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This threat is the onset of a new ice age.
It's not a very big change, as changes go. Only about 10-20F world-wide. But here's what we think will happen.
The ice age will start when oceanic and atmospheric currents reach their "tipping points" from water desalinization (from ice melt) and thermocline disruption. The circulations which mix warm and cool water, and air, will stop working.
Immediately, the polar areas will cool down and the equatorial areas will warm up -- dramatically, as much as 40F down (polar) or up (equatorial). With that kind of heat in the equatorial regions, the oceans will act like vast pans of hot water, sending huge volumes of water into the atmosphere. Thanks to global warming, this is happening already to a small extent -- it has already reduced sunny days by about 10% world-wide.
There will be a period of enormous heat waves, hurricanes, tornado outbreaks and violent thunderstorms. It will probably be a year-round thing until the snow-making "machine" gets cranked up.
Since there is a tendancy for heat to go where there is no heat, all this weather will head pole-ward at increasing speeds. So the ambient air temperature in the equatorial regions will be well above 100F, and the poles will be seeing temperatures fall as low as -150F on a regular basis. Once the hot, moist equatorial air hits the 32F "cold curtain", all this water will condense out, freeze, and fall as ice and snow. For centuries. It will take that long for the temperature gradients to reach a new equilibrium.
Meanwhile, anything above about 42N latitude in most places will become Siberian. Northern Europe, Russia, Canada, Mongolia and Northern China, will all have to "evacuate". A billion people or more will have to move, and move fast. The will also have to find places that won't become desertified. (See below.)
Since the flow of the water will be from equators to poles, most continental areas in lower latitudes will become parched. The worst drought in recorded human history will begin. We could lose as much as 90% of our food supply. The USA would be luckier, losing maybe as much as half.
In addition, we've been using oil so quickly that it's going to become much more expensive to get it out of the ground, real soon. And that also means no more petrochemical nitrogen fertilizer for "Green Revolution" crops.
If you have the resources, check out some of the climatological maps from the peak of the last ice age (about 28,000 years ago) and the Younger-Dryas "little ice age" of 9000 years ago. Four billion people depend on the cropland that will become desert within a decade of a climatic change. The Third World would fall into complete chaos within a year of this hitting. Population control is coming in a much nastier form than anyone expected.
Right now, our options seem to be whether we want a gentle kind of fascism, or a brutal kind of fascism. We have a decade or three to figure this out and make plans.
With any luck, I'm wrong.
--bkl Two inches forecast tonight for Philly -- the locals are in a panic.
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