Life on Earth depends on the protective properties of the ozone layer and the 'greenhouse' gases in the upper atmosphere (water vapour, carbon dioxide, methane, and nitrous oxide). The ozone layer filters harmful ultraviolet radiation while the greenhouse gases keep global temperatures within a liveable range. Recently, ozone levels have fallen while greenhouse gases have increased.
Potential problems arising from ozone depletion include the increased risk of impaired growth and reproduction in some plants, including crop plants, and of eye and skin problems in some humans and other animals as a result of more intense ultraviolet-B radiation.
Scientists working with the Intergovernmental Panel on Climate Change (IPCC) now believe that the release of greenhouse gases from human activities is causing global temperatures to rise and weather patterns to change. Potential problems arising from this are more frequent floods and droughts, land encroachment and coastal erosion from rising seas, more frequent invasions by tropical pests, weeds and infectious diseases, and the disappearance of some types of ecosystems. In New Zealand, patterns of agriculture may be changed and some fisheries may be affected.
Ozone concentrations in the upper atmosphere have been significantly depleted over the past 20 years by manufactured gases that contain chlorine (such as chlorofluorocarbons, known as CFCs). The total concentration of chlorine in the atmosphere has grown by more than 600 percent from its natural level of around 0.6 parts per billion to an unprecedented 4.0 parts per billion.
At mid latitudes, total ozone has declined by an average 4-5 percent over the past 20 years. Above the Antarctic, a 'hole' in the ozone layer has occurred every spring since the 1980s. By 1995, it had expanded to its maximum possible area but had not yet reached its lowest possible ozone concentrations. The hole (which does not directly affect New Zealand) is likely to keep reappearing until the middle of the next century, until atmospheric chlorine levels have been brought back down to about 2 parts per billion.
Coinciding with the depletion of the ozone layer, the levels of solar ultraviolet (UV) radiation in New Zealand have been increasing. Monitoring is relatively recent, and only limited data are available, but it is estimated that a 5-7 percent decrease in ozone since the mid 1970s has been accompanied by a 6-9 percent increase in ultraviolet-B (UV-B) radiation.
Greenhouse gases other than water vapour have increased worldwide in the past several hundred years, as a result of increased industrial and agricultural production, and motor vehicle use. Water vapour is also expected to increase as global temperatures rise, causing more evaporation.
Carbon dioxide (CO2) concentrations have increased by about 30 percent since pre-industrial times with Southern Hemisphere concentrations lagging slightly behind the Northern Hemisphere.
On a per capita basis, New Zealand emissions of human-induced carbon dioxide are 25 percent lower than the OECD average, but about 50 percent higher than the global average. At present, about half of these emissions are absorbed by forest growth which acts as a 'carbon sink'. The area of commercial pine forests is increasing at present, but in the long-term the forests will not be able to maintain the 50 percent absorption ratio unless reductions are made in actual carbon dioxide emissions.
Globally, atmospheric methane (CH4) concentrations have more than doubled in the past 400 years. Methane is produced by biological processes (e.g. sheep and cattle digestion, rice growing, and the breakdown of waste) and fossil fuel production and use. The global rate of methane increase slowed during the 1980s and early 1990s, but has been rising again since 1993.
New Zealand's per capita emissions of methane are almost six times the OECD average, and almost ten times the global average. Although cattle and deer numbers are increasing, the recent fall in sheep numbers caused a reduction in livestock methane emissions of 3.5 percent between 1990 and 1995.
Nitrous oxide (N2O) concentrations are increasing globally but the sources are dispersed and not well understood.
The atmospheric changes of the past century have coincided with small changes in average global temperature and sea level. Surface temperatures have increased by between 0.3°C and 0.6°C since 1900, and by between 0.2°C and 0.3°C in the last 40 years. Global sea level has risen by between 10 and 25 cm over the past 100 years, and much of that rise may be related to the increase in global mean temperature.
Scientists increasingly suspect that the two trends are linked, and have attempted to disentangle the climatic effects of human-induced atmospheric change from those of natural factors (e.g. the short-term effects of sunspot cycles and sulphate particles emitted by industry, volcanoes and marine algae, and the long-term effects of fluctuations in the Earth's axis and orbit).
Although there are still scientific uncertainties, and although recent global warming still falls within the range of natural variability, the Intergovernmental Panel on Climate Change (IPCC) has concluded that 'the balance of evidence suggests that there is a discernible human influence on global climate'. The latest IPCC report predicts global warming in the range of 1°C-3.5°C over the next century and a sea level increase of about 50 centimetres.
New Zealand's climate has warmed faster than the global average, gaining 1. 1°C since 1860, compared to the world's 0.7°C. Most of this warming has occurred in the last 50 years. Prior to 1940, there was little discernible trend. Our sea level has risen by 15 centimetres in the past century.
Scientists are uncertain about the potential interaction of climate warming with the El Niño-Southern Oscillation (ENSO). This recurrent climatic pattern periodically makes New Zealand cooler, drier, and more prone to cyclones. It is triggered by changes in surface air pressure over the Pacific Ocean and alters ocean currents, rainfall, winds and temperatures around the world. Some scientists suspect that ENSO events may increase in frequency or duration as the climate warms.
New Zealand's general response to human-induced pressures on the atmosphere has been to participate in international efforts to reduce damaging emissions and to contribute to international research on the problems (e.g. through regular monitoring of the atmosphere at Baring Head in Wellington, Lauder in Central Otago, and Scott Basein Antarctica).
In particular, New Zealand has responded to the ozone threat by agreeing to the restrictions on ozone-depleting substances set down in the international convention known as the Montreal Protocol, and has set even more stringent domestic goals for phasing out the use of chlorofluorocarbons (CFCs) and several other harmful substances.
CFC use is declining, both here and overseas. New Zealand's use per person in 1993 was half that of the OECD, but still twice the global average. Our national use of CFCs had always been below the internationally-agreed limits set by the Montreal Protocol, but in accordance with the Protocol, consumption (imports) ceased on 1 January 1996.
Ozone is a naturally occurring greenhouse gas and has always been part of the natural greenhouse effect. What effect the restoration of ozone to pre-CFC levels will have on global warming is unclear. This is because of the uncertainties relating to future levels of greenhouse gas emissions from human activities; the fact that although the substances replacing CFCs are not ozone-depleters, many of them are powerful greenhouse gases, and because of the overall complexities of atmospheric physics.
New Zealand has responded to the greenhouse threat by signing the Framework Convention on Climate Change (FCCC) and setting the domestic goals of reducing net carbon dioxide emissions to their 1990 levels by the year 2000 and stabilising them from then on.
Since 1990 our net carbon dioxide emissions have actually risen, despite increases in forest planting. The main causes have been increasing fossil fuel use associated with economic growth, and changing forest growth rates associated with age structure. Current projections indicate that, in the absence of further policy measures, our net CO2 emissions will be more than double 1990 levels by the year 2000.