President Barack Obama left Hawaii this morning for China after two days in his home state to promote conservation and combat climate change.

The presidential motocade arrived at Joint Base Pearl Harbor-Hickam at 10:34 a.m. and the president boarded Air Force One shortly afterward after shaking hands with service members waiting to see him off.

Obama, dressed in a dress shirt with no tie and khaki pants, waved before entering the plane.

Admiral Harry B. Harris, commander of U.S. forces in the Pacific walked him to Air Force One.

The president’s morning in Hawaii was uneventful. He exercised at Marine Corps Base Hawaii in Kaneohe before returning to the rented Kailua home where he is staying and departing for the airport.

Obama is flying to China after talking to Pacific island delegates ahead of the World Conservation Congress meeting in Honolulu and a trip to Midway Atoll for snorkeling and to showcase the island’s natural and mostly-untouched beauty.

“I look forward to knowing that 20 years from now, 40 years from now, 100 years from now, this is a place where people can still come to and see what a place like this looks like when it’s not overcrowded and destroyed by human populations,” Obama said on Midway, his shirt partially unbuttoned in the punishing island sun.

Squinting in the sunlight, Obama described Midway Atoll as “hallowed ground,” a nod to the place it occupies in Native Hawaiian tradition. Yet Obama had a policy argument to make, too: It was critically important to examine what damage climate change is inflicting on communities in the Pacific Ocean.

“There are countries that now are at risk, and they have to move as a consequence of climate change,” Obama said.

The president, who was born a short hop away in Honolulu, appeared particularly enthralled by a cluster of threatened green sea turtles — each the size of a tire — enjoying a bit of beach repose. “Unbelievable,” he called them, peering out through sunglasses.

“When I grew up, you’d see these turtles all the time,” Obama said. “But you’d never see them beaching like this, basking in the sun.”

In a prelude to his visit, Obama last week expanded four-fold the Papahanaumokuakea Marine National Monument that surrounds Midway. Of the 7,000 species that live in the waters, 1 in 4 is found nowhere else in the world, the White House said.

Yet for all the pristine splendor, there were poignant reminders that even this dot of far-flung land hasn’t been immune to human contamination. Along the muddy pathways the president traveled, brightly colored bits of plastic lined the ground — all remnants of albatrosses that had ingested waste washed ashore and then died with the bottle cap-shaped pieces in their guts.

The visit came as Obama uses his final months in office to try to lock in an aggressive legacy on climate change and environmental protection. After Midway, Obama planned to open his final trip to Asia on Saturday with a visit to China, a chance to showcase his unlikely partnership with President Xi Jinping on global warming.

Obama will meet with the Chinese president during this weekend’s summit of the Group of 20 industrialized and emerging-market nations.

During the meetings, he will also promote his trade agenda that appears imperiled by anti-globalization sentiment at home and abroad that could undo years of negotiations.

White House spokesman Josh Earnest said that Obama “is going to spend a lot of time on this trip in Asia advocating for the ratification of the Trans-Pacific Partnership, and underscoring how the United States and the U.S. economy benefit from deeper U.S. engagement.”

On the environment, Obama has one last item he may yet achieve on his wish list for China: sealing the deal with Xi Jinping for their nations to implement an international accord to combat climate change.

Ratification by the two presidents when they meet after Obama’s arrival Saturday in Hangzhou, China, would represent the high-water mark of the U.S. leader’s largely frustrated pivot to Asia. If achieved, it also would all but lock in the Paris agreement to reduce carbon emissions.

While Republican presidential nominee Donald Trump has pledged to “cancel” and renegotiate the agreement if he wins election, ratification by China and the U.S. would almost guarantee it would go into effect first. The accord provides for it to be implemented when countries producing 55 percent of global emissions ratify it; China and the U.S., the biggest emitters, together account for 38 percent.

“The U.S. and China jointly ratifying the Paris agreement is the first big step toward making that agreement a reality,” John Coequyt, the Sierra Club’s global climate policy director, said in an interview. “And having that space where we agree on a leadership level is really important because of all the difficult areas between the two countries. It’s allowing us to solve other issues on things in our national interest.”

For Obama, ratification would be the culmination of a years-long diplomatic effort to win over China for a joint effort to fight climate change, and an opportunity to cement his legacy on the issue.

“Climate will be a centerpiece of our agenda” at the G-20 meeting, Obama said Wednesday in Hawaii. “Joint U.S.-Chinese leadership was part of the reason that we were able to get Paris done.”

A 2014 agreement between the U.S. and China to cut greenhouse-gas emissions was seen as a necessary precursor to the broader Paris climate deal. Xi’s willingness to work with Obama on climate issues has helped Democrats parry Republican assertions that U.S. efforts to curb emissions would put American companies at a disadvantage while having little impact without similar restrictions on China.

It’s also a diplomatic and political accomplishment in a U.S.-China relationship that has been plagued by disagreements over cybersecurity and China’s territorial claims in the South China Sea. China’s increasing military assertiveness has undercut Obama’s pledge to refocus U.S. military and foreign policy toward Asia, as has opposition by Trump and Democratic presidential nominee Hillary Clinton to Obama’s signature trade deal, the Trans-Pacific Partnership.

Ratification of the climate accord by Obama and Xi would give Clinton and other Democrats a campaign season talking point. But Republicans say the president’s pledge to cut U.S. greenhouse-gas emission by 26 to 28 percent over the next 15 years would hurt American business, and they contend ratification by the president would circumvent a vote by Congress on the Paris agreement.

“While some members of the U.S. Congress still seem to be debating whether climate change is real or not, many of you are already planning for new places for your people to live,” Obama told a conference of Pacific island leaders during his stop in Honolulu. “Crops are withering in the Marshall Islands. Kiribati bought land in another country because theirs may someday be submerged. High seas forced villagers from their homes in Fiji. And no nation, not even one as powerful as the United States, is immune from a changing climate.”

U.S. and Chinese officials have said they hope to ratify the agreement this year, although the White House wouldn’t confirm plans for the two leaders to do so publicly upon Obama’s arrival in China. Brian Deese, a senior adviser to the president who focuses on environmental issues, traveled to China in advance to meet with Chinese officials about the possibility of ratification.

“I anticipate that we will be able to once again demonstrate our two countries working together on this issue when the presidents meet in China,” Deese told reporters in Washington on Monday.

Deese and Chinese officials also discussed other environmental efforts that are expected to be major topics of discussion when Obama and Xi meet Saturday evening.

The U.S. is hoping to recruit the Chinese to an international aviation agreement capping carbon emissions on international flights at 2020 levels. China has signaled some support for the effort, but also has lobbied that the deal should let nations opt out of initial, voluntary stages — a provision it and other developing countries, like India, would probably take advantage of.

“If China doesn’t join, the U.S. feels uncomfortable and then Europe feels uncomfortable,” said Jake Schmidt, director of the Natural Resources Defense Council’s international program. “They might not fully resolve stuff in two days, but the U.S. is seeking a quiet confirmation that both countries want this, and that political injection of energy into the process signaling the Chinese are a yes would be helpful.”

White House officials also are hoping to secure an amendment to the Montreal Protocol, an international treaty designed to phase out the production of chemicals that deplete the ozone layer. The U.S. and China are working together to grease the wheels for a potential agreement during an international meeting in early October that would phase out the production of hydrofluorocarbons, which are commonly used as refrigerants and can contribute to global warming.

U.S. officials are also likely to press Chinese leaders to make progress on their previous commitment to phase out financing for coal projects.

For Obama, the trip will be his last, best chance to make tangible progress on climate change, an issue that has become a cornerstone priority of his second term.

“It’s not until the bosses meet that there’s the last-minute scramble and it all has to come together,” Schmidt said.

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The Associated Press and Bloomberg News contributed to this story.