What is the atmosphere?
The atmosphere is the thin envelope of gases that surrounds our planet. It is divided into five layers, known as the troposphere (which is where we live), the stratosphere, mesosphere, thermosphere and, more than 400 kilometres above the surface of the earth, the exosphere.
The actual mixture of gases makes it quite different to the atmosphere of our nearest neighbours in the solar system. Those of Venus and Mars consist predominantly of carbon dioxide, with water vapour, nitrogen, oxygen and other gases accounting for only a few per cent between them. But the earth's atmosphere consists of 78 per cent nitrogen, 21 per cent oxygen and considerably less than 1 per cent carbon dioxide, with very tiny quantities of other gases and variable amounts of water vapour.
This delicate mixture has several functions, apart from providing oxygen for animals to breathe and carbon dioxide for plants to use in photosynthesis. The most important of these are absorbing the lethal ultra-violet rays of the sun and keeping the earth warm.
Environmental scientists have been aware of the fragility of the atmosphere for a long time. But it is only recently that world leaders have begun to acknowledge the serious and widespread atmospheric damage being caused by modern society and industrial processes.
Two of the most important problems are the depletion of the ozone layer and global warming. Widely considered to be among the most critical environmental issues facing the world today, they are both global in scope and can be solved only by an unprecedented level of international cooperation. The two issues often gets confused but, while there is a connection between ozone and global warming (ozone contributes to global warming), they are entirely different.
What is ozone?
Ozone is a gas, a form of oxygen. It occurs naturally in the upper atmosphere, where it acts as a shield protecting the earth from dangerous ultraviolet (UV) radiation; and it occurs at ground level, where it is man-made (a major source is vehicle exhausts) and toxic to both animals and plants.
The thin mist of ozone higher up in the atmosphere is essential to life on earth. Plants and animals depend on it as much as they depend on food and water. It occurs at an altitude of between 16 and 48 kilometres, with a peak concentration in the stratosphere some 20 kilometres above the surface of the earth. Even there, it is so diluted that its concentration is no more than ten parts per million; if it were all brought down to sea level, at atmospheric pressure, it would compress into a layer only 3 mm thick. Although a form of oxygen (it has three atoms in the molecule instead of two), ozone has very different chemical properties which are critical in its role of screening out UV radiation. It is so effective that it keeps 95-99 per cent of the sun's ultraviolet radiation from striking the earth.
A certain amount of UV is beneficial - it activates vitamin D in human skin, for example. But too much causes sunburn, skin cancer and eye damage, and may suppress the immune system. It is also believed to slow plant growth and to decrease the productivity of phytoplankton in the sea.
Concern about the ozone layer
Concern about ozone was first expressed in the early 1970s. But it was a discovery in 1985 which surprised the experts most - and shocked the world. A team of British scientists reported that they had found a hole in the ozone layer over the Antarctic. Since then, this hole has grown steadily larger and now sometimes covers an area the size of Antarctica itself. Meanwhile, a new hole, first detected by Canadian scientists early in 1989, has been reported over the Arctic. Both holes are moving around - the Antarctic one has even 'hovered' over Melbourne - and they fluctuate seasonally and according to climatic conditions.
How is the ozone layer damaged?
The holes appearing in the ozone layer - and overall depletion of the ozone layer - are man-made. The major culprit responsible is a group of chemicals known as chlorofluorocarbons, or CFCs, although halons (used in some fire extinguishers) are also to blame.
CFCs were invented in 1928 and quickly became an 'essential' ingredient in myriad industrial and domestic products. They are used as propellants in aerosol sprays, and as refrigerants, in insulation products, as coolants in air-conditioning plants, in cleaning agents (for everything from computer chips to dirty clothes) and in plastic foam packaging for things like the take-away cartons used by some hamburger restaurants.
As their name suggests, CFCs are molecules consisting of chlorine, fluorine and carbon atoms. When they are released into the air we breathe they cause no harm at all. They are non-toxic, have no smell and are even non-flammable. Unfortunately, the very same characteristics that render them so inert down here enable them to remain unchanged long enough to drift slowly towards the stratosphere. They may take as long as ten years to reach the ozone layer, which is where they begin to cause serious damage.
The problems are caused because, when a CFC molecule is exposed to the strong UV radiation high above the earth's surface, it is broken apart. This releases a chlorine atom which attacks the ozone. It is a complicated process but, in simple terms, a single chlorine atom can destroy between 20,000 and 100,000 ozone molecules.
Why does it matter if the ozone layer is depleted?
As the ozone layer is depleted, more UV radiation squeezes through the protective shield to reach the earth's surface. An American politician once commented that if too much gets through, we will all have to wear hats and sunglasses, and to apply very strong suntan lotions whenever we go outside. But the reality is much more frightening.
Research in the US has shown that a depletion of just 1 per cent of the ozone layer causes a 2 per cent increase in the incidence of skin cancer. It can also cause genetic damage and more people will suffer from cataracts and other eye diseases caused by the radiation. At the same time, a higher dose of UV may slow down plant photosynthesis, the process by which green plants change carbon dioxide and water into carbohydrate and oxygen. And the world's oceans could be affected. Although very little is known about this, it appears that too much UV radiation reduces the density of plankton. If this were to happen on a large scale, it would affect the entire ocean ecosystem. Increased UV radiation in the lower atmosphere can also result in increased amounts of photochemical smog, which is already a health hazard in many of the world's largest cities.
Patching up the ozone holes
If CFC emissions were reduced to zero today, the chemical reactions that are depleting the ozone layer would continue for at least a century. There are two reasons for this: the bulk of CFCs already released have yet to reach the stratosphere; and, once the chlorine atoms have broken away, they will remain active for a very long time. But this is not cause for complacency - quite the reverse, it makes the situation all the more urgent.
The obvious answer is to ban the production and use of CFCs. An agreement to do precisely this was signed in 1987 and has been amended several times since (each amendment is named after the city where the revision committee met). Called the Montreal Protocol on Substances that Deplete the Ozone Layer, it was an important first step in galvanising world action to deal with the problem. Governments committed themselves to reduce consumption and production of CFCs - and to phase them out by the year 2030. The latest revision of the Montreal Protocol was held in Egypt in 2009 and the resulting agreement was signed by over 100 nations representing 95 per cent of the world's current CFC consumption. But many experts believe that it is still too weak and that more drastic reductions should be negotiated as a matter of urgency.
Unfortunately, while many alternative materials are already available, there is still debate about what might be an acceptable substitute for CFCs - because none of the alternatives are entirely harmless.
What is global warming?
Temperatures on the moon swing from 100ºC during the day to minus 150ºC at night. But the earth's temperatures vary much, much less - typically by no more than about 50ºC. How can this be, when the two are both roughly the same distance from the sun?
The answer is simple: the earth has an atmosphere and the moon does not. Certain atmospheric gases regulate the temperature of the earth, maintaining it at a suitable level for life and calming fluctuations between night and day.
A staggering amount of energy enters the atmosphere from the sun. Some bounces back into space, reflected by clouds, but the remainder manages to reach the earth. Some of this heat is trapped, as if the earth were inside an enormous greenhouse. Indeed, certain gases in the atmosphere have similar properties to the panes of glass in a greenhouse: they are transparent to the sun's rays but allow little heat to escape. This is how the air inside the greenhouse, and the air around the earth, is kept warm. The gases are therefore called greenhouse gases, and their effect, the greenhouse effect. It is a perfectly natural phenomenon. Without greenhouse gases, in fact, the earth would be 33ºC colder - frozen and lifeless.
It stands to reason that the greater the concentration of greenhouse gases in the atmosphere, the more of the sun's heat will be trapped and the warmer the world will become. This is 'global warming'. The earth could be getting warmer on its own, but many of the world's leading experts point to the fact that human activities are spewing greenhouse gases into the air at an astonishing rate and they argue that we are the primary cause of global warming.
There will never be full agreement on something so complex and all-pervading, but there is no doubt that the average global temperature has been rising since about 1850. The accumulated evidence suggests that the world could continue to heat up by several degrees before the middle of the century. To put this into perspective, an increase of just 3ºC would make it warmer than it has been for 100,000 years. Unfortunately, though, we do not have the answers to two fundamental questions. How hot will the world become? And what will be the effect?
What are the greenhouse gases?
There are five main greenhouse gases - carbon dioxide, methane, CFCs and related compounds, ground-level ozone and nitrous oxide.
The most important is carbon dioxide, which most scientists insist is the chief cause of global warming. Natural carbon dioxide comes from volcanic eruptions, the decomposition of organic matter, gas exchange in the oceans and from respiration by animals. A certain amount is absorbed naturally, by 'sinks' or 'sponges' such as oceans, forests and peatlands. Man-made carbon dioxide is generated by burning coal, oil and natural gas (fossil fuels) in power stations, mostly in rich countries, and the burning of forests to clear land for agriculture, mostly in poor countries. Carbon dioxide concentrations started to rise significantly with the Industrial Revolution, when we began to mine and burn coal - releasing the trapped carbon back into the atmosphere. The extraction and burning of oil, which started about a century ago, released yet more carbon. According to the Royal Commission on Environmental Pollution, the concentration of carbon dioxide in the atmosphere is already higher than at any time for millions of years.
Methane is believed to be the second biggest contributor to global warming, and it is some 21 times more effective in absorbing heat than carbon dioxide. The main natural sources are still not entirely clear, but it is certainly produced by microbes in swamps and marshes, by termites, and by ruminants such as cattle (cows and other ruminants produce huge quantities of methane in their digestive systems and belch and fart it into the atmosphere). Methane also bubbles up from rice paddies, and builds up in landfill sites when household rubbish decays. There are huge stores of methane under the Arctic tundra and, if this is ever released, it might push global warming totally out of control.
Three more pollutants make up the remainder and are all man-made. CFCs are found in myriad industrial and domestic products; ground-level ozone is produced by the action of sunlight on certain vehicle emissions; and nitrous oxide is emitted by car exhausts, fossil fuel combustion, nitrogenous fertilisers, ploughing fields and the burning of vegetation.
How hot will the world become?
If atmospheric concentrations of the greenhouse gases continue to rise, average world temperatures are likely to rise as well. But we are not sure by how much.
Most estimates and forecasts are based on complex mathematical models and sophisticated computer simulations of the world's climate, but these are far from perfect. One alarming theory, for instance, is that there could be a sudden and very dramatic change - a kind of climatic backlash - which, by its very nature, is completely unpredictable.
The key policy forum for action on climate change is the Intergovernmental Panel on Climate Change (IPCC). The IPCC initially predicted a temperature rise of 3ºC but a few years later doubled that prediction. Events in recent years have followed a course even worse than the IPCC's 'worst case' scenario. The temperature in parts of the Antarctic, for instance, has risen five times faster than the global average; and, in early 2002, experts were astonished at the collapse of the Antarctic Larsen B ice shelf (five times the size of Singapore) in just one month. Scientists are constantly being surprised by such incidents.
Despite these prediction problems, many scientists and climatologists have considerable confidence in the broad messages of their models. They are certain that the earth is warming at a rate probably never experienced before - temperature records from around the world suggest that the Nineties were by far the hottest decade of the past century - and that the greenhouse future will bring some profound changes to the world.
What will be the effect of global warming?
While nothing is certain, in many parts of the world the changes are likely to be dramatic and even disastrous. Climatic belts are likely to shift, storms could be more frequent and more severe, monsoon rains may change in reliability and quantity, heatwaves could be longer and hotter, and droughts are likely to be more protracted.
There is intense debate about the impact of global warming on sea levels. A warmer world may result in the thermal expansion of the oceans, the melting of mountain glaciers in the Alps, the Rockies, the Andes and other mountain ranges, and a major retreat of the polar ice-caps in Greenland and Antarctica. And an increase of just a few degrees would be enough to melt all the permanent ice in the Arctic. Sea level rises of up to 1.5 metres by 2050 have been predicted. This would endanger entire nations - the Maldives, in the Indian Ocean, could disappear altogether, for instance, along with a large part of Bangladesh. It would also endanger low-lying cities such as London, New York and Venice. It could flood as much as one-third of all agricultural land worldwide and many important coastal wetlands, which support fish, shellfish, birds and other animals, would be inundated. It might also cause salt to seep into our drinking water supplies.
As sea levels rise, coral reefs are likely to die, because they have a very limited temperature tolerance. The health of coral reefs has been likened to the miner's canary - if the canary falls off its perch the miner knows he is in serious trouble - and, already, coral reefs are bleaching all over the world.
The effect on other wildlife could also be disastrous, largely because the conditions plants and animals have evolved to live in will be altering faster than they can adapt. They may be unable to exist within their present ranges and will have to colonise new areas; in many cases, particularly where they are surrounded by human development, this will be impossible. Shifting seasons could have an impact, too: early springs, for instance, may already be leading to a mismatch between the peak food demands of birds and the availability of insects. The flowering times of trees and other plants could change, preventing synchronisation with their pollinators. Also, if farmers cannot rely on weather patterns, the world food system could be threatened.
Some of the changes could be quite subtle. The tsetse fly belt in Africa could shift, freeing marginal areas for unsuitable development. A warmer climate may make a country like Britain more liable to invasions of exotic species or new diseases; some tropical diseases, such as malaria, are already spreading out of their normal areas. And the sex ratio of alligators, sea turtles and some other reptiles could be disastrously altered, since the sex of the offspring depends upon the temperature at which the eggs are incubated.
Scientists have long argued that global warming will lead to more extreme weather - because when any gas is heated it becomes less stable. And environmentalists say there is strong evidence that this is already happening. Studies in several parts of the world show that the climate is becoming more extreme and almost every week there are news stories of dramatic and sometimes unprecedented weather events; even Britain has had five big storms since 1987. If just half a degree of global warming is causing hurricanes, cyclones and floods, what will several degrees do? The prospects are frightening.
Global warming could itself precipitate further temperature changes, with a snowballing effect. If the Arctic permafrost melts, for instance, it may release billions of tonnes of methane into the atmosphere; or, if the polar ice-caps melt, less solar energy will be reflected back into space and the earth will automatically absorb more heat.
What can be done about global warming?
It is too late to do anything about the vast quantities of greenhouse gases which have been released into the atmosphere so far - we are already committed to a temperature rise. Therefore it is not a matter of 'How can we stop the climate changing?' but 'How can we slow it down?'
Global warming is a byproduct of modern society, and it can be solved in the long term only by dramatic changes in attitude and in the way we live.
One of the best examples is the need to reduce the impact of car traffic. Buying cars with smaller engines is obviously important - in some countries, huge gas guzzlers are becoming as socially unacceptable as fur coats. Car-sharing is also valuable and can reduce the number of cars on the roads quite substantially. Low speed restrictions on fast roads can help by reducing the amount of fuel being used; these were imposed in the US in the face of the oil crisis (although some states have since reverted to faster limits) and now are being considered in other countries for purely environmental reasons. But most of all, people can be enticed away from their cars by efficient and comfortable (and environmentally-friendly) public transport systems.
The ultimate solution, of course, is to find alternative sources of energy that use renewable resources and do minimal harm to the environment. Electricity production, for instance, will always be environmentally damaging in one way or another, but some natural sources of energy are cleaner, more efficient and safer than conventional power stations. There are no perfect alternatives, but harnessing wind, solar, hydroelectric, wave, tidal and geothermal energy has great potential.
The world must become energy-conscious in other ways as well. This requires political will, enlightened economic thinking and personal determination. The less energy each person draws from the national grid, the less power the world consumes and the less pollution there is from power stations.
There may also be ways of increasing the capacity of the world to absorb excess carbon dioxide by increasing the size of important carbon dioxide sinks, such as forests. Worldwide, there is a considerable net loss of forest every year - so massive tree-planting schemes (and, of course, better conservation measures) are essential to reverse this trend.