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Ice ages

I like ice. When I order a drink at the movie theatre, I always ask for “more ice than you think anyone can possibly want.” At home, I drink iced tea. When I finish my drink, I eat the ice.

But I prefer that ice keep to its proper place–which is not on the roads, on the sidewalks, or especially on my car.

Still, if you think chipping half a centimetre of ice off your driveway is a chore, just imagine trying to clear away three kilometres of the stuff. Twenty thousand years ago, that’s the task you would have faced.

Scientists believe the Earth is about 4.6 billion years old, and it’s mostly been much warmer than it is now. But periodically it has cooled off, for anywhere from 2.5 to 60 million years. These cold epochs are called ice ages.

Within these epochs, there are “glacials”–periods when glaciers cover much of the Earth–and “interglacials”–periods when the glaciers retreat. The term “ice age” can also refer to a specific glacial periods. In fact, most people refer to the most recent glacial, the Wisconsin, which ended 10,000 years ago, as “The Ice Age,” as though there had been only one, when in fact there have been many.

Glaciers grow when the amount of snow falling on them in the winter is greater than the amount that melts in the summer. The snow piles up, and is eventually compressed by its own weight into ice. Its vast weight also causes it to spread out, rather like certain of us do when we sit down.

During the Wisconsin glacial, all of Canada was buried under three kilometres of ice, just like Greenland and Antarctica are today. The ice spread out from the poles and mountains to as far south as Missouri.

This didn’t happen overnight. In fact, it happened at a, well, glacial pace: probably something between 50 and 150 metres a year.

What causes ice ages? Scientists aren’t sure. There are so many interrelated factors it’s hard to separate cause from effect.

For example, it’s known that the stratosphere was 10 times dustier during the last glacial. Dust could have blocked sunlight, cooling the Earth. Was volcanic activity high? There’s no evidence for it. Maybe the dust simply blew off of ocean shelves exposed by dropping sea levels, and is therefore an effect, not a cause.

Winds were stronger, causing more clouds and precipitation, further cooling the Earth. Cooler seas absorbed more carbon dioxide from the air, reducing the greenhouse effect. Ocean currents changed, no longer delivering warm water to the north. Highly reflective sea ice spread, bouncing energy back into space. And the vast sheets of ice themselves cooled the atmosphere flowing over them. All of these things contribute to an ice age, but none appears to be the trigger. For that, many scientists believe, you have to look to outer space.

Ice ages occur about every 150 million years. It so happens that the Milky Way galaxy rotates once every 300 million years, taking our solar system through denser and thinner regions of interstellar dust and changing gravity and magnetic fields. Twice an orbit, every 150 million years, a slight change takes place in the solar system’s galactic environment…which could cool Earth’s climate.

The glacials and interglacials within an ice-age epoch have their own cycle of about 100,000 years. These fluctuations appear to be related to a large extent (although not entirely) to variations in the Earth’s orbit: eccentricity, equatorial tilt and precession. Eccentricity, the variation of the Earth’s orbit from perfectly circular, has a cycle of 93,408 years. When the orbit dips closer to the sun, the spin rate of the Earth/Moon system slows, increasing Earth’s magnetic field. This more effectively screens high-energy particles from the sun and cools the Earth.

The planet’s tilt changes over a 41,000-year period; the greater the tilt, the more extreme the seasons. Finally, there’s the 25,920-year precession cycle. (Precession is the planetary equivalent of the wobble in a spinning top.) This, too, can effect the severity of the seasons.

Many scientists believe we’re living in an ice age right now; we’re only in an interglacial period, and the glaciers are due back in about 23,000 years.

The last glaciation shaped all of Canada’s geography, forming lakes, scouring valleys, even giving the southern Prairies their rich farmland (which formed at the bottom of giant Lake Agassiz, created by the run-off from melting glaciers).

This country was shaped by ice, and I’m sure we’re all very grateful…but as winter stares us in the face again, I’d have to say ice has out-stayed its welcome.

Let’s keep it in our drinks where it belongs!

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