Actor Leonardo DiCaprio, newly named Messenger of Peace for the United Nations, told world leaders convening in New York yesterday, Sept. 23, that climate change is "the greatest challenge of our existence on this planet."

Addressing the opening of the U.N. Climate Summit, he urged the attendees to fully address climate change.

"I play fictitious characters often solving fictitious problems. I believe that mankind has looked at climate change in that same way," he said. "As if pretending that climate change wasn't real would somehow make it go away, but I think we all know better than that now ... None of this is rhetoric, and none of it is hysteria. It is fact."

Before the opening of the summit, DiCaprio took part in a march through New York City on Sept. 21, a political demonstration intended to increase awareness of the issue of climate change.

Also at the summit, President Obama announced an initiative to put the technological and scientific capabilities of the United States at the disposal of populations around the world vulnerable to climate change.

He acknowledged that achieving steep cuts in carbon emissions will be a key step in dealing with the issue, but it remains economically and politically problematic in many parts of the world.

"Yes, this is hard," said the president, speaking in the U.N. General Assembly chamber, "but there should be no question that the United States of America is stepping up to the plate."

He made it clear, however, that the U.S. should not and would not shoulder the burden alone.

"We can only succeed in combating climate change if we are joined in this effort by every nation, developed and developing alike," he said. "Nobody gets a pass."

U.N. Secretary General Ban Ki-Moon emphasized the same point in his remarks opening the climate summit session.

"We need to take action now to limit global temperature rise," said the secretary general. "We need all hands on deck to ride out this storm."

The aim of the New York session was to kick off discussions that, it is hoped, will lead to a binding agreement reached in Paris next year that will hold the biggest carbon emitters on the planet -- including the U.S., China, Russia, Japan, the European Union and many others -- to a policy of capping their consumption of fossil fuels, which are driving climate change and global warming.

Climate activists lauded Obama's speech, which comes after this pledge of sweeping environmental measure in the U.S.

"You have to act domestically to have any credibility with international partners on this issue," said Jennifer Morgan, director of the World Resources Institute's climate and energy program.