Weather has played a huge role in the course of human events, usually unacknowledged. It was weather -- a chilly April in 1912 -- that allowed icebergs to drift exceptionally far south in the Atlantic, where one sank the Titanic. The briefest respite between gales allowed D-Day to take the Nazis by surprise. And it was the Russian winter that helped topple Napoleon and Hitler.

Weather, or rather its long-term average called climate, has shaped history with broader strokes, birthing and killing cultures. In America, pre-Inca empires rose and fell in sync with wet and dry periods. Vibrant urban societies like the Moche and Tiwanaku civilizations were wrecked by killer droughts that kicked out their agricultural underpinnings. Drought may have also destroyed Mexico's Maya. A two century-long drought, one theory says, escalated city state strife to horrific levels. Another theory claims the Maya's godlike leaders, unable to bring rain, were toppled by a disillusioned people.

Drought and deluge have triggered crop failures, starvation, revolts and the downfall of cultures in ancient Egypt, Mesopotamia, Africa, China and the U.S. Southwest. Dry times in particular, the historical record shows, make people thirsty and hungry, turning them eventually desperate and violent.

Despite humanity's often hostile relationship with weather, we've been the big winners in the single largest climate change of recent millennia. The end of the last ice age 12,000 years ago afforded our species the ideal conditions to civilize and enter our halcyon days, maybe better called our Holocene days.

The Holocene, as our current geological period is called by scientists, began when the world warmed drastically, melting away glaciers that covered America as far south as Manhattan. The Holocene has endured minor climatic shocks but it has stayed remarkably stable, allowing us to thrive, invent agriculture and swell our numbers to 6 billion -- until now.

If today's Earth suddenly experienced some of the radical climate changes that wrenched the world in previous ages (global average temperature fluctuations of 10 degrees Fahrenheit in a single decade, for example), it seems unlikely that civilization could prosper.

But as William Stevens notes in his book "The Change in the Weather," a colossal climate shift isn't needed to make us miserable. The Little Ice Age, which occurred in Europe after 1300, brought crop failures, starvation and civil strife, all triggered by an average temperature drop of just 1.5 degrees Fahrenheit. We can expect a human-caused shift larger than that in the next 20 to 30 years.

Two reports from Europe in April 2002 show that temperatures are rising fast. Great Britain's Hadley Center for Climate Prediction put average global temperatures in the decade between 2020 and 2030 at 0.5 to 2.3 degrees Fahrenheit warmer than between 1990 and 2000. Switzerland's University of Bern projects an increase of 0.9 to 1.9 degrees -- that's almost a doubling or tripling of the warming we've seen in the past hundred years, occurring in only 30.

By 2100, the United Nation's IPCC says, average global temperatures could increase up to 10.4 degrees Fahrenheit. The gravity of this worst-case forecast becomes clear when it is compared with the global rise of just 5 to 9 degrees Fahrenheit that ended the last Ice Age 12,000 years ago, making all the difference between the Big Apple of today and a Manhattan buried under a half mile-thick mountain of ice.

Unfortunately, worsening scientific projections have made barely a dent on the policies of our current president or the lifestyle of the U.S. populace. And while new, gloomier climate change predictions seem to be issued almost monthly, what remains unknown is how global warming will impact humanity: our food and water supply, our societies and political stability; could it catalyze revolution, civil war, world war, or global terrorism?

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