The atmosphere is the band of gases that surrounds the earth. Scientists have identified five layers based on temperature. Moving upwards from the earth, these are the troposphere (where weather occurs), stratosphere (where jets fly), mesosphere, thermosphere and exosphere.
The upper part of the mesosphere, and most of the thermosphere, is also known as the ionosphere, 80–400 kilometres above the earth’s surface. The air becomes noticeably ionised (electrically charged), caused mainly by ultraviolet light from the sun.
At a height of about 100 kilometres (known as the E-region of the ionosphere), the air becomes thin enough, and the ionisation strong enough for electric currents to flow along the earth’s magnetic field lines. It is in this region that auroras occur – beautiful undulating bands of light, visible in the night sky.
To understand how they form it is useful to think of the earth as a giant magnet, with magnetic field lines arching from one pole to the other. Auroras occur when charged particles in the solar wind (the stream of electrons and ions ejected from the sun) collide with the magnetic field (magnetosphere), causing electrons to be accelerated into the atmosphere around the north and south poles. In the northern hemisphere this phenomenon is known as the aurora borealis, and in the southern hemisphere as the aurora australis.
For reasons that are not entirely understood, the sun goes through a cycle of increased magnetic activity every 11 years. Quiet times are followed by more active times – the solar wind is fast moving and disturbed, and there are large flares and numerous sunspots. Auroras are much more likely to form during these active times.
The glow of the aurora australis, or southern lights, is usually seen south of the subantarctic islands, but when the sun is more active the aurora can be seen from mainland New Zealand. At this time, people in the southern South Island may see spectacular and colourful displays of the southern lights on clear nights.
The aurora can even be seen north of Auckland during active times. The next period of maximum solar activity should occur in 2012 or 2013. This will be a good time to look out for the waving curtains and moving rays.
In the 1860s five survivors of a shipwreck spent 20 months on the subantarctic Auckland Islands before being rescued. They had a hard time, but one midwinter night they were diverted by a spectacular display in the sky.
‘We saw before us a most magnificent spectacle. It was a Southern aurora in all its pomp of splendour … The stars paled before the sheaves of fire of different colours which rose from the horizon, and sprang toward the zenith, swift as lightnings’. 1
In the mid-1950s New Zealand scientists used radar to send radio waves into the auroral zone of the ionosphere, from Bluff and Slope Point on Southland’s south coast. Reflections of these waves from disturbances in the E-region were picked up on the radar receivers. Known as radio auroras, they are much more common during the sun’s most active period. The New Zealand observations and similar ones from around the north magnetic pole helped to show that radio auroras, like visible auroras, were caused by very fast electrons and ions moving down the earth’s magnetic field lines towards the poles.
The earth’s surface and lower parts of the atmosphere are warmed by radiation from the sun. This is concentrated in a region of short wavelengths that include visible light. Much of the short-wave solar radiation travels easily down through the earth’s atmosphere. Some of it is reflected straight back into space by clouds and by the earth’s surface. But much of the solar radiation is absorbed. The warmed surface then radiates energy upwards.
If there were no atmosphere, the incoming energy from the sun minus the upward radiation of energy from the earth would balance out, resulting in an average surface air temperature of around -18°C – far too cold for life to survive.
The difference between this scenario and reality is easily explained. The earth does have an atmosphere and it traps heat, just as heat is kept inside a greenhouse for cultivating plants. ‘Greenhouse gases’ such as carbon dioxide, methane and water vapour in the atmosphere absorb energy before it can escape into space. The heat emitted by the earth (infrared radiation) is concentrated at long wavelengths and is strongly absorbed by greenhouse gases in the atmosphere. Absorption of heat causes the atmosphere to warm and emit its own infrared radiation. The earth’s real average temperature is 15°C – allowing life to evolve.
If more greenhouse gases are added to the atmosphere from human activities (such as driving cars), they will absorb more of the infrared radiation reflected by the earth’s surface. The surface and the lower atmosphere will warm further, until a balance of incoming and outgoing radiation is reached. This extra warming is called the enhanced greenhouse effect.
The magnitude of the enhanced greenhouse effect is influenced by the concentration of greenhouse gases in the atmosphere, but also by various complex interactions between the earth, the oceans and the atmosphere. Feedback mechanisms play an important role. For example, as the earth’s surface warms up, more water is evaporated. Since water vapour is itself a strong greenhouse gas, this is a positive feedback, which will tend to amplify the warming effect of carbon dioxide emissions.
Clouds tend to both cool the earth because they reflect incoming sunlight, and warm it by trapping outgoing infrared radiation. The overall effect of clouds is cooling, but it is still uncertain whether this cooling effect will be stronger or weaker as the concentration of greenhouse gases increases.
Greenhouse gases can absorb infrared radiation because of their molecular structure. They capture the energy by rotating and vibrating.
Carbon dioxide (CO2) is produced mainly by burning fossil fuels such as petrol, coal and oil. Burning forests and replacing them with crops or pasture also adds carbon dioxide to the atmosphere.
Methane (CH4) is produced mainly by the breakdown of organic substances in anaerobic conditions, such as in the digestive systems of sheep, cows and other ruminant (cud-chewing) animals, and in swamps, where plant material decays.
Nitrous oxide (N2O) is released into the atmosphere as a by-product of farming. It is estimated that global releases of nitrous oxide have risen around 60% as a result of human activities, mainly the huge increase in the use of nitrogen fertilisers in agriculture.
As well as causing an increase in naturally occurring greenhouse gases, humans have released some completely new chemicals into the atmosphere, including sulfur hexafluoride and chlorofluorocarbons (CFCs). CFCs used to be used as a coolant in refrigerators, and are to blame for the depletion in the ozone layer. While CFCs have been banned in an attempt to save the ozone layer, they will remain in the atmosphere until at least 2050. Although these artificial gases are not particularly abundant, they are hundreds or thousands of times more efficient at absorbing heat than carbon dioxide.
In New Zealand the National Institute of Water and Atmospheric Research makes high-precision measurements of the three main greenhouse gases. Carbon dioxide, methane and nitrous oxide are measured at the Baring Head Atmospheric Research Station near Wellington, at Scott Base (Antarctica), and from ships and aircraft in the Pacific Ocean and Southern Ocean.
The institute has measured the background levels of atmospheric carbon dioxide since 1973, methane since 1989, and nitrous oxide since 1997. Most of the data is from clean-air samples collected at Baring Head.
The measurements record a steady rise in greenhouse gas levels over that period. For example, the release of methane into the atmosphere has increased from natural levels of about 250 million tonnes per year to about 600 million tonnes per year in the early 2000s as a result of human activities.
As a signatory to the United Nations Framework Convention on Climate Change, New Zealand must report annually on the quantities of greenhouse gases emitted through human activities. This inventory involves measuring emissions and determining their sources.
New Zealand is also a signatory to the Kyoto Protocol, drawn up in 1997 to implement the Convention on Climate Change. Under the protocol industrialised nations have committed to reducing their greenhouse gas emissions, between the years 2008 and 2012, to levels that are 5.2% below those of 1990.
Because many countries burn coal to generate electricity, their major greenhouse gas emissions are carbon dioxide. However, New Zealand is unique. Methane made up 37% and carbon dioxide 45% of greenhouse gas emissions in the early 2000s. This reflected the large number of farmed livestock and relatively low use of fossil fuels for generating electricity. All ruminant livestock (such as cows, sheep, deer and goats) produce methane by belching as a result of the action of anaerobic bacteria during digestion.
Nitrous oxide is also important in New Zealand’s emissions inventory, making 18% of greenhouse gas emissions in the early 2000s. It is produced through the breakdown of animal excreta and the nitrogenous fertilisers applied to farmlands.
Ozone (O3) is a form of oxygen generated by reactions between sunlight and oxygen (O2). This process is most efficient in the stratosphere at altitudes of 15–50 kilometres. Ozone is carried by natural air motions and destroyed by chemical cycles. As a result, its concentration is highly variable in space and time. The thickness of the ozone layer is significant because ozone absorbs ultraviolet (UV) radiation from the sun, acting as a filter against sunburn and other damage. Without the protection of the ozone layer in the stratosphere, terrestrial life as we know it could not survive.
The amount of ozone in an atmospheric column up to 50 or 60 kilometres is measured in Dobson Units (DU), where 1 DU corresponds to a layer of pure ozone 0.01 millimetre thick at surface temperature and pressure. The global average ozone is 300 DU. Over New Zealand, the ozone layer ranges from about 250 DU in autumn to 450 DU in spring.
Ozone can be destroyed by reactions with by-products of man-made chemicals, such as chlorine from chlorofluorocarbons (CFCs). Increases in the concentrations of these chemicals have led to the much-publicised ozone depletion.
Decreased ozone can have serious environmental impacts by increasing UV radiation at the ground. In humans, this causes eye damage (cataracts) and skin damage (sunburn and skin cancer). It has been calculated that for each 1% reduction in ozone there is an increase in UV radiation of about 1.2%, which could lead to a 2% increase in skin cancers. It also harms plants and animals, and damages materials such as plastics and paints.
The most dramatic ozone losses occur in Antarctica, where spring ozone columns can be less than 90 DU. This ‘ozone hole’ lies well to the south of New Zealand and does not pose a direct health risk. However, when it breaks up, filaments of ozone-poor air can sometimes pass overhead.
New Zealand’s summertime ozone has decreased by about 10% since 1970. During the first half of the 21st century it is expected to recover, albeit slowly, because the release of the damaging chemicals is now restricted by the Montreal Protocol on Protection of the Ozone Layer. The rate of recovery may be influenced by the changing climate.
Living in what has been called the melanoma capital of the world, Aucklanders face a 5.7% chance of developing skin cancer. Although the city's climate encourages outdoor activities, in summer UV intensities are high due to the low ozone and clean air, and because the earth is closer to the sun. People with fair skin are particularly susceptible to being harmed by the sun's radiation.
New Zealand’s death rate from skin cancer is about 300 per year, the highest in the world relative to population (and over half that from road accidents). This is due to the relatively high UV exposures and the high number of fair-skinned people. Peak UV intensities in New Zealand are about 40% greater than at comparable latitudes in Europe.
The UV Index (UVI) represents the intensity of UV radiation. A UVI greater than 10 is extreme, and skin damage can occur in less than 15 minutes. In New Zealand, the midday UVI can exceed 13 in summer. In winter, it rarely exceeds 2, although intensities increase with altitude and when the surface is snow-covered, as on ski fields.
The most important factor controlling UV radiation is the sun’s angle above the horizon. At low angles, the rays pass through much more of the atmosphere, reducing the radiation. As a rule of thumb, if your shadow length exceeds twice your height (when the angle of the sun is less than 30°), then there is little risk of UV damage. If your shadow is less than your height (the sun angle is greater than 45°), then protective measures are needed.
With the exception of some nuisance odours and agricultural spray drift, New Zealanders’ concerns about air quality used to be limited to visible pollution from smoke and fumes in winter. However, there is a growing awareness that air pollution does not have to be visible to cause problems.
By world standards, New Zealand has relatively good air quality. This is due to the coastal location of most of the main centres, the limited amount of heavy industry, the strong winds that disperse pollutants, and the country’s distance from other continents and sources of pollution.
However, some urban areas occasionally have quite high air pollution levels. Pollution typically occurs in Auckland, with its heavy traffic, and Christchurch, which because of its topography is prone to temperature inversion – a layer of warm air traps cooler air, and any pollution, underneath.
Domestic fires and motor vehicles contribute most of New Zealand’s air pollutants. Industry can also cause localised problems. It is important to realise that there is a complex relationship between emissions of pollutants and their presence in the air, because of dispersion processes and chemical reactions. Variations in emissions and the prevailing weather conditions mean that pollution levels are constantly changing during the day, from one day to the next, and from season to season.
Motor vehicles are a source of air pollutants, and yet very little is known about the emissions from New Zealand’s vehicles. A campaign in Auckland in 2003 used roadside sensors to measure emissions of carbon monoxide (CO), hydrocarbons, and nitric oxide (NO) as vehicles drove past. The measurements showed relatively high emission rates. The dirtiest 10% of vehicles were responsible for over 50% of the total carbon monoxide and unburned hydrocarbon emissions, and almost 40% of the total nitric oxide emissions. The cleanest 50% contributed less than 10% of the total emissions.
Air pollution is generally perceived as an outdoor issue, but poor indoor air quality can also cause serious health problems. The most common indoor air pollutants are smoke from cigarettes, malfunctioning gas appliances, woodburners and open fires. Emissions of carbon monoxide and nitrogen dioxide from gas appliances and kerosene heaters, and formaldehyde and other organic substances from building materials, can also cause problems. Indoor industrial worksites may generate dust, fumes or odours.
Until the introduction of the Resource Management Act in 1991, air-quality monitoring in New Zealand had largely been in response to specific, perceived problems, and so was quite limited. The act shifted responsibility to the regional councils, and monitoring networks were established and expanded throughout New Zealand during the 1990s. The act also controls emissions from large industrial sources through air-discharge permits, issued by regional councils.
In 1994 the Ministry for the Environment developed a set of guidelines for key air pollutants, including carbon monoxide, particulates, nitrogen dioxide, sulfur dioxide, ozone, hydrogen sulfide and lead. Updated guidelines, incorporating new pollutants (benzene, butadiene, formaldehyde, acetaldehyde, benzopyrene, mercury, chromium and arsenic), were released in 2002.
In July 2004, the government approved 14 national environmental standards aimed at air quality and landfill gas emissions. Replacing the previous air-quality guidelines, they comprised:
Austin, Jill, and others, eds. Air pollution science for the 21st century. Boston: Elsevier, 2002.
Bengtsson, L. O., and C. U. Hammer, eds. Geosphere–biosphere interactions and climate. New York: Cambridge University Press, 2001.
Eather, Robert H. Majestic lights: the aurora in science, history, and the arts. Washington: American Geophysical Union, 1980.
Scientific assessment of ozone depletion: 2002. Global Ozone Research and Monitoring Project, Report 47. Geneva: World Meteorological Organisation, 2003.
UV radiation and its effects – an update. Miscellaneous Series 60. Wellington: Royal Society of New Zealand, 2002.