Friday, April 3, 2009

the heart(s) of the world

If you were asked to identify the heart of the world, what would you say?

The Earth's inner core?
An indy flick?
Tibetan beyul?

The
anatomical heart of an organism is a muscular pump driving the
circulation of oxygen and nutrients. In a physiological sense, then, I contend that Earth has two hearts: Antarctica and the Arctic.

Here's
the story. The Earth is tilted. So it has seasons. Earth's seasonality
is most pronounced at the poles, which oscillate annually between all
day, all the time, and all night, all the time. During the light, warm
summer, the massive ice shelves floating in the polar seas melt and
retreat. During the dark, cold winter, the Arctic, Ross, and Weddell seas hunker down to hibernate under blankets of sea ice.

Freezing saltwater actually creates two
products: nearly-fresh sea ice, and hypersaline, supercooled seawater.
You may remember (and if not, take my word for it) that the colder and
saltier water gets, the denser it becomes. Dense water is heavy water.
It sinks. So every winter at the poles (and it's always winter at one
of the poles!) a continuous stream of very cold, very salty, and
incidentally very well-oxygenated (cold water can hold more oxygen)
water pours down into the deep basins of the polar seas. These basins
fill up, and then the cold salty water spills over the edges into other
ocean basins. From the Arctic, it all flows into the Atlantic, since
the shallow Bering Strait won't let it into the Pacific. From the
Antarctic, it flows everywhere.

This sinking, flowing water
leaves behind it an aching emptiness in the hearts of the poles. They
try to fill the hole with warm water from the tropics. And as the warm
tropical water rushes eagerly into lonely polar arms, the abandoned
equator replaces it with cold water from the depths, an emotional
rebound called upwelling. (If you don't like the anthropomorphic perspective, just think of the whole thing as some kind of boring convection current.)

Thus, formation of sea ice is the muscular pump for the world's thermohaline circulatory system,
which provides oxygen and nutrients to the ocean ecosystem and also
happens to regulate the entire Earth's climate, thereby allowing it to
support
life as we know it. All thanks to the world's two polar hearts, each
beating steadily once per year. Q.E.D.

What's
that? Organisms have only one heart? Look, okay, it's not the Earth's
fault she's bipolar! Furthermore, cephalopods have three hearts, and
nobody gives them a hard time about it.


2 comments:

  1. Lovely. Unfortunately, the oceanographers aren't quite as poetic and would like to disabuse us of the term thermohaline.

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  2. Thank you! Mark Denny's recent (2008) textbook "How the Ocean Works" uses the term themohaline circulation--I had no idea that the term "has disappeared almost entirely from the oceanographic literature"! I actually don't find their argument against it terribly compelling . . . Upwelling may be the motor that drives the circulation (killing my heart analogy, oh well!) but the cold, dense water formed at the poles is the fuel. Why not name it after that?

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