WASHINGTON (BLOOMBERG) - US President Donald Trump made a surprise appearance at a climate change summit at the United Nations General Assembly on Monday (Sept 23).

Trump faced criticism from Democrats and climate activists for scheduling an appearance at a religious freedom event scheduled for the same time as the climate summit.

The president in the past has questioned man-made climate change, at one point calling it a Chinese hoax.

But the president denied he intended to snub the climate event on Sunday, pointing to a briefing he received on flooding in the Houston area as evidence he is concerned about climate change.

"It's not a snub at all. I'm very busy, I'm going on the flights today," Trump said as he departed the White House on Sunday. "The floods are very important to me, and uh, climate change - everything is very important."

Trump spent about 15 minutes at the summit and didn't speak. He listened to remarks by Indian Prime Minister Narendra Modi and German Chancellor Angela Merkel, then departed.

UN Secretary General Antonio Guterres said Monday that reversing climate change "will require fundamental transformations in society," such as taxes for pollution and other measures.

"Science also tells us it is not too late, we can do it," he said.

The climate summit coincided with protests in cities around the world designed to draw attention to man-made climate change.