The United Nations Secretary-General's daylong Climate Summit in New York on Sept. 23 will draw about 120 heads of state, and many lower-ranking officials, according to a list of speakers the U.N. released this week.

President Barack Obama will attend on behalf of the U.S., but China and India — the first and third-largest emitters of carbon dioxide — will be represented by lower-level officials. Carbon dioxide, the main long-lived greenhouse gas that is causing global warming, stayed at or above 400 parts per million for several months in 2014 — the highest it has been in human history. A single molecule of carbon dioxide can remain in the atmosphere for hundreds to a thousand years, which makes the problem particularly challenging to solve.

China is sending Vice Premier Zhang Gaoli as a special envoy of President Xi Jinping, according to Xinhua. "The Chinese side will participate in the summit and related activities in a constructive manner," Foreign Ministry spokesperson Hua Chunying told the state-run media outlet. "The Chinese side expects to join efforts with the international community to push for positive results in the summit, advance international cooperation on climate change and contribute to addressing climate change."

The amount of carbon dioxide and other greenhouse gases in the atmosphere has skyrocketed in recent decades. Image: U.S. National Climate Assessment

The U.N. document says leaders will be given speaking slots that are just four minutes long, although Xinhua reported that a "keynote address" will be given by China's Zhang, suggesting a longer speaking allotment.

India, the third-largest emitter behind the U.S., is sending its environment minister to the meeting instead of newly elected Prime Minister Narendra Modi. However, Modi is traveling to Washington several days after the summit to meet with Obama, and climate change is expected to be one of the issues they discuss.

Many countries, including Australia, Russia, Pakistan and Ukraine will be represented by their foreign ministers, while other nations, such as Canada, are sending their environment ministers. Notably, Saudi Arabia, whose economy depends on its oil exports, is sending its petroleum minister to the environmental meeting.

With leaders facing a deadline of 2015 to negotiate a new climate agreement that will be enforced in 2020, the summit is seen as a momentum builder ahead of formal negotiations. It will be the largest gathering of world leaders to discuss climate change since the Copenhagen Summit in 2009, and depending on the final guest list, it may even draw more leaders than that meeting.

The Climate Summit will bring together political leaders, business executives and representatives of nongovernmental organizations. Those involved in the planning say that a series of commitments in the areas of emissions cuts and financial contributions will likely be announced. The announcements will be aimed at increasing the use of renewable energy, increasing energy efficiency, reducing deforestation and promoting climate action in the world's cities, among other goals, according to a release.