A third kind of nation is facing global warming.
The first kind includes nations like the U.S. -- wealthy enough to help their citizens adapt to the temperature rise of 2 degrees Fahrenheit that scientists say is likely within 40 years, and almost inevitable.
The second kind of nation includes the world's poor countries, such as Bangladesh. They are likely, say scientists, to suffer far more than the rich nations, unable to help millions of climate refugees seeking food and water ... but they don't face the prospect of disappearing altogether.
The third kind, they say, may disappear entirely -- and quite possibly within the lifetime of today's teenagers. They are made up of islands, threatened by rising sea levels. They say it is not a future risk, but something that is happening now.
Take the vast island nation of Micronesia, with an ancient culture that includes 607 islands, scattered across a million square miles of the western Pacific.
These islands are already being eroded away by fast-rising sea level -- so fast that graveyards are disappearing.
"Even the dead are no longer safe in my country," Micronesia's Ambassador to the UN told ABC News at his mission's offices on a rainy day in New York.
He gave us recent digital photos of his home islands.
In one, a man stands shin-deep out in a calm and sunny sea ... where a cemetery used to be.
In others, colorful traditional burial grounds spill out of a wave-eroded bank onto the tiny remaining beach, and water surges inland past tumbled houses.
Masao Nakayama, Permanent Representative of the Federated States of Micronesia, is a soft-spoken man, born on the tiny atoll of Onoun in the state of Chuuk.
"The threat is to our existence, survival, not only as a people -- as a culture. ... We now have just flat beaches -- the wash comes in and hits the roots of coconut trees," he said, describing more photos. "It's very scary, it's very frightening."
Scientists are telling Micronesians there will probably be 3 more feet of sea-level rise in less than 90 years, with 6½ more feet as an estimated "upper bound" -- a distinct possibility.
"Even a small rise of 1 meter ... would already have a devastating effect," he said. "If it gets to a meter or higher, the islands would get uninhabitable."
A rising water table is already turning salty in the center of islands, killing staple food crops like taro, and many other kinds of plants.
"Sea level rise is the most scary -- you cannot put sea walls on all the islands -- about 600 of them. 500 are small islands like atoll islands," said Nakayama.
Micronesia's culture is ancient. Its people, say historians, arrived about 3,500 years ago.
"It's going to be very sad for us to lose all that ancestry and homes -- where we've grown and maintain our culture," Nakayama said.
"Where will you go?" we asked.
"We don't want to go anywhere," he answered. "We want to stay on our islands, and this is what we want the international community to understand.
"I feel they have written us off because the kind of targets they are putting on the table is not going to save the islands," said the ambassador.