As the ice melted, the issue gelled.
This was the year that global warming hit the mass radar screen, driven by a drumbeat of catastrophic predictions from top scientists, a jaw-dropping acceleration in polar ice melt, Al Gore's Oscar - and then his Nobel.
His prize showed how important the issue was. His cameo on NBC's 30 Rock showed how mainstream it was.
As recently as 2003, when climatologist Heidi Cullen began her 90-second spots on the Weather Channel, global warming seemed fringe.
Now, her program is an hour, and global warming is a dinner-table topic coast to coast. "It's found its way into basic American dialogue," Cullen says.
When sea-level researcher Benjamin Horton talked climate change in 2006, his University of Pennsylvania students yawned.
This year, same class, seismically different evaluations. They "think it is the most important thing for science to understand," Horton says.
Indeed, researchers say, most Americans now understand the potential of global warming to affect every facet of society: energy production and use, population growth, water resources, storms, droughts, human health.
Few missed the import of the Nobel. It was the peace prize, a nod to fears of famine, environmental refugees and wars over water and other resources.
In years to come, climate change seems certain to alter America's very way of life - the houses people live in, the cars they drive.
This was the year world communities came together to debate the problem.
Washington vacillated - but states acted, often led by California's example.
Unlikely bedfellows - eco groups and corporations like DuPont, Dow and Ford - formed a climate partnership to demand government action.
Religious leaders ratcheted up the rhetoric.
Six hundred stalwarts stood naked on a glacier in the Swiss Alps.
And us? The gas-guzzling, thermostat-wielding public?
We bought energy-saving compact fluorescent lightbulbs - enough that Wal-Mart reached its year-end sales goal of 100 million by Oct. 2.
Some even took public transit to the Live Earth concerts on climate change.
"It was a cover story everywhere," says Greenpeace research director Kert Davies. "It was in your lap, in the dentist's office, on local news."
Attitudes continued a shift seemingly as inexorable as climate change itself.
According to a new GlobeScan poll, 65 percent of Americans believe climate change will directly threaten them and their families.
"We have what is essentially a global consensus that it's a problem, it's real, and people are willing to make a change in their lifestyles and accept increased cost of energy," says Steven Kull, director of worldpublicopinion.org.
Particularly intriguing, Kull says, is that in many countries - notably India, China and the United States - citizens don't think the government is doing enough.
What's not clear is whether this translates into personal action. Kull says people have a "virtue quota" for all the things they think they ought to be doing, from eating better to promoting human rights.
With so many demands, they want governments, individuals, corporations - everyone - to share the global-warming burden.
Nearly half of Americans feel guilty about their contributions to climate change, the GlobeScan poll found. But more than half feel confident technology will come to the rescue.
Meanwhile, news of global warming piled up relentlessly throughout 2007.
The U.N.'s Intergovernmental Panel on Climate Change - later to share the Nobel prize with Gore - released four exhaustive documents, each bleaker than its predecessor, all but sealing with epoxy the case that global warming exists and is caused by human activity.
In consensus reports widely viewed as conservative, the panel's thousands of scientists outlined an apocalyptic potential for buried coastlines, mass extinctions and spreading disease.
"This is as much as scientists will ever yell and scream from the rooftops," says Greenpeace's Davies.
What perhaps resonated most was a flurry of reports about the decline of polar ice in summer - likewise the penguins and polar bears that live on it.
At the end of this year's Arctic melt season, the National Snow and Ice Data Center found that sea ice receded to its smallest area since satellite measurements began in 1979 - suggesting a sharp acceleration of warming's impact.
The fabled Northwest Passage - a route atop Alaska and Canada, shortcutting the Suez and Panama Canals - opened briefly.
The U.S. Coast Guard is mulling a frontier base as mineral prospectors, oil seekers and scientists rush poleward.
Tourists, too. They are clambering ashore to see penguins waddle in Antarctica and to watch ice melt at Greenland's Ilulissat ice fjord, notorious for its big drip. (Air Greenland began nonstops from the United States this year, out of Baltimore.)
Ultimately, says Davies, it all led to "a huge year for changes in the zeitgeist," which made it possible for Congress to pass an energy bill, however watered-down, that President Bush signed last week.
Pennsylvania, with the third-highest greenhouse-gas emissions in the United States, now buys 30 percent of energy used in state facilities from renewable sources, making it the largest state purchaser.
The largest solar-power facility in the eastern United States, announced in August, will rise in Bucks County.
Both Pennsylvania and New Jersey joined California in adopting tailpipe-emissions standards for greenhouse gases. Washington thwarted that move last week, and it's headed to court.
Gov. Corzine signed an order to reduce greenhouse-gas emissions 80 percent by 2050. "We can't let the momentum stop," vows state Environmental Protection Commissioner Lisa Jackson.
Nine days ago, at the U.N. climate summit in Bali, came international agreement - sort of - on a road map for two more years of talks over who has to bite the biggest bullet.
"It was a decision to decide," says Eileen Claussen, president of the Pew Center on Global Climate Change, but significant nonetheless, if only because "developing countries showed a new willingness to be part of something."
Tipping point is not a phrase she uses easily. She nevertheless thinks it may apply to 2007 and global warming, which may become a defining issue of our time.
Deciding what to do about it will mark "the beginning of the end game," she says.
One way or another.