U.N. Secretary General Ban Ki-moon called Thursday for stepped-up efforts to fight global climate change, directly linking the phenomenon to drought in the Horn of Africa and severe flooding in Australia.

Speaking at the University of Sydney in Australia, Mr. Ban criticized skeptics who challenge the science behind global warming and said “climate change is real.”

A day earlier in New Zealand, Mr. Ban and the leaders of Pacific island nations issued a joint statement stressing the need for urgent international action to reduce greenhouse gas emissions. They said action is necessary to ensure the survival and viability of the island states.

On his way to the Pacific Islands Forum, Mr. Ban visited the island nation of Kiribati, where rising sea levels have already swallowed some homes. Describing that experience in Sydney, he said, “the high tide shows that it is high time to act.”

International efforts to negotiate a successor agreement to the Kyoto Protocol on climate change have been on hold since the failure of a climate conference in Copenhagen in 2009. Delegates from 193 countries will try again in November at a conference in Durban, South Africa.