U.N.'s Climate Change Push Gains DiCaprio, but Loses India, China and Russia

United Nations Secretary-General Ban Ki-moon at a news conference before a General Assembly meeting on September 16, 2014. Mike Segar/Reuters

Global turmoil and violence in Iraq and Syria will join climate change as key topics at the 69th session of the United Nations General Assembly, which opened Tuesday.

Calling it "the defining issue of our time," U.N. Secretary-General Ban Ki-moon said climate change will join the "horrendous violence in Iraq and Syria" carried out by Islamic militant group ISIS, the shaky cease-fire between Ukraine and Russian-backed rebels and the worst Ebola outbreak in history.

"Action on climate is urgent. The more we delay, the more we will pay in lives and money," Ban said.

But the U.N. Climate Summit, set for September 23, is likely to be hampered by the failure of leaders from the U.N.'s three largest member nations—China, Russia and India—to attend. China's Vice Premier Zhang Gaoli will represent the country at the summit, as well as the September 24-30 General Debate, instead of President Xi Jinping, and the Russian Foreign Ministry said Foreign Minister Sergei Lavrov will lead the Russian delegation. The top leaders of China and Russia won't be coming because their schedules are too demanding, ThinkProgress reports.

Indian Prime Minister Narendra Modi will address the General Assembly but is not expected to attend the Climate Summit, even though a report earlier this year by the U.N.'s Intergovernmental Panel on Climate Change listed India as the world's third highest carbon polluter, at 6.6 percent, behind China and the United States. Modi is expected to discuss climate change when he meets with President Barack Obama a few days after the summit, Mashable reports.

Ban also announced that Leonardo DiCaprio is the newest United Nations Messenger for Peace, with a special focus on climate change, and will be speaking at the opening of the Climate Summit.

The Titanic and Wolf of Wall Street star launched his environmental nonprofit, the Leonardo DiCaprio Foundation, in 1998 and is a "credible voice in the environmental movement," said Ban.

"It's an honor to accept the role of U.N. Messenger of Peace on Climate Change and to support the secretary-general in his efforts to address one of the most important issues we face as a global community," said DiCaprio. "I feel a moral obligation to speak out at this key moment in human history—it is a moment for action. How we respond to the climate crisis in the coming years will likely determine the fate of humanity and our planet."

The Climate Summit will focus on reducing greenhouse gas emissions, limiting deforestation and mobilizing financial support for action on the climate. The U.N. hopes countries will agree to limit the rise in global temperature to less than 2 degrees Celsius by 2015.

Ahead of the Climate Summit, New York City will host the People's Climate March on Sunday. Ban will "link arms with those marching for climate action" as they walk through Manhattan.

More than 140 heads of states and governments will be attending the General Assembly session.