The greenhouse effect.

Over the past two centuries, the average temperature of our planet has been rising faster than at any time during the last 1,000 years

Anybody who gets into a car that's been sitting in the sun all afternoon understands the "Greenhouse Effect." Sunlight pours into the vehicle through the windows, but the resulting heat is trapped inside. The temperature in the car soars and turns chocolate bars to goo.

On a larger scale, this Greenhouse Effect is harnessed to work for us in real greenhouses. By trapping heat from the sun behind glass, we can grow tender fruits and vegetables in places where it would normally be too cold for such activities.

On an even larger scale, the Greenhouse Effect may be the downfall of the human race and every other species on planet Earth. The end of life on Earth is the disaster scenario of some scientists and environmentalists. At best, the Greenhouse Effect as it's applied to global warming is a theory; and this is how the theory goes.

Human industrial activity has created many by-products, among them large quantities of so-called greenhouse gases. Chief among these gases are carbon dioxide ([CO.sub.2]) and water vapour. These are produced when fossil fuels such as coal, petroleum, or natural gas are burned. Other greenhouse gases such as nitrous oxide and methane are also by-products of this combustion. However [CO.sub.2] and water vapour have been picked out of the line-up as the leading villains.

The Greenhouse Effect theory says that the gases mentioned mix with the Earth's atmosphere to change its composition. Higher concentrations of [CO.sub.2] and water vapour mean that a gaseous ceiling has formed all around the globe. Sunlight penetrates this ceiling, but some of the heat that comes from the sun's rays is trapped by it. The effect of this, says the theory, is a gradual rise in the Earth's average temperature.

Some experts fear the process may start feeding on itself. As the air warms up more water evaporates from soil, lakes, and oceans. Warmer air can hold more water vapour which intensifies the Greenhouse Effect. The really gloomy people fear that once this process gets underway nothing can stop it. The heating process will just accelerate until every living thing reaches its boiling point.

It's only since the Industrial Revolution of the late 1700s that we've been burning fossil fuels in large quantities. So, if the theory holds up in practice, we should be able to see some upward movement in temperatures over the last couple of centuries. And, that's exactly what scientists have found.

By 1865, factories and steam locomotives had been belching out vast quantities of steam and [CO.sub.2] for several decades. At that time, the average air temperature at the surface of our planet was around 13 [degrees] C. By the 1990s, millions of gasoline-powered engines were burning fossil fuels and most of our buildings were heated or cooled by similar energy sources. In addition, the number of people in the world has grown from one billion in the late 1770s to around six billion today. And, during the last two centuries average global temperatures have risen by 0.6 [degrees] C.

You might think that an increase of a tad over half a degree Celsius in 200 years isn't worth loosing sleep about. But, the Greenhouse Effect theorists warn that this is just the start of something bigger.

Most of the greenhouse gases are produced in industrialized areas such as North America and Europe. As developing nations, such as India and China, become more industrialized they will add vast new amounts of greenhouse gases.

At present, human activity adds about three billion tonnes of carbon (in the form of [CO.sub.2]) to the atmosphere every year. If we carry on chucking goop into the air like this we're going to see much more dramatic temperature increases. We could see average temperatures rise by as much as 3.5 [degrees] C during the next 100 years. That's the high-end estimate made by a United Nations agency in 1995 after many years of intense study.

If that happens, our descendants could be staring a major disaster straight between the eyes. There would be massive flooding, frequent and extreme weather catastrophes, dramatic shifts in vegetation zones, and an enormous loss of lives.

In the end, the Greenhouse Effect theorists say we have no choice but to head off the crisis by reducing greenhouse gas emissions aggressively. If the theory proves to be wrong there's no harm done. But, what if we do nothing and the theory proves to be right?

SUGGESTED ACTIVITY:

Assign a team of students to research Milankovitch Cycles and to conduct a seminar on the phenomena for the class. Milutin Milankovitch was a Serbian scientist who, in the 1930s, first calculated fluctuations in the Earth's orbit and related them to climate changes. The "eccentricity cycle" takes 93,408 years to complete and describes how the Earth's orbit around the Sun is not perfectly circular. The second cycle shows how the Earth's tilt changes over a 41,000year period. The third orbital phenomenon is the 25,920year precession cycle, which is similar to the wobble of a spinning top. According to Mr. Milankovitch's calculations the Earth should he in the middle of a cooling period at present.

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