Recently, it was another beautiful, sunny day out on the Arctic tundra.
It may sound nicer that way -- but it's a big problem for the Earth.
Scientists say the warm weather adds to global warming because of "feedback loops."
In a feedback loop, the rising temperature on the Earth changes the environment in ways that then create even more heat. Scientists consider feedback loops the single-biggest threat to civilization from global warming.
Past a certain point -- the tipping point, they say -- there may be no stopping the changes.
Scientists working in the Arctic report that feedback loops are already under way. As the frozen sea surface of the Arctic Ocean melts back, there's less white to reflect the sun's heat back into space -- and more dark, open water to absorb that heat, which then melts the floating sea ice even faster. More than a third of summer sea ice disappeared in the past 30 years.
In the ground next to the ocean, scientists say, warming has also awakened another enormous danger -- billions of tons of carbon locked up for eons by what was once frozen ground.
"I feel very uncomfortable about it," says Walter Oechel, a scientist studying the problem. "I mean, it's not the way the Arctic should be."
Oechel discovered that as global warming thaws and dries out the vast tundra, old decayed vegetation releases carbon dioxide. That's the same greenhouse gas that comes from car and plane exhausts, and power-plant chimneys -- and the tundra releasing carbon dioxide warms the atmosphere even more.
It's a slow-motion time bomb that's speeding up and could become self-generating, Oechel says.
"Humans are putting about 6 billion or 7 billion metric tons of carbon in the atmosphere a year," he adds. "And we are standing on 200 billion tons here. If any significant portion came out, that dwarfs the current human injection into the atmosphere. And once that runaway release occurred, there'd be no way to stop it."
Oechel and other scientists report that there are an additional 200 billion metric tons of carbon now beginning to leak from the northern boreal forests that encircle the Arctic tundra -- apparently for the same reason: The rising temperatures are drying out these forests, which means more decayed vegetation releasing yet more carbon dioxide.
Oechel says new carbon-free energy technologies, such as injecting greenhouse gas from power plants back into the ground, or zero-emissions cars, will be vital for maintaining a livable planet -- eventually, once they're developed.
But first, he adds, "The longer we wait, the worse the situation gets, and the harder it's going to be to crack."
Oechel says that, by his calculations, the only possibility for preventing a runaway greenhouse effect on Earth is to start reducing the use of fossil fuels like oil, gas and coal immediately.
ABC News Bill Blakemore originally reported this story for "World News Tonight."