The atmosphere is a thin shroud of gas surrounding our planet. By day it shields us from the worst of the Sun's rays. By night it acts as a blanket, keeping temperatures on Earth relatively stable. As they swirl above us, the atmospheric gases create the world's weather patterns, and at ground level they make up the air we inhale ten to twenty times every minute. This chapter is about the atmosphere in general, but concentrates more on the non-breathable but highly-protective part which begins several hundred metres above us and extends far out to space. We discuss its nature, the pressures on it, its current state, and the responses New Zealand is making to those pressures. Chapter 6 focuses more narrowly on the ambient atmosphere, the air we breathe.
Within our solar system, Earth's atmosphere is unique and comparatively recent. Unlike our neighbouring planets, Venus and Mars, Earth has an atmosphere that includes water vapour (H2O), and free oxygen (O2 and O3). Molecular oxygen (O2) and ozone (O3) are absent from the other planets, but make up one-fifth (21 percent) of Earth's atmosphere. Oxygen is a highly reactive gas. It readily couples up with other molecules to form new compounds, a process known as oxidation (see Chapter 6, Box 6.5). For free oxygen to occur on such a scale, something has to be decoupling (or, in chemical terminology, reducing) it from other compounds faster than it can oxidise them.
Composition of dry air on Earth:
- Nitrogen 78.08%.
- Oxygen 20.95%.
- Argon 0.93%.
- Carbon Dioxide 0.03%.
- Other trace gases <0.01%.
That something is life-specifically, photosynthesis, the process in which plants, algae and cyanobacteria use sunlight to break down carbon dioxide (CO2) so that they can 'eat' the carbon while releasing the unwanted oxygen into the environment. Life is what makes Earth's atmosphere so dramatically different from its sibling planets. Whereas the atmospheres on Venus and Mars are composed of about 95 percent carbon dioxide (CO2) and 3 percent nitrogen (N2), on Earth nitrogen is the dominant gas, making up more than three-quarters (78 percent) of the atmosphere, and carbon dioxide is a mere trace gas comprising less than 1 percent. (see Figure 5.1).
In recent decades, scientists have discovered that the composition of the Earth's atmosphere, which has remained quite stable for the past 10,000 years or more, is being altered by human activities. Mass-produced chemicals containing chlorine and bromine are eating away at the ozone layer, and widespread fossil fuel use, rice growing, waste disposal, deforestation, and livestock belching are pumping increasing amounts of greenhouse gases, particularly carbon dioxide and methane (CH4), into the atmosphere. At the same time, solar radiation has increased and global temperatures have risen slightly. Scientists suspect that these atmospheric and climatic trends are linked, and this has led to growing concern that humans are triggering worldwide climate change (Wratt et al., 1991; Houghton et al., 1996).
At present, scientists are intensively studying the links between atmosphere and climate to see how serious the risk might be. Much of their work is aimed at disentangling the effects of atmospheric change from the many other factors that influence climate. These other factors include:
- Milankovitch cycles-small but regular fluctuations in the Earth's orbit and rotational axis which have the potential to trigger ice ages every 100,000 years or so by reducing the amount of sunlight that reaches temperate zones;
- Sunspot cycles-regular variations of about 0.1 percent in the intensity of solar radiation which recur every 11 years and may influence climate by altering the amount of ozone; and
- The El Niño-Southern Oscillation (ENSO)-a climate pattern with global impacts that is triggered every 2-10 years by fluctuations in surface air pressure over the Pacific and Indian Oceans.
The scientists are also discovering things in the atmosphere other than gases which appear to influence climate. These are airborne particles or aerosols of solid matter, particularly sulphates, which arise from both human activities (e.g. the burning of coal, oil and diesel and the manufacture of fertilisers) and natural sources (e.g. volcanoes and sulphate emissions from marine algae). They form a light haze which has an anti-greenhouse effect by scattering solar radiation before it can reach the Earth's surface.
The most recent and comprehensive studies of the global atmosphere are those conducted by the Intergovernmental Panel on Climate Change (IPCC) in 1990, 1992, and 1995. The IPCC scientists compiled data on the known atmospheric changes caused by gas and particle emissions and made a detailed study of past climate records. This information was fed into a series of computer models of the Earth's climate. These 'general circulation models' (GCMs), as they are called, attempt to simulate the world's climate by combining data on atmospheric gases, aerosols, weather patterns and ocean currents.
Although individual GCMs vary in detail, and many have trouble simulating prehistoric climatic conditions on Earth, they are in broad agreement on recent climate patterns and likely future changes. Drawing on a number of different models, the IPCC scientists have concluded that, in the past century, humans have increased the level of greenhouse gases and ozone-destroying chemicals in the atmosphere to their highest levels in 10,000 years (Houghton et al., 1996).
The scientists have estimated that, during the next century, the global average temperature is likely to rise by between 1°C and 3.5°C and the average sea level is likely to rise by about 50 centimetres. These changes are unlikely to occur uniformly and may result in greater weather variability, wind turbulence, and temperature extremes in some areas. Some parts of the Earth may even become a little cooler as a result of changed wind currents and evaporation and rainfall patterns. The IPCC acknowledges the scientific uncertainties in these predictions, and the need for further research to refine them (Houghton et al., 1996).
An important finding of the IPCC studies is that the predicted climate changes, and the atmospheric changes driving them, are not limited to one particular part of the globe. Unlike nations, and even continents, the atmosphere has no borders. New Zealand's geographical isolation, therefore, affords no sanctuary. In fact, the rate of temperature increase has been greater in New Zealand than for the planet in general. Nor does our isolation protect other countries from the changes that we New Zealanders are causing to the atmosphere's trace gases (Bouma et al., 1996).
Faced with this information, and expecting others to play their part in reducing emissions of possibly harmful gases into the atmosphere, New Zealand, as a party to the Montreal Protocol on Substances that Deplete the Ozone Layer, is committed, with other countries, to phasing out the production and use of ozone-depleting substances. New Zealand has stopped the importation of the main ozone-depleting chemicals, halons on 3 October 1990 and chlorofluorocarbons (CFCs) on 1 January 1996, and is in the process of phasing out less damaging CFC replacements. Industries using CFCs and other ozone-depleting chemicals have taken a positive approach to the use of new technology and chemicals, and New Zealand has kept well ahead of the phase-out deadlines.
New Zealand has also started to address the issues of greenhouse gas emissions, and the prospect of climate change. It has ratified the United Nations Framework Convention on Climate Change (FCCC), which aims to stabilise greenhouse gas levels before they trigger dangerous climatic effects. New Zealand has set itself the domestic objective of reducing net carbon dioxide emissions to 1990 levels by the year 2000, and maintaining them at that level from then on. Objectives for beyond the turn of the century are currently being negotiated. Net emissions are those emissions that remain in the atmosphere after the amount of CO2 absorbed by forests has been taken into account. On current trends, there is no sign of this objective being achieved without further policy measures being taken.