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What is Snowball Earth?

Was the Earth once entombed in ice and snow? And, if so, why would that be important for us today? It is thought that 600 million years ago the Earth experienced the most violent and dramatic climate change in its history, caused by carbon dioxide. Worried? You should be.

There is a theory that no matter how advanced an ice age becomes, there will always be parts of the Earth that will always be warm and ice-free. During an ice age, cooler temperatures allow ice to spread from the Poles towards the Equator. However, the ice can never quite reach tropical regions due to the strength of the sun at these latitudes.

This basic law of nature cannot, however, explain findings which suggest that Namibia was once covered in ice. It is thought that the northern half of Namibia has been in the tropics for hundreds of millions of years and conventional wisdom says that ice coverage during an ice age could never spread this far.

Quick Links:

Evidence of Ice
Origins of the Snowball Earth Theory
Scepticism of the Snowball Earth Hypothesis
Nuclear Winter and Proof that Snowball Earth was Possible
Evidence of Glaciation in the Tropics
How Could Snowball Earth Have Come to an End
Evidence of an Immense Greenhouse Effect
How Could Life Survive the Snowball Earth Scenario?
A Catalyst for Life
Was the Earth Really Frozen Solid?
What Relevance does this have Today?
References and Further Reading

Evidence of Ice

Hundreds of millions of years ago, the rock which currently forms craggy outcrops in Namibia's desert was part of the sea floor. Layers of sea floor sediment can be seen in the rocks, and one of these layers contains something called 'drop stones'. These can be seen in cliff-faces, where layers of what was once the sea floor are exposed for all to see. The drop stones themselves are rocks of various sizes, the larger specimens typically measuring tens of centimetres across. As the ocean floor is usually made up of much smaller particles such as sand and silt, these stones stand out in a clear band in the rock.

Drop stones are usually caused when glaciers cut across the Earth, picking up rocks as they go. In places such as Antarctica or Greenland, the ice from these glaciers flow from the land onto the ocean surface. Once in the sea water, the ice starts to melt and the rocks fall to the bottom of the sea. If the climate becomes warmer and the glaciers disappear, layers of sand and sediment are then deposited over the layer of rocks, preserving them in a discrete band beneath the ocean floor.

The bending and buckling of the Earth's crust, combined with changing sea levels mean that the ocean floor is often forced upward above sea level to form rock on dry land. The most extreme example of this can be seen on Mount Everest, where layers of sea-floor sediment can clearly be seen running throughout the upper reaches of the mountain. This is because what was once rock that formed the floor of the Indian Ocean has been forced upwards to form the Himalayan Mountains by the Indian subcontinent moving north into Asia.

The result of these processes can be seen in many parts of the world. The cliffs in Namibia show large drop stones contained within a layer of rock. Drop stones are thought to be a clear indicator of glaciation in the area at a certain point in the past. Carbon dating can then be used to find out at what point in time the layer was formed, showing when the drop stones were deposited. This gives the date of the glaciation in that region.

Origins of the Snowball Earth Theory

The theory of Snowball Earth was first seriously posed by Brian Harland, a Geologist at Cambridge University. During his field trips in the Arctic, Harland became accustomed to seeing evidence of rocks being moved by ice, and then being deposited where the ice had melted. The presence of drop stones was, to him, an obvious sign that ice had been in an area at some point. When he studied rocks from around the world, he was astonished to find drop stones in some of the hottest places in the world that were not thought to have ever been glaciated. He realised that he had found evidence of a global glaciation.

At the height of the last ice age, 20,000 years ago, ice spread south from the North Pole as far as where New York is today. Harland suggested a hypothesis of a global glaciation, a 'Snowball Earth', with ice extending across the equator. This hypothesis attracted much hostility and scepticism.

Scepticism of the Snowball Earth Hypothesis

The reason for this scepticism is clear: The tropics always receive the full glare of the Sun. Even during the last ice age, geological evidence proves that the tropics were pleasantly warm. And, there was an alternative explanation for drop stones in the tropics - an explanation that Harland himself had developed years earlier. The theory was continental drift.

Continents currently move at a speed of between 1 to 10 centimetres per year. The sceptics pointed out that it was obvious that the continents would have been orientated differently 600 million years ago, and that the parts of the world that contain drop stones from these periods must have been located nearer to the poles when this happened.

The theory hit a roadblock.

Nuclear Winter and Proof that Snowball Earth was Possible

During the 1960s, at the height of the cold war, the governments of the world were busy calculating the chances of surviving a nuclear holocaust. The smoke that would have been caused by the intense bombing was predicted to be so thick and widespread that it would have blocked out the sun for years, causing a nuclear winter. This would have resulted in widespread crop failure, leading to famine.

Mikhail Budyko, a Russian climatologist working for the State Hydrological Institute in St. Petersburg, was one man given the task of calculating the extent of a nuclear winter. He was able to determine that the original predictions, which stated that the topics could not freeze over, were complacent and over-assumptive.

He pointed out that sheets of ice covered with snow reflect sunlight and heat back into space because they are bright white. This is known as an 'albedo' effect. Land and oceans, however, are much darker: they absorb heat from the sun. As ice coverage becomes more and more extensive during a period of cooling (such as the cooling produced by the start of an ice age, whether natural, or man-made as in the case of a nuclear winter), more and more sunlight is reflected back into space, cooling the Earth even further in a vicious cycle.

Budyko also worked out that the Earth's climate system has a breaking point: If the ice were to advance as far south as Texas is today, enough sunlight would be reflected back into space that the climate would pass a tipping point and create a runaway cooling effect, creating even more glaciation and, eventually, a complete glaciation of the planet $#45; a Snowball Earth. He also predicted that a snowball earth would reflect so much solar radiation that it would never warm up again; it would be frozen for eternity.

Whilst this gave weight to the hypothesis of Snowball Earth, it also appeared to disprove it. If the Snowball Earth theory was correct, surely the Earth would never have warmed again, never broken out of its snowball state and we would never have existed? The Snowball Earth hypothesis became a supposed impossibility.

The theorists who supported the snowball theory clearly had more work to do. They had to prove that not only had the continents been located in the tropics when the drop stones had been deposited, but that it was also possible for the Earth to have recovered from the snowball state and warmed itself back up again.

Evidence of Glaciation in the Tropics

During the 1970's, geologist Joseph Kirschvink set out to prove that the presence of drop stones in tropical regions such as Namibia could be explained by continental drift. He was of the opinion that the tropical regions of today must have been located near the icy poles at the time when the drop stones were deposited, and that continental drift had allowed the areas with the drop stones to be transported to the tropics over the last 600 million years.

Through his extensive work with magnets he had devised a revolutionary method for determining the latitude (how far north or south) at which any rock had been formed. His process was based upon the principle that, when a rock is formed out of molten lava, the liquid magnetic components of the rock line up with the magnetic field of the Earth before it cools and solidifies. This magnetic signature is then fixed from the moment it solidifies. Rocks that are formed at the poles have a magnetic alignment that points up and down, whilst rocks formed at the equator have an alignment that points from side to side.

Kirschvink was able to determine that the drop stones found in tropical regions were, in fact, formed near the equator. This meant that, 600 million years ago, there must have been glaciation in the tropics. The idea of continental drift explaining the location of the drop stones had taken a huge blow. But the question of how the Snowball came to an end still remained.

How Could Snowball Earth Have Come to an End?

To end the Snowball Earth state, a source of heat would have been required to persist throughout the ice age, despite average global temperatures of -50 degrees Celsius. It became apparent to Kirschvink that volcanoes could have provided this heat. They would have continued to erupt whether the planet was in an ice age or not. However, even a large volcano would only melt the ice in the immediate area. Something else would have been needed to raise temperatures globally.

His attention turned to something else that volcanoes provide: carbon dioxide. In today's climate, the small amount of carbon dioxide produced by volcanoes is quickly washed out of the atmosphere by rain. Added to this is the fact that volcanoes also produce sulphur dioxide, which mixes with clouds at high altitudes, making them shinier. This, combined with thick, dark ash that volcanoes often produce blocks out sunlight, produces a cooling effect which is much greater than the warming effect produced by the carbon dioxide.

However, during the Snowball Earth state, nearly all the water on the planet would have been frozen solid. There would have been hardly any clouds to be made shinier by the sulphur dioxide and almost no rain to wash the carbon dioxide out of the atmosphere. With the cooling effect of the volcanoes removed and the opportunity for carbon dioxide to build up in the atmosphere over a period of millions of years, the conditions would have been set for global warming on an unimaginable scale.

Evidence of an Immense Greenhouse Effect

Currently, there is about 0.04% carbon dioxide in the atmosphere and this figure is rising. But, Kirschvink found calculations that showed that, over millions of years, enough carbon dioxide would have built up in the atmosphere during the Snowball Earth period to make it over 10% carbon dioxide. He worked out that this would have been enough to raise temperatures from -50 to +50 degrees Celsius, a change of 100 degrees and more than enough to melt the ice. This is hotter than the Earth has ever been. All that was needed now was proof that this had happened.

In 1992, Professor Paul Hoffman of Harvard University entered the picture. He wanted to find hard evidence that proved Kershwin's theory that carbon dioxide induced global warming was responsible for ending Snowball Earth.

He went back to Namibia and noticed that immediately above the layer of drop stones were huge layers of carbonate. These 'carbonate caps' had puzzled geologist for generations. They usually form in warm water, but in Namibia they had formed immediately over the ice cold conditions of the drop stone layer. It showed that the climate had suddenly changed due to a colossal greenhouse effect.

Hoffman turned to geochemist, Dan Schrag. They painstakingly went through the final stages of the snowball period, trying to work out what processes were at work. They worked out that firstly, the ice would have melted to form liquid water. This water would then have evaporated to form clouds. The formation of clouds would have eventually led to rain.

The rain would have mixed with carbon dioxide in the atmosphere to form carbonic acid, which would then have rained down heavily on the Earth. The carbonic acid rain would have broken down the newly exposed rocks into their constituent parts, one of which is calcium. The calcium then would have fused with the carbon in the carbonic acid rain to form calcium carbonate. This is exactly what was found immediately above the layer of drop stones in Namibia. The layers of calcium carbonate in the rocks indicated extremely high levels of carbon dioxide in the atmosphere, a gas which causes global warming. This was hard evidence that supported Kirschvink's theory.

Atmospheric carbon dioxide levels of that magnitude would have changed the weather more dramatically that anything ever seen. It would have been the 'mother of all climate changes', according to Hoffman, as the Earth went from the coldest conditions it has ever experienced to the warmest it has ever experienced. The change would have been unbelievably violent, with rain storms, hurricanes and ocean waves of over 100 metres high, turning the sea-ice off the coast of Namibia into a tropical ocean in a flash.

How Could Life Survive the Snowball Earth Scenario?

In 1998, botanists claimed that the Snowball Earth scenario was impossible. One scientist, Guy Narbonne, pointed out that at the time of Snowball Earth, all life lived in the oceans. Cyano bacteria and green algae were living before and during the snowball period, proven by fossil records. These are both photosynthesising organisms that use sunlight to gain the energy they need to grow in the same way that plants do. Narbonne believed that they would not have survived if they had been covered by ice more than a few tens of metres thick, as they would not have received enough sunlight. This produced a paradox: the geological evidence pointed to global glaciation; the fossil records pointed to open expanses of water.

Biologists believed that Hoffman and Schrag's predictions of ice sheets over 1km thick, and tens of metres thick at the tropics, would have prevented sufficient sunlight from penetrating the oceans. This in turn would have prevented photosynthesising organisms from surviving. Botanists believed that, if the Earth had been covered completely by ice, life would have been wiped out completely, even in the oceans. Maybe there was a severe ice age, but not a complete covering of ice?

Along came Chris McKay, a planetary scientist with an interest in dry and cold conditions, from the Space Science Division at NASA. He heard of the Snowball Earth theory and wondered how life could have survived these conditions.

He had visited Antarctica, which is an environment so cold that it can be compared to equatorial regions during the snowball period. In Antarctica, there are the dry valleys where there are lakes covered by ice many metres thick. Scientists decided to dive beneath the ice on these lakes to see if any life existed. To their surprise there was lots of light, even though the ice above was over 5 metres thick. There was so much light that they found cyano bacteria and green algae living quite happily in the water.

McKay realised that the reason there was so much light in the lakes was that the ice itself was like glass. It was totally transparent due to the fact that it was freezing very slowly because of its insulating snow cover. By freezing slowly, the water had rejected the dirt, salt, air and other impurities to produce very transparent ice. They realised that even during the coldest times of the snowball period, there would have been areas where the ice was thin enough for life to survive.

A Catalyst for Life

Any life that did survive would have emerged from the snowball period to a time of almost no competition from other species. This would have been a perfect time for a period of explosive development of life and evolution. The fossil records show that the complexity of life dramatically increased after the snowball glaciations. It is therefore said that although the snowball was the greatest catastrophe that the Earth has ever seen, almost wiping out life entirely, it actually served as a catalyst for the development of complex life. Indeed, the first multi-celled organisms appeared after the snowball period.

No one knows what caused the Snowball Earth period, if it did happen for certain, or if it could happen again. The sun is hotter now than it was 600 million years ago, and is getting hotter. Climate trends would have to change dramatically for it to happen again. But, if it did happen again, it would mean the death of almost everything, including us.

Was the Earth Really Frozen Solid?

This is the subject of much debate. The theory of Snowball Earth is still controversial, as many scientists believe that some sort of hydrological cycle would have been necessary during the glaciation, meaning areas of ice-free ocean. It may have been more of a 'Slushball Earth' than a Snowball Earth, with much of the tropics containing thin, broken ice or no ice at all.

Some scientists believe that the drop stones found in the tropics may have been caused by the splitting apart of major continents. This could have caused uplift of the ground to high altitudes where glaciation could have been possible due to the lower temperatures at those heights. The Great Rift Valley in Eastern Africa is a good example of this type of continental 'unzipping' taking place, leading to an area of higher ground.

It is also thought that the Earth's magnetic field may have been different during the snowball period, which could potentially lead to problems with Kirschvink's measure of the magnetic signature of the rocks. In addition to this, there appears to have been at least three potential snowball states separated by brief warm periods, called 'Interglacials'. There is also the tricky problem of verifying the facts and data from that time period.

The general consensus appears to be that there were extensive ice ages between 600 and 800 million years ago, which may have reached tropical areas in places. These were, in all likelihood, reversed suddenly by extremely high levels of carbon dioxide and possibly methane. This process appears to have ultimately provided the catalyst for complex life. The arguments over whether the Earth really was a 'Snowball' at some point will probably continue for decades to come.

What Relevance does this have Today?

If true, the escape from the snowball scenario shows just how powerful greenhouse gasses can be. If the temperature of the planet can be raised by 100 degrees Celsius in approximately 1000 years, we are certainly at its mercy. Today we are undergoing a period of global warming due to carbon dioxide emissions from industrial and agricultural sources. So far, warming is about 0.5 degrees Celsius compared to pre-industrial levels, with further warming predicted to be 3-4 degrees this century if left unchecked.

The emergence from the Snowball Earth scenario shows that this is more than possible during the timeframe of our own lives. It also shows the effect that the growth and recession of ice coverage has on temperature. The current thinning and rapid disappearance of large areas of Arctic sea ice and the recession of mountain glaciers should be ringing alarm bells.

Given that Antarctica gained its ice cap when carbon dioxide concentrations fell below 420 parts per million, and that we are currently at a level of around 380 parts per million and rising (up from 280 parts per million before industrial times), it shows how close we are to a major tipping point for the Earth's climate. Whilst the Antarctic ice sheet is probably thick enough to survive carbon dioxide concentrations above 420 parts per million for a short time, there is no reason to believe that it will last indefinitely.

Climate change has gone berserk in the past due to greenhouse gasses; we must not push it too far again.

References and Further Reading

This article was based largely upon the television programme 'Snowball Earth', from the Horizon series by the BBC, with additional information from other sources (see links in the main text). You can view the programme's website here and a full transcript of the programme here.

This programme has recently featured on the UKTV Documentary channel, so look out for it if you want to watch!

Further reading on the Snowball Earth theory, and criticism of it, can be found at the following websites:

www.snowballearth.org - The 'official' Snowball Earth site.

http://en.wikipedia.org/wiki/Snowball_Earth - Discusses evidence for and against the Snowball Earth theory and counter-theories.

http://www-eps.harvard.edu/people/faculty/hoffman/snowball_paper.html - Snowball Earth paper by Paul Hoffman and Daniel Schrag.

Photo Credit: Stephen Hudson (used under GNU License Agreement). Source: http://en.wikipedia.org/wiki/Image:AntarcticaDomeCSnow.jpg