For the people of Kiribati, global warming is not a distant, theoretical hazard. In this remote nation of 33 islands, located in the central Pacific Ocean, the land is disappearing one inch at a time, as ocean waves slowly creep inland.
ABC's Bill Weir toured Kiribati for the first part of his series, "Key to the World," which highlights major challenges to our time.
Kiribati occupies two million miles, mostly made of salt water and the inhabitle land of these islands could fit inside New York's city limits. Most of the 100,000 residents have never been off its necklace of coral, although many may have to move in this century as the region's president expects it will become unlivable in 50 years.
"We don't know how much longer we're going to have it," Kiribati's President Anote Tong told Weir.
Right now, the children of Kiribati, who have a literacy rate of 90 percent, spend their free time playing along the shore. But their playground could be eliminated if expected global warming trends progress, and the surrounding ocean water becomes warmer, pushing the seas higher as waves strengthen.
After narrowly defeating his brother for the presidency in 2003, Tong went to the United Nations in 2004 and 2005, and stunned world leaders by claiming his country will be gone by mid-century -- and would become the first sovereign victim of man-made climate change.
"Our very lives are at stake," he told the U.N.
When Weir asked for a time frame, he said it is a "certainty" that his land will disappear.
"I think it's a question of time," Tong said. "So, for planning purposes, I like to set a time at 50 years."
And he's concerned it could be too late to save Kiribati. "I believe that that is the case. The momentum has already been set … some villages that have been there for centuries, they've had to move."
As the ocean challenges the land, another problem has emerged. While people scramble to build stronger homes and sea walls, they mine the beaches for bits of coral and rock to mix with mortar, and further challenge the island.
"Last year we had to move an uncle of ours who died 10 years ago because the grave was coming close to being … engrossed by the sea," explained resident Linda Uan. "Perhaps it hasn't affected people in your country but we in the low lying islands are really affected by what is happening."
Still, not everyone believes in global warming. School principal Zita Lopez said she does "not really" believe in the theory.
She said she takes comfort in the Old Testament story of Noah, and God's promise never to flood the Earth. It is a view she shares with her students.
But while many choose to wait and see, a few are taking no chances. Some Kiribati residents, whose ancestors arrived to this land in canoes 3,000 years ago, are now leaving on jets, for good.
"There is no hope for Kiribati anymore if it is sinking," said former resident Charles Inoka. "I guess our government has to look for alternatives."
Inoka moved to Auckland, New Zealand, which is the only country open to climate change refugees. It now receives 75 emigrants from Kiribati each year.
Tong believes there will be tens of thousands more who will want to relocate and challenge the future of his nation, and he knows that no sovereign nation has ever peacefully relocated intact.
"We are very conscious of the fact that neighboring countries will be reluctant to add us to their existing problems at the moment. It's a humanitarian problem," Tong said. "The question as to what happens to our sovereignty? I don't think anybody has the answer."
When asked if he's willing to split his country up to save it, he replied, "The question is, do I have a choice?"
Watch "World News with Charles Gibson" every Monday in April and see where else Bill Weir travels for the "Key to the World" series.