The earth’s climate is the result of a finely balanced system. Incoming radiation from the sun and infrared radiation emitted by the earth are approximately equal. Anything that upsets this exchange results in altered temperatures around the planet. For example, large volcanic eruptions in the tropics deposit gases and dust particles in the stratosphere, reflecting some of the incoming solar energy and leading to worldwide cooling. Following such eruptions, temperatures in New Zealand can drop by a few tenths of a degree Celsius for up to three years.
Large volcanic eruptions near the equator can affect the world’s climate for several years. The impact of the blast on the Indonesian island of Krakatoa, which occurred around 536 AD, was still being referred to by European historians many centuries later. According to the 11th-century scholar Michael the Syrian, ‘The sun became dark and its darkness lasted for eighteen months ... The fruits did not ripen, and the wine tasted like sour grapes’. 1
The amount and distribution of solar radiation reaching the earth can vary over a wide range of timescales.
Measurements taken from space since the late 1970s, for example, show that the solar constant (the average amount of solar radiation that reaches the earth's upper atmosphere) varied by less than 0.1% over the approximately 11-year sunspot cycle. Reconstructions of past variations, however, suggest that changes 5–10 times larger have occurred over the last 400 years, sufficient to produce noticeable changes in the earth’s mean temperature.
Over tens of thousands of years the well-documented ice age cycles, caused by systematic and predictable variations in the earth’s orbit, alter the distribution of solar radiation at polar latitudes in summer.
Ozone is a form of oxygen produced by reactions between sunlight and oxygen. This occurs most efficiently in the stratosphere (at an altitude of 15–50 kilometres). Since the 1970s there has been a long-term trend towards a stronger Antarctic Oscillation (stronger westerly winds at 50° south), which has been attributed to both an increase in greenhouse gases and a depletion of stratospheric ozone.
Increasing concentrations of greenhouse gases have serious consequences for the future climate. Most of the gases occur naturally – water vapour, carbon dioxide, ozone, methane and nitrous oxide – but some are man-made, such as chlorofluorocarbons (CFCs). A build-up of these gases traps infrared radiation in the lower atmosphere, leading to a warming of the earth’s surface. This is called the enhanced greenhouse effect, since it is an amplification of a natural process that has operated for billions of years and kept the planet habitable.
Since 1750, atmospheric concentrations of carbon dioxide have increased by 31%, methane by 151%, nitrous oxide by 17%, and tropospheric ozone by 36%. There has been a reduction in stratospheric ozone since 1979, partly because of the ozone hole. However, it is expected to recover during the 21st century if countries comply with international targets on CFC emissions.
In the past two million years there have been 30 major oscillations between cold glacial and warm interglacial periods. The present interglacial era has so far lasted 15,000 years.
When the last glacial period peaked 20,000 years ago, temperatures were 4–5°C below what they are today. New Zealand’s glaciers were at their largest, and the sea level was 120–130 metres lower. Inhibited by the cool, harsh, windy climate, forests grew only in the top half of the North Island, while the majority of the South Island lay under ice.
About 18,000 years ago, the climate began to get warmer and wetter. The sea level rose as the glaciers melted and began to rapidly retreat, separating New Zealand’s two main islands some 12,000 years ago. The forests began to recolonise the grasslands, soon covering the entire North Island and the top of the South Island.
Glaciers were at their smallest between 9,500 and 5,000 years ago, when temperatures may have been as much as 1–1.5°C higher than they are at present. The climate continued to fluctuate during the interglacial period. In the last 5,000 years, prolonged intervals of cool weather led to renewed glacial advances. By about 500 BC New Zealand’s present climate was established, with its characteristic strong west and south-west windflows.
Between 850 and 1850 the climate was variable, with cooler periods occurring about every 100–150 years.
New Zealand’s climate patterns can be reconstructed from land and marine observations collected since the beginning of the 1860s. From 1860 to 1990 there was a mean temperature increase of 1.1°C across the country. Temperatures increased sharply after the 1940s. Records of sea-surface temperatures taken between 1900 and 1980 show an increase of 0.8°C.
Distinct changes in rainfall have occurred since 1930. In particular, between 1951 and 1975 there were increased east and north-east airflows, the north of New Zealand got wetter, and the south-east became drier. Between 1976 and 1994 there were several strong El Niño weather events, resulting in decreased rainfall in the North Island and an increase in much of the South Island. Winter rainfall increased over almost all of the country.
The late 1970s saw a long-lasting shift in climate, characterised by more persistent westerly winds around central New Zealand. This resulted in the west and south of the South Island becoming wetter and cloudier, with greater incidence of major floods. By contrast, the north and east of the North Island were drier and sunnier.
In addition to meteorological data collected over the past 140 years, information about past climate can be obtained by piecing together evidence taken from the land.
Glacial ice cores can provide information back to 230,000 years. The ratio of oxygen isotopes in ice cores, for example, indicates what the temperature was when that ice first fell as snow. Air bubbles in the ice are analysed to measure carbon dioxide and methane concentrations, and trapped dust may indicate windy, arid conditions.
Variations in the size of past glaciers can be deduced from features in the nearby landscape and the location of moraines (rocks and debris deposited by glaciers). The width of tree rings reflects soil moisture, temperature and other conditions in which the tree grew.
Cores taken from lake and ocean sediments carry the fossil remains of plankton, and indicate the physical and chemical conditions of the water. Pollens show the type of vegetation present. Since certain plants favour particular climate conditions, fossil pollens can provide clues to the climate of the time.
In 1988, international concern about global warming led to the establishment of the Intergovernmental Panel on Climate Change (IPCC). This group was to assess all the latest scientific, technical and socio-economic research on the topic. The IPCC’s Third Assessment Report in 2001 identified sufficient evidence to blame global warming on human activities. Scientists had carried out extensive modelling to understand the global climate, but were unable to reproduce the warming observed since 1950 without increasing carbon dioxide concentrations. Neither the natural variability of weather systems, nor changes in incoming solar radiation were shown to be sufficient to cause the observed changes.
New Zealand, together with many other developed countries, is taking action to minimise greenhouse gas emissions and therefore reduce the effects of climate change. It has ratified the Kyoto Protocol, researched emission reduction, and struck up international partnerships (with, for example, Australia and the US) in order to collaborate and exchange ideas. Many of New Zealand’s local and regional governments are involved in initiatives.
The prediction by scientists in the 1970s of an impending ice age is often raised by skeptics to discredit talk of global warming. However, the cooling was calculated to occur gradually, reaching glacial conditions in 20,000 years’ time. This long-term cooling would be due to natural variations in earth’s orbit of the sun. By contrast, global warming is largely a consequence of human activity.
Just under half of New Zealand’s total greenhouse gases are produced by agriculture in the form of methane and nitrous oxide. The agricultural sources of methane are ruminant animals like sheep and cows, while nitrous oxide is produced in the soil by bacterial breakdown of animal excreta and nitrogenous fertilisers. Forty-three per cent of emissions come from carbon dioxide produced by the energy sector (mainly transport and electricity generation). Industrial processes and waste account for 8%. On the upside, New Zealand's abundant forests help to absorb carbon dioxide from the atmosphere. However, New Zealand's total emission of greenhouse gases is estimated to have increased by about 21% from 1990 to 2004.
Developed from the earlier United Nations Framework Convention on Climate Change, the Kyoto Protocol is an international agreement to address global warming and delay climate change. It aims to reduce the total greenhouse emissions of participating countries to below their 1990 levels by 2012. However, each country has its own target – New Zealand’s is to reduce emissions to the same level they were in 1990.
The Kyoto Protocol was designed as a first step towards a future with lower emissions. While targets beyond 2012 have not yet been agreed, it is clear that greater cuts in greenhouse gases and broader participation will be needed.
Research conducted by New Zealand’s government, education and private sectors aims to understand how sensitive the country is to climate change and variability. In addition, there are a number of initiatives to reduce emissions. These include strategies to improve energy efficiency, increase renewable energy sources, use more energy-efficient transport, and reduce emissions from landfills.
The earth’s climate is the result of complex interactions between many processes in the atmosphere, ocean, cryosphere (snow, ice and permafrost) and on land. Predictions about how the climate is likely to respond to increased greenhouse gases must therefore consider a number of variables. For example, the oceans hold heat and transfer it around the globe, so it is essential to consider the effects of this along with the atmospheric processes.
Our present understanding of the climate system would be impossible without global climate models (GCMs). These are powerful computer programs that simulate climate systems in three spatial dimensions and in time. Climate modelling gauges interactions between the land, ocean and cryosphere. Comparisons between different models and a wide range of data allow scientists to usefully predict changes in climate for future decades or centuries.
To accurately predict human-induced changes in New Zealand’s climate, scientists need to know the global extent of greenhouse gas emissions, future changes in carbon dioxide concentrations, and the influence of New Zealand’s topography on local climate. Each of these factors comes with uncertainties. For example, gauging future emissions relies on anticipating human behaviour, including the success of constraints negotiated under the United Nations Framework Convention on Climate Change. Our understanding of the carbon cycle and of sources of non-carbon dioxide greenhouse gases is also incomplete.
Forecasting regional climate changes in New Zealand from global projections requires complex adjustment, since the global average does not necessarily apply to a given location in New Zealand. A variety of approaches are used to do this, although they all combine global model projections with higher-resolution local climate information.
New Zealand’s climate varies from year to year from natural processes. Some parts of the country, for example, have dry summers and autumns when an El Niño weather pattern is present. Natural fluctuations need to be considered alongside human-created climate changes when developing plans and policies. Beyond the next few decades, however, global warming, caused mostly by human activity, will begin to dominate. To understand the range of possibilities for future climate in New Zealand, it is helpful to first look at projections of global change.
The Intergovernmental Panel on Climate Change (IPCC) was established in 1988 by the World Meteorological Organisation and the United Nations Environment Programme. It analyses the most up-to-date research on climate change, and reported findings in 1990, 1995 and 2001.
Projections developed by the Intergovernmental Panel on Climate Change (IPCC) suggest that between 1990 and 2100 the average surface temperature around the world will increase by 1.4–5.8°C. This rate of warming is probably without precedent during at least the last 10,000 years. This same period will also see increases and decreases in annual rainfall (depending on location) of 5–20%, a rise in global mean sea level of 9–88 centimetres, and the continued widespread retreat of glaciers. These changes will bring a range of beneficial and adverse effects to environmental and socio-economic systems.
The range of predicted changes is broad, for two reasons:
There are a number of scenarios for New Zealand’s climate. One study completed by the National Institute of Water and Atmospheric Research for the Ministry for the Environment Climate Change Office concluded that if future greenhouse gas emissions were in the middle of the range projected by the IPCC, it was likely that New Zealand's mean temperature would rise by up to 2°C by the 2080s. Other likely changes included a rise in sea level between 9 and 88 centimetres, increased rainfall on the south and west coupled with a decrease in the north and east, long-term reduction in glacier length, and a greater westerly windflow across New Zealand.
With changing climate, some crops may no longer be grown in some areas. Health risks could change. Local government may alter their regulations for building development and use of water resources. For pastoral farming, increased droughts expected on the east of both islands and in Central Otago could lead to a reduction in grass growth. Subtropical grasses are expected to spread south, with pastures extending to higher ground. Warming will increase the incidence of agricultural pests and diseases, but it will also allow arable and fruit crops to spread south. One study suggests that warming in winter could begin to restrict kiwifruit production in the Bay of Plenty in the second half of this century.
Acknowledgements to David Wratt