Pressures on the atmosphere
Although New Zealand has a relatively small population, we, like the people in other developed countries, make a disproportionate contribution to greenhouse gas emissions. At the peak of its consumption in 1986, New Zealand was also a major user of ozone-depleting substances on a per capita basis. In 1993, New Zealand's 3.5 million people represented only 0.06 percent of the world's population, but our contribution to all human-related carbon dioxide emissions was closer to 0.10 percent and our per capita share of methane emissions was ten times the global average (see Table 5.1).
The reasons for this are emissions from livestock and fossil fuel use.
Although CFCs and halons are potent greenhouse gases, they are much better known for their impact on the ozone layer. New Zealand had made extensive use of halons in fire extinguishers up until 1990 when it became the first country to eliminate their consumption by banning imports. In 1986, New Zealand's share of global CFC use stood at 0.20 percent, but by 1992 it had fallen to 0.10 percent. This was significantly faster than the global reduction over that period. New Zealand's 1986 per capita consumption of CFCs was very high at 700 grams per person compared to the global average of 210 g/person. However, by 1992, New Zealand had reduced its consumption rate by nearly 70 percent to 200 g/person. This was still nearly twice the global average, which, by then, had fallen to 120 g/person, but around half the 1992 OECD average of 400 g/person. Since 1 January 1996, there have been no more CFC imports into New Zealand, though products containing CFCs (e.g. refrigerators) are still imported.
Pressures from ozone-depleting substances
Over the past five decades the ozone layer has been under attack from manufactured chemicals used in such diverse areas as refrigeration and air-conditioning, packaging, fire fighting, market gardening and industrial cleaning. For 20-30 years the chemical attack on ozone was silent, unseen, and unrecognised. The world only learned of the extraordinary longevity of CFCs in 1971 when Dr Jim Lovelock studied their global distribution to see if they would be useful marker chemicals for tracking air movements (Gribbin, 1988). Lovelock later became famous as the co-author of the Gaia hypothesis (the idea that Earth's atmosphere acts like a self-regulating organism).
Other scientists quickly seized on the fact that these apparently indestructible chemicals must eventually float up to the stratosphere and the fragile ozone layer. The suggestion that CFCs may be destroying the ozone layer was published the following year by two University of California scientists, Sherwood Rowland and his Ph.D. student, Mario Molina (Molina and Rowland, 1974). Controversial at the time, the idea was soon tested and confirmed.
It turned out that CFCs were not alone. A number of other halocarbons have the same destructive properties, though their Ozone Depleting Potential (ODP) varies according to chemical structure and longevity (see Table 5.2). A chemical's ODP is calculated by measuring it against CFC-11 which has been assigned an ODP of '1'. A chemical with an ODP of 2, for example, would mean it is twice as destructive as CFC-11. What these chemicals have in common are highly reactive atoms, particularly chlorine and bromine, which remain rigidly attached to the parent molecule as it floats up through the troposphere, but then become detached under the impact of ultraviolet radiation in the stratosphere. At this point, the free molecules began attacking ozone molecules (see Box 5.4).
Figure 5.10: The global increase in atmospheric chlorine
See Figure 5.10
at its full size.
Textual description of figure 5.10
Global atmospheric chlorine has increased from approximately 0.6 parts per billion prior to the industrial age to almost 4 parts per billion today.
Source: Houghton et al. (1996)
Before these chemicals were invented the chlorine level in the atmosphere was 0.6 parts per billion (ppb). The natural chlorine was mostly in molecules, such as methyl chloride, which caused no damage to the ozone layer. Today, chlorine levels have risen to almost 4.0 ppb - a six-fold increase (see Figure 5.10). Eighty percent of the chlorine in the stratosphere is now directly attributable to human activities (World Meteorological Organisation, 1995).
The ban on the manufacture and importation of ozone-depleting chemicals (with the exception of hydrochlorofluorocarbons and methyl bromide) took effect on (and in some cases before) 1 January 1996. However, their past use and continued presence in manufactured products means that their impact on the atmosphere will continue for several decades.(See Figure 5.18).
Figure 5.11: CFC-12 concentrations over Tasmania, 1982-91
See Figure 5.11
at its full size.
Textual description of figure 5.11
CFC-12 concentrations over Tasmania have increased from approximately 325 parts per trillion in 1982 to 480 parts per trillion in 1991.
Source: Fraser and Derek (1994)
Chlorofluorocarbons
Chlorofluorocarbons (CFCs) were invented in 1928. At the time they seemed like magic molecules. They are colourless, odourless, and non-toxic. They do not burn or explode and they do not break down this side of the stratosphere. They vaporise easily at low temperatures, making them ideal coolants in refrigerators and air-conditioners, and ideal propellants in spray cans. However, the 80 grams of CFC-12 in one refrigerator can destroy three tonnes of ozone.
Throughout the 1980s, the atmospheric concentrations of the principal chlorofluoro-carbons, CFC-11 and CFC-12, increased by as much as 5 percent per year. The annual increase through 1990-1992 was more modest: about 0.9 percent for CFC-11 and 2.6 percent for CFC-12 (World Meteorological Organisation, 1995). CFC monitoring at Cape Grim in Tasmania shows a steady rise in the concentrations of these compounds in the Southern Hemisphere (see Figure 5.11). In 1986, New Zealand's estimated consumption of CFCs was 2,300 tonnes. By 1989, this had fallen to around 1,211 tonnes. The reduction was largely due to New Zealand phasing out the use of CFCs in spray cans (other than those used for medical purposes). Consumption dropped to under 375 tonnes in 1994 and the importation of new CFCs ceased altogether at the end of 1995.
Box 5.4: Ozone destruction by CFCs
Ozone depletion is caused mainly by synthetic chemicals containing chlorine and bromine, particularly chlorofluorocarbons (CFCs) and halons. The key to their destructiveness is their long atmospheric lifetimes. Most chemicals break down in the lower atmosphere, but CFC and halon molecules are so stable that they hold together long enough to reach the upper atmosphere. This can take up to 100 years. Once there, individual chlorine atoms are torn from the parent molecule by the strong ultraviolet radiation. They then become free to bond with other molecules. They can destroy thousands of ozone molecules before eventually finding a more stable chemical to bond with.
Destruction of Ozone by Chlorine Atoms (eg.CFCs)
1 Cl + O3 (ozone) 
ClO + O2
2 ClO + O Cl + O2
The process of ozone destruction is as follows. Ultraviolet radiation splits individual chlorine (Cl) atoms away from the CFC molecule.. The freed chlorine atom (Cl) encounters an ozone molecule (O3) (step 1 in the diagram), 'steals' one oxygen atom (O) to form chlorine monoxide (ClO), and leaves behind a common oxygen molecule (O2).
As explained in Box 5.2, ultraviolet radiation also breaks down ozone molecules, creating free oxygen atoms. When a free oxygen atom (O) encounters the chlorine monoxide molecule (ClO), it seizes the already 'stolen' oxygen atom (O) and forms molecular oxygen (O 2) (step 2). At the same time, the chlorine atom is set free to restart the whole process, breaking down molecule after molecule of ozone and converting it to molecular oxygen.
This example is only one of several chemical processes involving chlorine and/or bromine that are now known to catalytically destroy ozone. The reason chlorine atoms in bleach or swimming pool chemicals don't destroy ozone is because the molecules in these compounds are less stable and never reach the ozone layer. They quickly dissolve in the lower atmosphere, and get washed back to Earth when it rains. The same applies to most chlorine released from volcanoes and sea salt.
Halons
Halons are similar to CFCs but contain bromine (and sometimes chlorine as well). Bromine also destroys ozone. Halons were developed during World War II to put out fires in tanks and submarines. They became more widely used from the mid-1970s and are commonly found in yellow fire extinguishers. They have not been used as extensively as CFCs, but are much more destructive (see Table 5.2). In fact, Halon 1301, with an ODP of 12, is more lethal to ozone than any other pollutant. Halon imports in 1986 were 142 tonnes. This rose to 264 tonnes in 1989. Imports were prohibited from 3 October 1990.
Hydrochlorofluorocarbons and hydrobromofluorocarbons
Hydrochlorofluorocarbon (HCFC) and Hydrobromofluorocarbon (HBFC) have similar properties to halons and CFCs, but they are not as damaging to the ozone layer. They are being used by some industries as transitional substances to ease the difficulties caused by phasing out CFCs. In 1989, the base year used for Montreal Protocol phase-out calculations, 416 tonnes of HCFCs were imported. This rose to 472 tonnes in 1993, and 695 tonnes in 1995, as more of the chemical was used to make the transition from ozone-depleting CFCs to ozone-benign substances. A minimal amount (2 tonnes or less) of HBFCs may have been brought into the country before 1992 when imports were banned, but there are no official data.
Carbon tetrachloride
Carbon tetrachloride is a solvent once used by dry-cleaners, but now used mainly in chemistry laboratories. It is toxic and causes cancer. It has an ozone-depleting potential slightly greater than that of CFC-11, and an atmospheric lifetime of 42 years. Between 1989 and 1 January 1996, when imports ceased, less than one tonne was imported annually.
Methyl chloroform
Methyl chloroform is an industrial solvent used to degrease metal. It is also used as a solvent in white-out correction fluids for typing. It has an ozone-depleting potential roughly one-tenth that of CFC-11, and an atmospheric lifetime of 5-6 years. In 1989, the base year for the Montreal Protocol phase-out calculations, 982 tonnes were imported. By 1994 this figure had dropped to 101 tonnes. Imports ceased on 1 January 1996.
Methyl bromide
Methyl bromide kills all pests from microbes to insects and weeds and is used by horticulturalists to fumigate soil (often for crops such as strawberries and tomatoes). It is also used to fumigate imported goods being held in quarantine, and some export products, such as logs and fruit. Since CFC imports stopped in 1996, methyl bromide and HCFCs are the only ozone-depleting substances still imported into New Zealand. Of these, methyl bromide is the most significant because its ozone-depleting potential is only slightly less than CFC-11 (see Table 5.2).
In 1991, the official estimate for methyl bromide imports was 165 tonnes. Of this, 15 tonnes were used for quarantine and pre-shipment (QPS) purposes and the remaining 150 tonnes for all other non-QPS purposes (primarily horticulture). Those figures are estimated to have remained constant through until 1994, when a total 186 tonnes were imported, with 38.5 tonnes of that used for quarantine and pre-shipment purposes. In 1995, New Zealand capped its imports for non-QPS uses at 1991 levels (150 tonnes) as required under the Montreal Protocol. In the event, only 129 tonnes were imported for non-QPS uses in 1995, but 56 tonnes were imported for quarantine and pre-shipment use. This pushed the total for the year to 185 tonnes.
Pressures from greenhouse gases
It is well established that the concentrations of some greenhouse gases are increasing as a result of human activities (e.g. Wratt et al., 1991; Houghton et al., 1996; Battle et al., 1996). Many of the world's climate scientists are concerned that an 'enhanced greenhouse effect' could lead to a temperature increase for the surface of the Earth, changes in other aspects of climate (such as rainfall) in some regions, and rises in sea level.
Scientists cannot yet say with certainty whether the observed global and regional temperature trends are caused by natural climate phenomena, by enhanced emissions of greenhouse gases produced by human activity, or by a combination of the two. However, in its most recent report, the Science Working Group of the IPCC concluded that "the balance of evidence suggests a discernible human influence on global climate" (Houghton et al., 1996; Kerr, 1995b). This conclusion has received added support from the latest study of recent temperature trends, which found a close relationship between actual temperature rises and those predicted by state-of-the-art computer models (Santer et al., 1996; Kerr, 1996c; Nicholls, 1996).
The rise is not predicted to be at a steady rate, because natural fluctuations in climate will be superimposed on this change. However, the predicted warming and sea level changes summed up over time intervals of 30-100 years are much greater than corresponding natural rates of change, and are four to five times those experienced during the last century.
In the past 100 years the average temperature of the Earth has risen by 0.5°C. Even small temperature variations can produce large changes in climate. The temperature during the last Ice Age (which ended about 14,000 years ago) was only 3-5°C colder on average than it is now. Many scientists expect temperature increases of between 1°C and 3°C by the year 2050, and the IPCC, in its 1996 report, concludes that global temperatures are likely to increase by between 1°C and 3.5°C over the next 100 years (see Box 5.5).
The pattern of temperature change in New Zealand has been similar to the global pattern over the past 140 years, but recent evidence suggests that New Zealand's temperature is rising at a rate 50 percent higher than the global average. Global mean surface temperatures for the land and sea have increased by 0.45°C over the past 100 years, but New Zealand's temperature in the same period has risen by 0.7°C (Salinger, 1995). However, there is no firm scientific basis for predicting future rates of temperature increase based on past trends.
The warmest decade in New Zealand was the 1980s (see Figure 5.21). The very much cooler years of 1991 and 1992 are directly attributable to the effects of the eruption of Mount Pinatubo, in the Philippines, in June 1991 (see Figure 5.22).
Although the temperature changes for both New Zealand and the globe still fall within the ambit of natural variation, IPCC scientists now say evidence of the human influence on global climate is starting to emerge from the 'noise' of natural variability (Houghton, 1996).
Figure 5.12: IPCC projections of possible increases in global average temperatures
See Figure 5.12
at its full size.
Textual description of figure 5.12
There are three levels of IPCC projections for possible increased in global average temperatures. By 2100, low estimates increase to around 1 degree; mid estimates to between 2.25 and 2.75 degrees; high estimates to between 4 and 5 degrees. The lower range for each estimate is if aerosol emissions increase beyond 1990 levels.
Figure 5.13: IPCC projections of possible increases in sea level
See Figure 5.13
at its full size.
Textual description of figure 5.13
There are three levels of IPCC projections for possible increased in sea level. By 2100, low estimates increase to around 20 cm; mid estimates to between 40 and 50 cm; high estimates to between 65 and 85. The lower range for each estimate is if aerosol emissions increase beyond 1990 levels.
Source: Houghton et al. (1996)
Different greenhouse gases vary in their ability to trap heat. The Global Warming Potential (GWP) index depicts these differences using carbon dioxide as a benchmark for comparison (see Table 5.3). Factors which affect the GWP of a gas include its chemical make-up and the length of time it can remain intact in the atmosphere. Although carbon dioxide is the most notorious of the greenhouse gases it is actually very weak in comparison to the others.
In fact, most halocarbons are many times more potent greenhouse gases than carbon dioxide. CFCs, for instance, which are better known as ozone-depleters, have several thousand times the warming power of carbon dioxide. This is known as the 'direct global warming effect'. Indirectly, however, by destroying large amounts of ozone, which is also a greenhouse gas, the CFCs actually cancel out their own direct effect. Consequently, the warming effect of the CFCs is close to zero, and they may, in fact, have a slight cooling effect. Other halocarbons, such as hydrofluorocarbons (HFCs) and the perfluorocarbons (PFCs)-perfluoromethane and perfluoroethane-which do not 'eat' ozone, retain their very high GWP (see Table 5.3). Methyl bromide (CH3Br) is similar to CFCs. It is a powerful ozone-depleting substance, but does not act as a greenhouse gas and therefore has a GWP of zero.
Gram for gram, the most potent greenhouse gas of all is not a halocarbon, even though it does contain a halogen, in this case, fluoride. It is sulphur hexafluoride (SF6) with a GWP nearly 35,000 times greater than carbon dioxide.
However, the GWP alone is not the full picture. Greenhouse gases also vary in their abundance. After water vapour, carbon dioxide is by far the most abundant of the greenhouse gases and therefore has the greatest total impact on atmospheric temperatures. It is, therefore, is said to have a high warming 'commitment', despite its relatively low warming potential. The global warming commitment of a gas is simply its GWP multiplied by its abundance.
At present the global warming commitment of the super-heater, SF6, accounts for about 1 percent of the total commitment arising from human-induced greenhouse gases. However, the SF6 contribution is climbing, at 9 percent a year, with some users coming from an unexpected quarter-environmental agencies. A number of environmental monitoring programmes in North America and Europe have used the chemical as a marker, adding it to other pollution emissions in order to track their movements (Pearce, 1996b). Emissions of SF6 in New Zealand are not monitored at present, but work is underway to monitor consumption.
Table 5.3: The Global Warming Potential (GWP) of selected greenhouse gases, relative to Carbon Dioxide (CO 2) ( ±35%).
Box 5.5: The scientific assessment of climate change
In the 1980s, several major conferences around the world revealed that many scientists were concerned about the possible influence of air pollution on the climate. The idea of such an effect had been around for more than a century, but rising world temperatures were now persuading more scientists to take it seriously. By 1988, temperatures and concern had grown sufficiently for the United Nations to set up the Intergovernmental Panel on Climate Change (IPCC). The IPCC's role is to examine the best evidence available, draw scientifically-based conclusions, and develop suitable research and policy responses.
The IPCC's Scientific Assessment Group (known as Working Group 1) involves about 170 scientists from 25 countries. Its first report, Climate Change - The IPCC Scientific Assessment (Houghton et al., 1990; IPCC, 1990), was followed in 1992 and 1994 by updates incorporating the latest scientific knowledge (Houghton et al., 1992 and 1994). In 1996 the Working Group released its second assessment (Houghton et al., 1996).
In the original 1990 assessment, the Scientific Working Group scientists concluded that human activities are substantially increasing the atmospheric concentrations of the greenhouse gases: carbon dioxide, methane, chlorofluorocarbons (CFCs), and nitrous oxide. They were also certain that those increases would enhance the greenhouse effect, making average temperatures warmer than they would otherwise have been at the Earth's surface. Furthermore, as temperatures increased, more water would evaporate from the oceans, thereby increasing the main greenhouse gas, water vapour (i.e. clouds), which would further enhance the greenhouse effect in a process referred to as positive feedback.
The 1992 Supplementary Report again predicted a warming rate of 0.2°C to 0.5°C per decade arising from human-induced greenhouse gases, but suggested this would be masked to some extent by the cooling effects of industrial emissions of sulphate aerosols and by ozone depletion in the upper atmosphere. Aerosols decrease temperature by forming cloud cover (if water vapour is present) or a whitish haze which blocks or scatters light, thereby reducing the amount of solar radiation that reaches the Earth's surface (Taylor and Penner, 1994; Jones et al., 1994; Stephens, 1994; Kerr 1995a, 1995f) . The extent of this cooling effect is highly uncertain, making climate change predictions also uncertain (Houghton et al., 1996; Schwartz and Andreae, 1996). One estimate suggested that aerosols may reduce temperature rises by 20 percent, and sea level rises by 25 percent (Wigley and Raper, 1992). More recent assessments by the IPCC and the US National Research Council indicate a level of uncertainty ranging from a very minor effect to a very strong one (Houghton et al., 1996; Seinfeld, 1996).
In its second assessment the IPCC Working Group concluded, among other things, that "most ... studies have detected a significant change [in the global mean surface air temperature over the last century] and show that the observed warming trend is unlikely to be entirely natural in origin" (Houghton et al., 1996). The Working Group further concluded that, despite the uncertainties, "the balance of evidence suggests that there is a discernible human influence on global climate." The Working Group said that "the increasing realism of computer simulations of current and past climate has increased our confidence in their use for projection of future climate change. Important uncertainties remain, but these have been taken into account in the full range of projections of global mean temperature and sea level change" (Houghton et al., 1996).
Based on this, the Working Group predicted an increase in average global temperature of between 1°C and 3.5°C by the year 2100, and an increase in average sea level of about 50 cm. This is slightly lower than earlier estimates, mainly because of lower estimates of future greenhouse gas emissions and an increased awareness of the potential cooling effect of sulphate aerosols. Six different projections were made of future temperature and sea levels, and the middle ones were taken as best estimates (see Figures 5.12 and 5.13).
Carbon dioxide
For the past few million years, carbon dioxide levels in the atmosphere have been relatively stable at about 280 ppm. This means that human beings and most of the other species existing today have never experienced CO2 levels above 300 ppm-until this century. In the past 100 years, the concentrations have increased by about 25 percent, from 280 to over 350 ppm and are still rising. The present rate of increase is between 0.4 percent per year, with the Northern Hemisphere leading the Southern Hemisphere by about 3 ppm (Houghton et al., 1994). This reflects a lag effect in which the Northern Hemisphere concentrations are running one to two years ahead of the Southern Hemisphere, with bigger seasonal variations.
In New Zealand the main sources of carbon dioxide are motor vehicles, electricity generation, and the petrochemical, steel, and dairy industries (see Tables 5.4 and 5.5). Our gross carbon dioxide emissions per person are currently about 7,500 kg (reducing to approximately 2,000 kg of net emissions). This compares to gross global emissions of 5,000 kg per person and an OECD emission rate of 11,000 kg per person (see Table 5.1).
In the 24 years that the Baring Head record has been kept, the concentration of carbon dioxide in the Southern Hemisphere has increased by nearly 10 percent (see Figure 5.14). The rate of increase was about 1.2 ppm per year in the 1970s, and 1.6 ppm per year in the 1980s. Carbon dioxide growth rates slowed during the early nineties, but have now started to rise again. Such short-term fluctuations in carbon dioxide growth have been observed before.
Table 5.5: The main sources of human-induced carbon dioxide emissions in New Zealand
| Emitter | Carbon Dioxide kilotonnes (kt) | Data Year |
|---|---|---|
| Transport | 10,983 | 1995 |
| Electricity generation | 3,263 | 1995 |
| Synthetic petrol | 666 | 1995 |
| Chemical methanol | 1,249 | |
| Steel Importing | 1,668 | 1995 |
| Dairy Industry | 1,060 | 1991 |
| Cement Industry | 981 | 1995 |
| Oil Refining | 955 | 1995 |
| Forest Industry | 940 | 1992 |
| Aluminium production | 532 | 1995 |
| Meat Industry | 413 | 1992 |
| Ammonia/urea production | 157 | 1995 |
| Total | 11,939 |
* Gross carbon dioxide emissions 1995: 27,368 kilotonnes (kt)
Source: Ministry of Commerce
Figure 5.14: Carbon Dioxide concentrates over the Southern Hemisphere, 1971-94
See Figure
5.14 at its full size.
Textual description of figure 5.14
Carbon dioxide concentrations have increased over the Southern Hemisphere between the pre-industrial age (around 280 parts per million) and today, when concentrations have steadily increased from around 330 parts per million in 1971 to 355 parts per million in 1994.
Source: Houghton (1995): NIWA unpublished data
Methane
Global methane (CH4) concentrations have always fluctuated on a scale of about 1,000 years. As noted earlier, however, before about a.d. 1600 the concentrations had not exceeded 800 ppb for over 100,000 years (and probably much longer). Since then, mainly as a consequence of increasing global agriculture and industry, atmospheric methane levels have more than doubled to over 1,700 ppb.
In New Zealand, the methane emissions from sheep and cattle (and also deer and goats) exceed all other sources (see Table 5.4). The methane produced in the stomachs of ruminants (and in the paddy-fields of large rice-producing countries) is the by-product of anaerobic (non-oxygen using) bacteria breaking down organic material. Primary production processing waste (e.g. dairy factories and meatworks) and rotting waste at landfills are New Zealand's next largest sources.
The global rate of increase in atmospheric methane was about 0.8 percent per year in the 1980s. In the early nineties the rate slowed considerably, then picked up from 1993 (Houghton et al., 1994). Methane concentrations have been measured in southerly, clean air, conditions at Baring Head since 1989 (see Figure 5.15). The monitoring shows unexpectedly high variability in spring. This is probably caused when the seasonal burning of tropical forests emits large amounts of methane which are carried south by air currents.
New Zealand's large number of ruminant animals makes our methane emission rate per capita about ten times higher than the global average (see Table 5.1). In the early 1990s, methane emissions from New Zealand livestock declined sharply leading to predictions of a 10 percent reduction by the year 2000 (Ministry for the Environment, 1994). However, the increase in cattle numbers, particularly dairy cows, in the mid-1990s has partially reversed this trend so that the overall methane reduction between 1990 and 1995 was 3.5 percent.
Figure 5.15: Methane concentrations over New Zealand, 1989-94
See Figure
5.15 at its full size.
Textual description of figure 5.15
Methane concentrations fluctuate seasonally with higher concentrations occuring during summer. Concentrations have gradually increased over New Zealand from the pre-industrial age (around 720 parts per billion) to today, when concentrations are between 1,695 and 1,720 parts per billion in 1995.
Sources: Lowe et al. (1994); NIWA, unpublished data
Nitrous oxide
Nitrous oxide (N2O) is another important long-lived greenhouse gas. The annual global increase is 0.2 percent to 0.3 percent. Atmospheric concentrations are believed to have increased by about 15 percent since the pre-industrial era. The nature of human-related sources (land-use change, fossil fuel combustion, biomass burning, nylon manufacture) remains very uncertain, but the emission rates generally track those of carbon dioxide, suggesting some common factor (Houghton et al., 1992; Battle et al., 1996).
In New Zealand most of the nitrous oxide is thought to come from the soil. The rate of emission is determined by rainfall, temperature, soil texture, drainage, and farming practice. The Cape Grim record, which is representative of a large part of the Southern Hemisphere, shows that nitrous oxide concentrations have been increasing through the 1980s, though not at a constant rate. (See Figure 5.16)
Figure 5.16: Nitrous oxide concentrations over Tasmania, 1982-91
See Figure
5.16 at its full size.
Textual description of figure 5.16
Nitrous oxide concentrations have steadily increased over Tasmania from the pre-industrial age (around 175 parts per billion) to around 308 parts per billion in 1991.
Sources: Lowe et al. (1994); NIWA, unpublished data
Halocarbons
Many of the most potent greenhouse gases are halocarbons (see Table 5.3). Perfluorocarbons (PFCs) are molecules containing only fluorine and carbon. They are not ozone depleting, despite their molecular structure. However, they are extremely potent greenhouse gases. Using a 100 year time horizon, their Global Warming Potential (GWP) ranges from 6,500 for perfluoromethane to 9,200 for perfluoroethane. Ten tonnes of emitted PFC can have the same long-term greenhouse warming effect as 92,000 tonnes of carbon dioxide.
Current emissions of PFCs are small, both globally and domestically, so the present global warming commitment of these gases is much less than that of carbon dioxide and methane (see Table 5.3). However, if emissions were to increase, their contribution to future climate change would become more important (UNEP/World Meteorological Organisation, 1994). The main source of PFCs in New Zealand is the aluminium smelter at Tiwai Point, near Bluff. Since 1994, there has been a significant decrease in smelting emissions, although some of this decrease has been offset by a small increase in PFCs imported for industrial purposes in 1995. The uses for industrial purposes is expected to increase slightly over the rest of the decade. The total amount emitted from both sources was estimated to be 0.089 Gg (89 tonnes) in 1990 (New Zealand Aluminium Smelters Ltd, 1993) and 0.029 Gg (30 tonnes) in 1995, a decrease of 67 percent.
Emissions of hydrofluorocarbons (HFCs) in New Zealand are thought to be small compared to carbon dioxide, although there are no precise data. They are, however, potentially important because of their increasing use and their high GWP/long atmospheric life. Approximately 120 tonnes of HFC-152a are imported into New Zealand every year. Most is used at New Zealand's only oil refinery at Marsden Point. Less than 20 kg of HFC-134a were imported in 1990, enough for research purposes only. In 1993 about 7.5 tonnes of HCF-134a were imported. However, in 1994 and 1995 the amounts imported rose sharply to 63 tonnes and 141.5 tonnes respectively as refrigerator manufacturers, seeking ozone-friendly alternatives to CFCs, switched to HFCs (Ministry for the Environment, 1994). Imports are expected to rise for some years to come as the use of CFCs and HCFCs decline.
