The Triangle

Issue date: 7/9/04 Section: Sci-Tech

Media Credit: http://www.thedayaftertomorrow.com/

It doesn't take a rocket scientist to figure out that weather isn't constant.The weather changes from day to day, and even more noticeably from season to season. But climates also change appreciably over much longer periods of time, ranging from thousands to millions of years.

The climate of a region can be thought of as the pattern of weather over a particular length of time, usually something like a year. The climate of the region takes into account all its seasonal variations.

Ice ages are caused by changes in the Earth's climate over tens of thousands of years. We are all aware that our ancestors came to North America tens of thousands of years ago sometime during the last ice age, when glaciers extended down into all the current temperate latitudes.

But ice ages have been occurring since long before then, and there was once an ice age which made the previous one seem like a summer beach outing in comparison. Compelling geological evidence exists that until about 600 million years ago, the Earth has entered at least one such period of dramatic climate change and alternated between deep freeze and greenhouse.

The last of these "mega ice ages" was thought to be responsible for killing off most life on the planet, and paving the way for larger multi-cellular organisms to evolve. What caused the Earth to enter these periods of dramatic climate change?

One of a number of factors that may have triggered climate change is small variations in the angle of Earth's rotation on its axis or around the sun itself. Though the Earth is at an angle of 23.5 degrees, it does wobble around its axis, which is responsible for the shifting of magnetic poles over time.

This wobble can result in different amounts of solar radiation reaching the surface, thus affecting surface temperatures. Volcanic eruptions also cause a cooling of the planet. The dust that is emitted into the atmosphere reflects solar radiation back into the atmosphere. In fact, the explosion of Mount Krakatoa in Indonesia in the early 1800s caused the famous "year without a summer" in the northern hemisphere, and that year it snowed in New England in July.

In any case, we know that there was some factor that caused a cooling of the planet and thus the ice caps started expanding slowly. Since snow and ice are white, they are very good at reflecting solar radiation back to space. So in a snowball effect, the more the ice caps expanded, the more light they reflected, and the cooler the planet became, the more ice caps expanded, and so on.

At first, the expansion happened slowly. As the ice caps started approaching the tropics, however, they started expanding at an alarming pace, since now a large portion of the sunlight reaching the surface was being reflected back. Eventually even the tropics froze over, and all of the world's oceans were covered with kilometer-thick ice. The Earth remained this way, a giant ball of ice hurtling through space, for tens of millions of years.

The average global temperature plummeted to an inhospitable minus 40 degrees Celsius (minus 40 degrees Fahrenheit). Compare that to the average global temperature today of 17 degrees Celsius (63 degrees Fahrenheit). The only type of life that survived would have been extremely hardy microorganisms growing near deep sea volcanic vents and other such places.

Media Credit: http://www.cdli.ca/ The glaciers that covered the Earth eventually altered overall climate.

It was a puzzle to scientists how the Earth was able to come out of this ice age, since it was not possible to imagine the Earth warming itself when the global ice sheet would have reflected most of the light back into space. The Earth did eventually warm of course, otherwise I wouldn't be sitting here today typing this. So what exactly happened?

I'm sure you're all aware that rocks are constantly being eroded by the oceans. This weathering of the rocks results in the formation of calcium carbonate, which is washed into the oceans. To form calcium carbonate, carbon dioxide is pulled out of the air. As the oceans froze over, the weathering process of water on rocks also ceased.

Carbon dioxide, a greenhouse gas, was not being pulled out of the air anymore and was allowed to build up in concentration. To bring Earth out of this deep freeze, a significant amount of carbon dioxide was required. It would have taken a concentration of carbon dioxide far greater than what exists today to compensate for the reflected light from the ice.

Geological records indicate that carbon dioxide levels in the atmosphere reached as high as 1 percent, compared to about 0.037 percent today.

This carbon dioxide was supplied through geological processes such as volcanic eruptions. It took tens of millions of years to accumulate enough carbon dioxide to break the shell of ice encasing the planet.

While the Earth did come out of an ice age, it was a case of going from the freezer into the frying pan. The high carbon dioxide concentration and the receding ice caps, and thus less reflected sunlight, turned Earth into a hothouse.

Average global temperatures went up to a sizzling 40 degrees Celsius (104 degrees Fahrenheit). When this happened and all the ice caps melted, renewed weathering of rocks would have once again started the process of removing carbon dioxide from the atmosphere and depositing it in the form of calcium carbonate. In any case, before this catastrophic ice age happened, little complex life existed on Earth. This event proved to be a jolt for life, and afterwards, flora and fauna alike flourished, blossoming into amphibians, giant reptiles in the form of dinosaurs, giant ferns and eventually flowering plants.

You may wonder that if such a catastrophic ice age happened then, why doesn't a similar ice age happen again? The main difference between then and now is that at that time, most of the Earth's land mass was clustered around the equator as opposed to now, when a significant portion of it is near the poles.

Take a look at Antarctica, for example. Here you have a large continent that is permanently covered by a sheet of ice and thus no weathering of rocks goes on. If most of the Earth's landmass was near the equator however, the polar ice sheets could expand into the temperate latitudes without affecting the weathering process much.

So even as the polar ice caps grew, carbon dioxide was still being pulled out of the air. With the continents in the position they are in now, such a scenario is not possible.

Even at the height of the last ice age, the ice caps were nowhere near the threshold limit necessary for a runaway global deep freeze to occur. In the coming hundreds of millions of years, as the continents realign themselves, it will have to be seen whether we will enter another mega ice age.

If we do, it will only inevitably jolt life on Earth in some new direction.

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