April 1, 1994

Ocean Currents

I wonder that any human being should remain in a cold country who could find room in a warm one.

Thomas Jefferson, 1801

On returning from Europe early one summer, I flew over the southern end of Greenland. From 30,000 feet I could see the immense glaciers flowing to the sea. The rest of this largest of islands was completely white. I wondered where it got the name Greenland. I have read that Vikings colonized Greenland. It would take a great leap of the imagination to think of that icy wilderness as green, or to imagine a climate change that converted a once green coastal area to what it is today.

One of the major factors in determining climate is ocean currents. They are, in part, the reason that Washington, D.C. has a mild climate relative to those midwestern areas at the same latitude.

Ocean currents are now a hot area of research. One of the most fascinating mechanisms is the postulated conveyor belt effect.

If you take fermented apple cider, which has an alcohol content similar to a dry wine, and put a plastic bottle of it into the freezer, you will end up with a block of ice and some liquid. Drink that liquid and it will knock your socks off. That is because when water freezes, it incorporates only the water into the ice block. What is left is mostly alcohol. The same thing happens with salt water; ocean water that is frozen is just water, with little or no salt.

Probably due to the prevailing winds, warm tropical water flows toward the poles as warm ocean currents. In the process, water is lost by evaporation, concentrating the salt. When the water reaches cold arctic regions, the water freezes, extracting still more water. This leaves a more concentrated salt solution. Concentrated salt water is heavier than normal ocean water, so it sinks. By some mechanism, which I don't understand, that concentrated salt water flows, as a cold current, back toward the equator, picking up fresh water in the process and diluting the salt. The result is a continuous current that is described as being like a conveyor belt. In our hemisphere, warm water flows to the north and cold water flows south. It is, of course, much more complicated than that.

Suppose that we get some global warming and more polar ice melts, what then? The melting ice could dilute the salt water, which will not sink and the conveyor belt will stop. When it stops, it will no longer bring warm water toward the poles. Without the warm water, the polar areas will get colder and that will stop the ice from melting. It will also freeze out more water, re-starting the sinking of salt water and starting the conveyor belt again. Darn clever, isn't it? This mechanism has been postulated to be responsible for short sharp periods of cold. One of these ice ages started 13,000 years ago and lasted for a mere 1,500 years. At the end of this, global temperatures rose suddenly by a spectacular 7 degrees C. Changes of as little as 1 or 2 degrees C. were believed to be responsible for the Little Ice Age that occurred about 750 years ago.

More spectacular drops in temperature have been attributed to the eruption of Mt.Toba in Sumatra, 73,500 years ago. In a year or less, temperatures may have dropped 3 to 5 degrees C. globally, and as much as 15 degrees in northern regions.

This report doesn't mean that you should run out and stock up on warm clothing. If you listen to the global warming people, you might decide to build a swimming pool.

You might well ask: is the climate going to get warmer or colder? Don't ask me, because I haven't the faintest idea. I suspect that the best climatologists don't know either. As with any predictions of the future, just stick around for a few hundred thousand years and you'll find out.

(Most of the information in this column came from an article by Keith Stewart Thompson, which appeared in the December '93 issue of American Scientist.)

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