It dawned with the warmest winter on record in the United States. And when the sun sets this New Year's Eve, the decade of the 2000s will end as the warmest ever on global temperature charts.
Warmer still, scientists say, lies ahead.
Through 10 years of global boom and bust, of breakneck change around the planet, of terrorism, war and division, all people everywhere under that warming sun faced one threat together: the buildup of greenhouse gases, the rise in temperatures, the danger of a shifting climate, of drought, weather extremes and encroaching seas, of untold damage to the world humanity has created for itself over millennia.
As the decade neared its close, the U.N. gathered presidents and premiers of almost 100 nations for a "climate summit" to take united action, to sharply cut back the burning of coal and other fossil fuels.
Secretary-General Ban Ki-moon told them they had "a powerful opportunity to get on the right side of history" at a year-ending climate conference in Copenhagen.
Once again, however, disunity might keep the world's nations on this side of making historic decisions.
"Deep down, we know that you are not really listening," the Maldives' Mohamed Nasheed told fellow presidents at September's summit.
Nasheed's tiny homeland, a sprinkling of low-lying islands in the Indian Ocean, will be one of the earliest victims of seas rising from heat expansion and melting glaciers. On remote islets of Papua New Guinea, on Pacific atolls, on bleak Arctic shores, other coastal peoples in the 2000s were already making plans, packing up, seeking shelter.
The warming seas were growing more acid, too, from absorbing carbon dioxide, the biggest greenhouse gas in an overloaded atmosphere. Together, warmer waters and acidity will kill coral reefs and imperil other marine life - from plankton at the bottom of the food chain, to starfish and crabs, mussels and sea urchins.
Over the decade's first nine years, global temperatures averaged 0.6 degrees Celsius (1.1 degrees F) higher than the 1951-1980 average, NASA reported. And temperatures rose faster in the far north than anyplace else on Earth.
The decade's final three summers melted Arctic sea ice more than ever before in modern times. Greenland's gargantuan ice cap was pouring 3 percent more meltwater into the sea each year. Every summer's thaw reached deeper into the Arctic permafrost, threatening to unlock vast amounts of methane, a global-warming gas.
Less ice meant less sunlight reflected, more heat absorbed by the Earth. More methane escaping the tundra meant more warming, more thawing, more methane released.
At the bottom of the world, late in the decade, International Polar Year research found that Antarctica, too, was warming. Floating ice shelves fringing its coast weakened, some breaking away, allowing the glaciers behind them to push ice faster into the rising oceans.
On six continents the glaciers retreated through the 2000s, shrinking future water sources for countless millions of Indians, Chinese, South Americans. The great lakes of Africa were shrinking, too, from higher temperatures, evaporation and drought. Across the temperate zones, flowers bloomed earlier, lakes froze later, bark beetles bored their destructive way northward through warmer forests. In the Arctic, surprised Eskimos spotted the red breasts of southern robins.
In the 2000s, all this was happening faster than anticipated, scientists said. So were other things: By late in the decade, global emissions of carbon dioxide matched the worst case among seven scenarios laid down in 2001 by the Intergovernmental Panel on Climate Change, the U.N. scientific network formed to peer into climate's future. Almost 29 billion tons of the gas poured skyward annually - 23 percent higher than at the decade's start.
By year-end 2008, the 2000s already included eight of the 10 warmest years on record. By 2060, that trajectory could push temperatures a dangerous 4 degrees C (7 degrees F) or more higher than preindustrial levels, British scientists said.
Early in the decade, the president of the United States, the biggest emitter, blamed "incomplete" science for the U.S. stand against rolling back emissions, as other industrial nations were trying to do. As the decade wore on and emissions grew, American reasoning leaned more toward the economic.
By 2009, with a new president and Congress, Washington seemed ready to talk. But in the front ranks of climate research - where they scale the glaciers, drill into ocean sediments, monitor a changing Earth through a web of satellite eyes - scientists feared they were running out of time.
Before the turn of the last century, with slide rule, pencil and months of tedious calculation, Svante Arrhenius was the first to show that carbon dioxide would warm the planet - in 3,000 years. The brilliant Swede hadn't foreseen the 20th-century explosion in use of fossil fuels.
Today their supercomputers tell his scientific heirs a much more urgent story: To halt and reverse that explosion of emissions, to head off a planetary climate crisis, the 10 years that dawn this Jan. 1 will be the fateful years, the final chance, the last decade.