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November/December 1997 issue	In this issue Table of Contents Editor's Notebook Feature: Mussel bound Feature: Global warming Write to Know À la carte Our Home and Native Tongue

What causes ice ages?

THE TRIGGERING OF ICE AGES remained a mystery until recent discoveries confirmed that their timing matches recurring changes in the Earth's orbit around the sun. These cycles cause long-term climate changes by altering the amount and distribution of sunlight on the planet.

The effect of the cycles (called the Milankovitch Cycles for the Yugoslav mathematician who computed them 75 years ago) was confirmed by the analysis of deep-sea core samples taken in the 1970s.

The most significant cycle governs the timing of the seasons — where along Earth's elliptical orbit the poles tilt toward or away from the sun. The angle of this tilt varies according to a second cycle. In a third cycle, Earth's orbit changes shape from nearly circular to a more elongated ellipse. The first cycle occurs over periods of 19,000 and 23,000 years; the tilt varies over a 41,000-year cycle; and the ellipse changes over periods of 100,000 and 400,000 years.

Summers turn cooler when Earth is farthest from the sun and the axis is less steeply tilted toward it. Unmelted winter snow and ice accumulate from year to year, and an ice age can begin. Debate continues on whether human-induced global warming will alter the naturally occuring climatic cycles.

The colours in these images indicate the amount and distribution of sunlight reaching the planet at various latitudes. Red indicates greater amounts of sunlight; green and blue lesser amounts. Grey indicates areas where calculations could not be made.

Present day

Earth is strongly tilted, creating hotter summers and colder winters. Summer temperatures in the northern hemisphere, however, are moderated by a long earth-sun distance. Earth's orbit is nearly circular.

Ice house conditions
20,000 years ago

Cool summers in the northern hemisphere prevent the melting of winter snows. The accumulating snow and ice in turn reflect more of the sun's heat away from Earth's surface, and an ice age is under way. The trigger? During the northern summer, Earth is at its farthest point from the sun. Also, northern latitudes are tilted less steeply toward the sun.

Hot house conditions
125,000 years ago

Orbital conditions create unusual warmth in the northern hemisphere. Summer in the north occurs when Earth is at its nearest point to the sun and the north pole's sunward tilt is at its maximum. Thus the warm summers and extensive snow melting of Earth's last major interglacial period.

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