Ice Ages

Perhaps it was the breakup of Rodinia which triggered the formation of the ice about 700 mya. This was an example of an ice age or glaciation.

Snowball Earth controversy

This early ice age is controversial because some people claim that, unlike more recent ice ages, the whole Earth was covered in ice. However the thickness and extent of the ice at that time are difficult to read in the record of the rocks.

It was probably not the first and certainly not the last glaciation. These cold episodes in history have had profound effects upon life on Earth, and will no doubt continue to do so in the future. Therefore it is important that we try to understand what causes them.

So what is an ice age and what causes them?

Causes of Ice Ages

Unfortunately, the causes of ice ages are complex, and seem to involve a number of factors, including:

►    Continental Drift

►    Changes it the Sun's activity

►    Change in concentration of greenhouse gases in the atmosphere

►    Periodic changes in the Earth's orbit round the Sun, called Milankovitch cycles

►    Effects of the Sun's orbit round the Galaxy (not well established)

Could continental drift cause an ice age?

When continental drift creates mountains, or when a continent drifts over a pole, then snow starts to fall. Snow reflects the Sun's heat back into space, so cooling the Earth. The ice also weathers the rocks, so minerals are washed out of them and carried by rivers to the sea. Here the minerals join with carbon dioxide from the air. The amount of carbon dioxide in the air is reduced so the greenhouse effect is reduced and the Earth cools still further.

Moving continents also change the flow of water from one ocean to another and mountains affect the winds. So in many ways continental drift affects the climate. Eventually continental drift separates the continents, weathering wears down the mountains, and the climate goes back to its normal warm state. If continental drift happens in cycles, that would explain why ice ages seem to occur every 250 million years or so.

However, sometimes in history there were mountains without an ice age. Perhaps changes outside the Earth also played a part in starting ice ages. Changes in the Sun or its movement round the Galaxy might have lessened the sunlight reaching the Earth. It takes about 250 million years for the Sun to travel once round the Galaxy. Is this a coincidence? Nobody knows.

Sunlight

Sunlight is a form of radiation energy reaches the Earth from the Sun. The amount of energy which arrives varies with time and place. At the equator at midday enough energy reaches each square meter of the Earth's surface to power a 1 kilowatt bar of an electric fire. Because of the curvature of the Earth, the amount of energy falls off as you move nearer to the poles.

The Sun is believed to be a variable star. The number of sunspots (darker areas caused by magnetic fields) varies every eleven years, and the intensity of sunlight reaching the Earth also varies but only by a small amount (only 1 Watt in the 1000 per square meter at the equator). Accurate long-term measurements do not exist, so we cannot be sure of the amount of solar variation, but many scientists consider it is not a major influence on the climate.

Greenhouse Effect

One of the factors affecting the climate is the greenhouse effect. There is much debate about it, and how it relates to human activity. So what is it?

Most sunlight energy passes straight through the atmosphere and warms up the Earth's oceans and continents. These get hot and they give out infrared radiation. This cannot travel through some gases in the atmosphere, so it gets trapped. So some gases trap heat in the same way as glass traps heat inside a greenhouse. Hence the name greenhouse effect.

Gases which trap heat are called greenhouse gases. When there is more greenhouse gas in the atmosphere the Earth heats up. When there is less greenhouse gas the Earth cools. So which gases are these greenhouse gases?

There is much talk of the greenhouse gas carbon dioxide, and evidence from carbon dioxide in air bubbles trapped in ice shows that the amount of CO₂ in the air has increased by about a quarter since the industrial revolution began around 1750.

But you need to consider all greenhouse gases to get the complete picture. Other greenhouse gases include methane and nitrous oxide and water vapor. The most important greenhouse gas is water vapor, and the amount of this varies much more than the amount of carbon dioxide. Also water vapor creates cloud, which stops sunlight coming down to the surface, so the true picture is complex.

The greenhouse effect raises the average temperature of the Earth by more than 30 degrees, so without it the Earth would be a very cold place!

Weather

Weather is the short term local changes to the Earth's atmosphere, such as wind and rain.

Weather is driven by sunlight. Hot air expands and rises. Cold air sinks down and moves in to take its place, creating wind. Winds are also affected by the spinning of the Earth, so that instead of going straight they spiral around.

Water evaporates from the oceans to form clouds which are carried around by the winds. When clouds pass over mountains they rise, cool and rain falls. Evaporation and condensation absorb and release heat, so making the picture more complex. If it rains on high mountains or near the Earth's poles the rain will fall as snow which reflects back the sunlight, so cooling the Earth.

Just as the Sun makes the air move, so it also makes the oceans move. Their movement is controlled by how much salt they contain too. Wind and water act together to produce a complex pattern. Even with weather stations and satellites, with the best computers in the world and years of practice, experts still can only predict the weather for a few hours ahead.

Climate

Where weather is short term and local, the long term average of weather over a wide area is called climate. Changes in climate have very important effects upon all life on Earth, including humans. We need to understand the long term patterns of the climate, and use those patterns to predict what will happen.

This is especially important today, as the climate is changing. There is widespread agreement that, despite minor annual fluctuations, average global temperatures are warmer now than at any time since global measurements began in 1860. The results are large El Nino events, forest fires, torrential rains, flooding in some places, droughts in others, melting of glaciers and ice-caps and other problems.

Perhaps we should think about climate in two ways, the short term climate which covers tens or hundreds of years, and the long term climate which covers thousands or millions of years. It turns out we are able to make computer models of short term climate which are fairly accurate. Current predictions are that the short term global climate will continue to get warmer, due to human production of greenhouse gases. The rise is predicted to be about 4°C by 2100.

However there are indications that the long term global climate might be cooling.

Ice Ages in History

At some periods in history the continents were flat and mostly covered by warm shallow seas, the climate was warm and there was no ice anywhere on Earth. But every 250 million years or so an ice age begins.

Data from Shaviv 2003

During an ice age, ice forms on the mountains and flows down in frozen rivers called glaciers. They wear away the mountains, grinding out valleys and carrying bits of rock down to the lower land. There the ice spreads out in sheets. Some of it reaches the sea and breaks off as icebergs. Remember that ice expands when it freezes into ice. That is why icebergs float. This is very lucky. If ice sank then all the oceans would freeze solid and life would be impossible.

Sometimes the ice is thick and covers a lot of land. This is a glacial period and it lasts about ten thousand years. At other times most of the ice melts, covering only the tops of mountains and the poles. This is an inter-glacial, and it lasts about twenty thousand years or more. We are in one of these now.

Each ice age is made up of several glacials and interglacials.

Data from European Project for Ice Coring in Antarctica (EPICA)

Glacial Period

A glacial period (or glaciation) is a time during an ice age when the ice gets thicker and glaciers advance down mountains. They typically last tens of thousands of years. The last glacial ended about 15 thousand years ago.

Interglacial Period

The changes from glacial to interglacial and back again are probably caused by changes in the way the Earth moves round the Sun. This changes the amount of sunlight reaching the Earth. They are often shorter than glacials, lasting only for some thousands of years.

After a few million years the ice age ends, all the ice melts, and the weather goes back to its normal warm state.