 2015, concluding 20 years of climate negotiations," Ban said.

He was speaking at an event to mark the release of the Korean-language edition of the US-published book on Ban's decade-long term, 'Furthering the Work of the United Nations; Highlights of the Tenure of Secretary-General Ban Ki-moon 2007-2016.' 

"Many people around the world assess that the climate change accord is the greatest achievement of my 10 years as secretary general. This is by any standards, one of the proudest moments for me," he noted. "I have exerted all my energy and passion to achieve the climate change agreement ... we must ensure that the commitments are kept."

During the initial period of his term, UN senior staff warned Ban that his climate change initiative may fail since it is too complicated and politically risky, he said. "At that time, I was saying that we do not have a plan B, because we do not have a planet B." 

The former South Korean foreign minister also stressed the indispensability of the world body in dealing with global issues. 

"The world is yet to be peaceful ... Terrorism and radicalism continue to expand their scope of activity. The role of the UN is more demanding than ever," Ban said.  

"Many people are questioning whether the UN has been an effective and appropriate body in dealing with the complexities of the world today ... I strongly believe the answer is yes," Ban said. 

"I have always been told that if we abolish the UN today the world will have to form another UN tomorrow ... This is because the UN is the only institution that pursues the most common values and ideals of the world and is the only common platform where each and every member state has an equal voice and equal vote," he said.  

Retiring from his UN position at the end of 2016, Ban was on the verge of launching his presidential bid at a time South Korea was gripped by a massive corruption scandal which later led to the ouster this month of President Park Geun-hye. Amid emerging influence-peddling allegations surrounding his own relatives, however, Ban announced in February that he will not run for the upcoming presidential election. 

An aide to Ban told Yonhap News Agency on Thursday that he will leave for the US on March 24 to take up a position as a visiting professor at Harvard University. (Yonhap)