The atmosphere enveloping the earth is a vital part of the world we live in. It contains the oxygen we need to breathe, protects us from ultraviolet radiation and the extreme cold of space, and plays an essential role in recycling energy, water, and other essentials for life. Weather is a direct product of atmospheric processes, and influences our pattern of living.
As a result of industrial and other activities, human society is now emitting gases into the atmosphere in such quantities that the composition and dynamics of the atmosphere are changing. The changes occurring in the atmosphere as a result of human activity can have significant environmental, health, and economic effects.
In recent decades, global attention has focused on two environmental issues in relation to the atmosphere: climate change (often referred to as ‘global warming’) and depletion of atmospheric ozone. This chapter discusses the New Zealand aspects of these two environmental issues.
Greenhouse gases in the earth’s atmosphere trap warmth from the sun and make life possible. Without them, temperatures at the surface of the earth would be about 30°C colder. The major greenhouse gases include:
carbon dioxide (CO2)
nitrous oxide (N2O).
Other greenhouse gases at lower concentrations include:
sulphur hexafluoride (SF6)
The ‘enhanced’ greenhouse effect
The Intergovernmental Panel on Climate Change (see box ‘International initiatives on climate change') recently brought together the most up-to-date knowledge on climate change in its Fourth Assessment Report (Intergovernmental Panel on Climate Change, 2007a; 2007b). Since the industrial revolution of the early 19th century, the concentration of greenhouse gases such as carbon dioxide, methane, and nitrous oxide has increased in the atmosphere. They now far exceed pre-industrial levels as determined from ice cores that span thousands of years.
Global increases in atmospheric carbon dioxide concentration are caused primarily by fossil fuel use and land-use change, while increases in methane and nitrous oxide are primarily due to agriculture (Intergovernmental Panel on Climate Change, 2007a). This increased concentration of gases traps more of the sun’s warmth than normal, leading to a gradual warming of the atmosphere (the ‘enhanced’ greenhouse effect).
Fluctuations in the ‘natural’ atmospheric system bring about changes in climate (climate variability). However, changes in the climate as a result of increased concentrations of greenhouse gases in the atmosphere are expected to be much greater, and to happen more quickly, than any natural changes in the past 10,000 years.
Most of the increase in global temperatures since the mid-20th century is attributed to increased greenhouse gas concentrations caused by human activity. Human influence is also discernible on other aspects of the climate, such as ocean warming, temperature extremes, and wind patterns (Intergovernmental Panel on Climate Change, 2007a).
Impacts on New Zealand’s climate
New Zealand’s average surface temperature increased by 0.9°C between 1920 and 2006. This increase is consistent with increases in global temperature. The average global temperature has warmed by 0.76°C in the past century (Intergovernmental Panel on Climate Change, 2007a). The average minimum temperature has increased by 1.2°C and the maximum has increased by 0.7°C over the same period for New Zealand. Frost frequency has decreased since the 1950s (National Institute of Water and Atmospheric Research, pers comm).
For the next two decades (to 2030), global increases in temperature of about 0.2°C each decade are projected for a range of expected levels of greenhouse gas emissions (Intergovernmental Panel on Climate Change, 2007a). Best projections of global temperature increases by 2100 are between 1.8 and 4.0°C.
These changes suggest that we can no longer rely on using historical climate data to predict our future climate patterns. Figure 8.1 illustrates how current climate variables such as temperature differ from the historical pattern.
Text description of figure
The difference in temperature in an individual year compared with an average for the 1971-2000 period is plotted for each year between 1853 until 2006. From 1853 there is a predominance of years during which the average temperature was lower than the 1971-2000 period. Between 1970 and 2006, 23 years have shown average temperatures greater than the 1971-2000 period. From 1920 until 2006 there is a linear trend in increasing temperature anomaly.
Changes in annual rainfall in New Zealand are expected as a consequence of climate change. A warmer atmosphere will hold more moisture, and so increased rainfall amount and intensity are likely (Intergovernmental Panel on Climate Change, 2007b).
However, Figure 8.2 shows that these expected changes in annual rainfall are not yet evident for several locations in New Zealand. No consistent, country-wide trend has so far been discernible in the rainfall record.
Other climate impacts
Rising global temperatures are expected to lead to other changes in climate patterns that could impact significantly on the global environment, economy, and human society. As an example, the average global sea-level rise is expected to be between 0.19 metres and 0.58 metres by 2100 (Intergovernmental Panel on Climate Change, 2007b). This rise in sea level has the potential to affect coastal erosion and cause saltwater intrusion into aquifers.
Climate models also predict that New Zealand will experience an increased intensity and frequency of extreme weather events, including more droughts in already drought-prone areas, and larger and more frequent floods in regions already vulnerable to flooding. An increase in westerly winds is likely to result in more rainfall and flooding in the west, and less rainfall and more droughts in the east of the country (Ministry for the Environment 2004).
Sea level rise has the potential to impact on coastal erosion.
Source: Courtesy of Peter Arnold, National Institute of Water and Atmospheric Research.
More about climate change impacts in New Zealand
As an island country reliant on primary production and tourism for its economic well-being, New Zealand stands to be significantly affected by climate change.
Rising temperatures are likely to lead to greater risks to our agricultural, horticultural, and forestry sectors, as extreme weather events become more frequent and tropical-pest plants and insects become established here. The type of, and suitable location for, key crops may also change. Higher temperatures could cause problems for crop production. For example, crops such as kiwifruit require cold winters for fruit development.
Climate change is also likely to accentuate existing pressures on New Zealand’s native species (see chapter 12, ‘Biodiversity’) and natural ecosystems. Subtropical diseases may become a problem for human health if carrier insects, such as mosquitoes carrying the Ross River virus, become established here.
Predicted extreme weather events have significant costs associated with them, particularly when infrastructure and economic productivity are damaged. As an example, the drought in 1997–1998 is estimated to have cost the New Zealand economy a billion dollars, and the floods of February 2004 are estimated to have cost about $400 million. An increase in forest and grass wildfires in drier eastern areas of the country is likely to have a significant economic effect.
Coastal erosion and sea level rise can pose a particular problem for Māori, whose spiritual sites (urupā and wāhi tapu) are often located close to sea level.
However, a changing climate may also provide opportunities, such as new crops, and faster crop growth rates, made possible by elevated levels of carbon dioxide in the atmosphere. Warmer winters are expected to relieve the pressure on electricity and heating supplies, as well as to provide health benefits (and a reduction in heating costs). Increasing rainfall in the Southern Alps could boost electricity supply by raising water levels in our major hydro storage lakes.
Ozone is a gas present in trace quantities in the atmosphere. It is a form of oxygen with three oxygen molecules instead of the normal two. More than 90 per cent of ozone is found in the stratosphere, 20–25 kilometres above the earth, in what is known as the ozone layer.
Ozone plays an important role in protecting the earth from the sun’s harmful effects. Ozone is produced when ultraviolet (UV) radiation from the sun meets oxygen molecules. Once formed, ozone molecules absorb UV radiation before it reaches the surface of the earth. Without this process, life as we know it could not exist (National Institute of Water and Atmospheric Research, pers comm).
Although ozone is constantly created and destroyed in the stratosphere through natural processes, some chemicals increase the amount of ozone destroyed. This is particularly true of chlorofluorocarbons – chemicals that were used widely in the past as refrigerants and in some industrial processes.
When gases containing chlorine and bromine reach the stratosphere, they break down to release reactive molecules of chlorine or bromine, which alter the natural balance of ozone creation and destruction. As the concentrations of chlorine and bromine in the stratosphere rise, there is a consequential increase in the depletion of the ozone layer, and in the amount of UV radiation reaching the ground.
Atmospheric ozone and ground-level ozone
While ozone depletion in the upper atmosphere can cause harm to both the environment and human health, an increasing quantity of ozone at ground level can be a concern for air quality. Chapter 7, ‘Air’, describes the effect of increasing ground-level ozone.
What is the ‘ozone hole’?
Over the past 30 years, ozone levels over Antarctica have dropped by almost 60 per cent during the spring of each year, and a ‘hole’ in the ozone layer is clearly visible in satellite observations. This hole does not extend over New Zealand. In fact, New Zealand experiences its highest ozone levels in October, at the same time as the ozone hole occurs over Antarctica.
Nonetheless, summertime ozone levels over New Zealand continue to be strongly influenced by Antarctic ozone depletion. When the ozone hole over Antarctica breaks up in November or December, ozone-depleted air moves into surrounding areas in the southern hemisphere, including New Zealand. The later the ozone hole breaks up, the higher the sun is in the sky over New Zealand and so the larger the effect on UV levels. If New Zealand experiences a combination of lower ozone with high sun and few clouds, then skin-damaging UV levels can be extreme (National Institute of Water and Atmospheric Research, pers comm).
Increased UV levels
New Zealand’s location in the remote southern Pacific and Southern oceans and its low population density mean it has an exceptionally ‘clean’ atmosphere over most of the country. For example, New Zealand has little of the particulates and shorter-lived industrial gases that impact on heavily industrialised countries in the northern hemisphere (see chapter 7, ‘Air’, for further detail on air quality).
Our latitude and clarity of atmosphere mean that high levels of radiation from the sun reach the ground relatively unhindered. This results in higher levels of UV radiation at ground level in New Zealand than in other developed countries (National Institute of Water and Atmospheric Research, pers comm). Ozone depletion further exacerbates this effect, resulting in increased intensity of the UV radiation that causes sunburn.
The environmental and health impacts of raised levels of UV can be severe, particularly for New Zealand where high levels of UV are already experienced. The impacts include increases in the rates of skin cancers and eye cataracts. Raised levels of UV also suppress human and animal immune systems. Plants can also be affected. For example, high UV levels reduce the growth of plankton, a critical building block of the marine food chain. Increased exposure to UV damages some man-made materials such as paints, plastics, and construction materials.
Seasonal variability in UV levels
In New Zealand there is high seasonal variability in UV levels, especially in the south of the country, where:
winter levels of sunburn-causing UV are less than 10 per cent of those in summer
winter intensities of UV that produce vitamin D are probably less than 5 per cent of their summertime peaks.
This variability means that there are two concerns relating to UV in New Zealand. In the summer months, high UV intensities increase our risk of skin damage compared with corresponding northern hemisphere countries. But during the winter months, there may not actually be enough UV radiation to produce sufficient vitamin D in our bodies (McKenzie et al, 2006).
Links between climate change and ozone depletion
Climate change and ozone depletion are accelerated by human activity. Many ozone-depleting gases are also greenhouse gases. By reducing the use of ozone depleting gases, we can both protect the ozone layer and reduce climate change.
At the same time, climate change is likely to accelerate the recovery of the ozone layer, at least outside polar regions. While the earth’s surface is expected to warm in response to increases in greenhouse gases, the stratosphere is expected to cool. Outside polar regions, this combination results in a decrease of the rate of ozone depletion. However, in polar regions, the lower stratospheric temperatures and stronger polar winds could extend the period over which stratospheric clouds are present, which in turn promotes chlorine-caused ozone destruction.