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Understanding Greenhouse Effect


We often speak of the Greenhouse effect as something that is always undesirable or dangerous to life on earth.  However, without the greenhouse effect, the earth would be very cold, with an average temperature of around minus 18 degrees centigrade. The greenhouse effect is an atmospheric condition created by the heat energy radiated by the sun and greenhouse gases that are normally present in the atmosphere. Briefly, sunlight passes through the atmosphere and warms the Earth, which radiates this energy back into space. As the energy passes through the atmosphere, part of it is absorbed by greenhouse gases (water vapour, carbon dioxide, methane and nitrous oxide), and the rest is reflected back towards the sun. The trapped energy from the sun warms the earth.

The greenhouse effect becomes a problem when this balance is disturbed by human intervention.

The natural balance

Venus’s atmosphere is high in carbon dioxide (96%), a greenhouse gas, so that its average surface temperature is 450 degrees centigrade. If the earth’s atmosphere trapped all the sun’s heat that passes through it, the earth’s temperature would continue to rise to those levels. However, the earth’s atmosphere contains only around 0.03% carbon dioxide, which allows much of the sun’s heat to be reflected back into space.

On our planet, the temperature only rises until the amount of infrared or long wave radiation leaving the Earth equals the amount of energy coming in from the sun. This means that the earth receives the same amount of the sun’s energy as it reflects. As long as the amount of greenhouse gases in the atmosphere stays the same, and the amount of heat arriving from the sun stays the same, there is a balance. In this state of balance created by the natural greenhouse effect, the earth has an average temperature of 16 degrees centigrade.

Atmospheric change

The earth's atmosphere is composed of 78 % nitrogen, 21% oxygen, and only about 1% of greenhouse gases (water vapour, carbon dioxide, methane and nitrous oxide). By analysing air bubbles trapped in ancient glaciers, scientists have learned that before the Industrial Revolution began place in England around 200 years ago, the combination of atmospheric gases was relatively constant. However, with the rapid growth of industries, agriculture, and population over the last 200 years, more and more greenhouse gases were released into the atmosphere.

Many scientists believe that these increases will cause the earth’s temperature to rise, resulting in an enhanced greenhouse effect. Other scientists believe that these changes are just part of the natural cycles of change that continuously occur on earth. While most agree that earth’s temperature is rising, they disagree about how much it will rise, how fast it will rise, and what the effects will be on earth and on living things.

El Niño

The El Niño is an abnormally warm current that occurs every three to eight years. It begins with a small change in atmospheric pressure and sea temperature off the coast of Australia and develops into a major effect in the waters of South and Northern America. El Niño prevents the upwelling of cold water that brings nutrients to the surface to feed surface fish. As a consequence, thousands of fish die, seabird depending on them for food die, and fishermen have no income. In addition, erratic weather and storm patterns, some of great ferocity, can result from the atmospheric effects of El Niño. For this reason, El Niño is used by many as an illustration, even a precursor, of the devastation that would result from global warming.

Ozone as a Greenhouse Gas

Ozone is a form of oxygen formed when oxygen atoms combine oxygen molecules (which have two atoms each) to create molecules made of three oxygen atoms. Unlike oxygen, ozone is poisonous. However, most of it is found in that layer of the atmosphere called the stratosphere, where it blocks out the sun's ultraviolet rays. Since ultraviolet radiation can damage DNA (genetic material) and is harmful to most living things, including plants, ozone’s blocking action is an important and beneficial atmospheric effect.

It is believed that the amount of ozone required to shield the earth from lethal UV radiation existed 600 million years ago, and enabled life forms to develop and live on land. Previously, life had only occurred in the oceans.

Synthetic chemicals called chlorofluorocarbons (CFCs) are now well-known as destroyers of ozone. Although some ozone-depleting substances occur naturally, the human-produced ones are much more abundant, and they are long lived. Over several years, they rise the stratosphere, where they are broken down by the ultraviolet radiation, releasing chlorine atoms. These chlorine atoms react with ozone to form new molecules.

These reactions only occur in conditions of extreme cold, darkness and isolation, followed by exposure to light. Such conditions can be found over polar regions just at the beginning of Spring. In Antarctica, the worst affected area, scientists have found a vast region of the upper atmosphere where there is less ozone than elsewhere. Although it is called a hole, it really is an area where the ozone layer has become thin. This ozone-.poor air can spread out into other areas. Also, ozone loss is occurring in other places, though not as much as over the poles.

Despite the fears regarding the thinning of the high altitude ozone, there is another fear that the low level ozone is increasing in the middle latitudes of the Northern Hemisphere because of industrialisation. The low level ozone affects the eyes, making them smart. The low level ozone produced when vehicle and factory emissions "cook" in the sun is increasing over industrialised areas by 0.5 to 4% per year depending on location. Gribbon says that at the beginning of the industrial revolution there was probably only one quarter of the low level ozone that exists now, and that by the year 2030 the amount can be expected to double. Gribbon also says that low-level ozone is a far bigger problem than either nitrous oxide or methane as an agent in the greenhouse effect.

The Montreal Protocol, an international agreement, limits the production and use of ozone-depleting substances. CFCs and most other ozone-depleting substances are banned or strictly controlled. Their replacements, the hydrochlorofluorocarbons, are less ozone-depleting, but are also being phased out. This has already resulted in less ozone loss, but because of CFCs still in the atmosphere, the ozone hole will continue for some decades after that.

Further Information:
The authors of this article are staff orf ACS Distance Education. This school operates from both Australia and the UK, offering over 350 courses (Hobby and Vocational). See www.acs.edu.au, www.acsedu.co.uk
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