What drives Glacial cycles?

Answer 1

Factors that contribute to "Ice Ages" or global glaciation include: (1) atmospheric composition (greenhouse gases and airborne particulates); (2) variations in solar output; (3) the motion of tectonic plates resulting in changes in the relative location and amount of land mass on the Earth's surface; (4) ocean water circulation; and (5) changes in the Earth's orbit around the Sun.

The current period of ice ages started about 3 million years ago and tends to recur in 40,000 and 100,000 year cycles. In earlier times, the earth was much warmer due to much higher levels of volcanism that spewed carbon dioxide and sulfur dioxide into the atmosphere. The Sun was also about 25% brighter when dinosaurs first roamed the earth. The shape of the earth's land mass and water mass is important, since these determine temperature and rainfall/snowfall variations around various parts of the world.

The earth currently rotates around the sun on a 23.4 degrees tilt with the north-pole pointing close to the star Polaris. That has not always been the case since this inclination wobbles with a 26,000 year period. The degree of tilt also varies 21.5 degrees to 24.5 degrees on a 41,000 year cycle.

Answer 2

Think of a glacier as a river of ice making it's way down the mountain.

High up in the mountains where the air is cold the precipitation falls as snow, in time more snow falls, then more and more and more. The weight of the accumulated snow compresses the lower layers squeezing the trapped air out, it compresses it so much that it turns to ice.

Due to gravity the ice slowly makes it's way down the mountainside just like a slow moving river. Quite often glaciers converge to form bigger glaciers.

As the glaciers descend the air becomes warmer until it's warm enough to melt the ice. During the cold winter seasons the snout (front) of a glacier advances down the mountainside and in the warmer seasons it retreats back up the hill. The distance between furthest advance and retreat is usually several hundred meters, in some cases 2 or 3 kilometers.

The melt water from the glaciers runs off in streams and rivers eventually flowing into the seas and oceans where it's evapourated into the atmosphere to fall as rain or snow somewhere else.

Glaciers are natures way of removing snow from mountains (along with avalanches, cornice collapses, insolation etc). If snow remained where it fell it would accumulate at an incredible rate and the oceans would dry up. Imagine this, a metre of snow falls each year, once compressed to ice its 125mm thick, after just a million years it would be 125 kilometres (78 miles) thick, a million years is nothing in geological time.

Answer 3

nobody knows... but a good guess could be that it has a lot to do with the sun.
In any case, it is NOT human activity.