Indian Prime Minister Narendra Modi welcomes Chinese President Xi Jinping at a hotel in Ahmadabad, India. Neither plans to attend the climate summit. (Ajit Solanki/Associated Press)

This week, the United Nations will host a huge and well-publicized one-day summit on climate change. The public is likely to be watching it closely: It comes just days after thousands of people in New York and around the world took to the streets, demanding more political action to help fight global warming.

The climate summit's organizer, U.N. Secretary General Ban Ki-moon, took part in the New York march, and for Tuesday's event, he is promising to bring together some of the most powerful people in the world with a common purpose. "I have invited leaders from government, business, finance and civil society to present their vision, make bold announcements and forge new partnerships that will support the transformative change the world needs," he wrote in a blog post on the summit for the Huffington Post.

It's certainly true that the climate summit has an impressive guest list: More than 120 world leaders are heading to the United Nations in New York for the event, including President Obama and many big names from the private sphere. Actor Leonardo DiCaprio will give one of the opening speeches.

But as impressive as that guest list is, what's more interesting is who is missing.

Notably, Chinese President Xi Jinping and Indian Prime Minister Narendra Modi are skipping the event. In empirical terms, it's hard to think of two more important leaders in the world right now: Together they lead more than 2.5 billion people, more than a third of the world's population.

And the two countries are not only the first and second most populous countries on Earth; research shows they also were the first- and third-biggest producers of carbon dioxide emissions (the United States holds the No. 2 spot). That figure can only partly be explained away by their huge populations: One study showed that per capita emissions from China recently surpassed that of the European Union, and India is predicted to follow suit in five years.

Both China and India will have representation at the summit: Vice Premier Zhang Gaoli will attend for China, and Prakash Javadekar, minister of environment and forests, will be represent India. Officially, Xi and Modi are missing the event because of scheduling conflicts. However, given the high status of some of the other guests in attendance, their absence has been interpreted as a snub.

For example, when news first spread about the leaders' decision, Tony de Brum, foreign minister of the Republic of the Marshall Islands, said in a statement that he was "shocked and very disappointed" to learn that Xi and Modi would not attend. “Every study I have ever read makes it clear that developing countries have the most to lose from runaway climate change," he said. "And for the small island states of the world, the science says we might be forced to pay the biggest price of all – the loss of our countries."

The Marshall Islands are one of many smaller island nations at risk from rising sea levels. "We expect solidarity from our developing country compatriots, not excuses," De Brum said.

Both China and India's leaders' views on climate change policy are hard to pin down. Xi has recently spoken about China's need to do more to curb global warming, but China's delegate has said that no new emissions reductions targets will be proposed at the summit (the United States also is taking this line). Instead, China's foreign ministry has said that they are seeking "win-win and sustainable paths in achieving economic development and combating climate change that fits China's realities." Modi was once vocal about climate change, but recent comments have suggested that he may view the issue less seriously than other issues facing India.

Xi and Modi are not the only notable world leaders missing the summit. Russia is the 10th-most-populous country in the world and the fourth-largest producers of carbon dioxide emissions, yet President Vladimir Putin won't be in attendance (Putin has shown little interest in climate change, and Russia's economy is reliant on carbon-producing industries). Both Canadian Prime Minister Stephen Harper and Australian Prime Minister Tony Abbott, two leaders known for their relative skepticism about climate change, will be missing the event, too. All are sending representatives.

Stephane Dujarric, spokesman for Ban, concedes that the absence of certain world leaders is not a positive, but he argues that it is the outcome that is important. "Obviously, it's always better to have the highest possible representation," he said at a briefing Sunday, "but it is really the commitments that are made, [more] than who delivers them on behalf of the country." Ban has spoken about the climate issue with China and India and understands the "diplomatic issues" that prevented them from making it, Dujarric said.

For those hoping for a quick international consensus on climate change, those words may fail to comfort. And the United Nations' own research is likely to make them more anxious. One recent report found that carbon dioxide levels in the atmosphere had already begun to increase at a dramatic rate.

Actor Joseph Gordon-Levitt, U.N. Secretary General Ban Ki-moon and Sen. Chuck Schumer (D-N.Y.) were among the estimated 100,000 demonstrators who joined the People's Climate March in New York on Sunday, demanding action on the issue of climate change. (Divya Jeswani Verma/The Washington Post)