While referencing other major challenges facing the world -- including disease, economic inequality and the threat of terrorism -- President Barack Obama declared climate change to be the "one issue that will define the contours of this century more dramatically than any other" at a United Nations summit Tuesday afternoon.

Climate change, Obama said, is an "urgent and growing threat," and it "is changing faster than our efforts to address it."

"The alarm bells keep ringing. Our citizens keep marching," he said. "We cannot pretend we don't hear them. We have to answer the call."

In his remarks, Obama touted actions his administration has taken to address rising greenhouse gas emissions, including the release in June of draft rules limiting carbon emissions from power plants. He also announced a new executive order that directs federal agencies to consider climate change in their international development projects.

Obama said he met with Chinese Vice Premier Zhang Gaoli shortly before his address, and emphasized that as their countries are the two largest emitters, "we have a special responsibility to lead. It's what big nations have to do."

Obama's speech repeated that emphasis, arguing that a new global agreement -- which negotiators are working to complete by the end of 2015 -- should include commitments from all nations. "We can only succeed in combating climate change if we are joined by every nation, developed and developing alike. Nobody gets a pass," Obama said. "It is those emerging economies that are likely to produce more and more carbon emissions in the years to come. Nobody can stand on the sidelines on this issue."

In separate remarks, China's vice premier Zhang reiterated the country's commitment to cutting its carbon intensity by 40 to 45 percent by 2020, and to increasing the share of non-fossil fuels in use there. He also said China would announce "as soon as we can" any additional actions it will take to cut emissions after the year 2020. And while he did not offer a more specific timeline, he said China "will also try to bring about the peaking of total CO2 emissions as early as possible."

Zhang emphasized that an agreement reached at the United Nations Framework Convention on Climate Change meeting in 2015 should "uphold common but differentiated responsibilities" for developed and developing nations. "All countries need to follow a path of green and low-carbon development that suits their national conditions," he said.

Others at the climate summit criticized the U.S. and other developed nations for not putting more money into the Green Climate Fund, a pool of money created in 2009 to direct $100 billion to climate mitigation and adaptation work in poorer countries. Prime Minister of Antigua and Barbuda Gaston Browne said that he is "angry and frustrated" that only Germany had committed significant resources to the fund prior to Tuesday's meeting, while "others have been deafeningly silent." He added that it is "immoral" to demand that developing nations commit to cutting emissions while developed nations -- those with the responsibility for historic emissions -- have not contributed to the fund. (France, South Korea and others announced commitments to the fund on Tuesday.)