The earth’s climate is the result of a finely balanced system, and natural events and human activities can tip the balance. When incoming radiation from the sun and infrared radiation emitted by the earth are equal, average temperatures are constant. Anything that upsets this balance results in altered temperatures around the planet.
Large volcanic eruptions near the equator, carrying fine ash and acidic gas up into the atmosphere, can affect the world’s climate for several years. This became clear to scientists after the eruption of Mt Agung in Bali, Indonesia, in 1963, which affected the climate around the globe. A massive volcanic explosion in Mount Tambora, Indonesia, brought about the disastrous changes in the weather in 1816, known as ‘The Year without Summer’, causing famine throughout Asia, Europe and North America.
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 of those are increased by human activity, and others are only manufactured, 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 pre-industrial times, the atmospheric concentration of carbon dioxide has risen from around 280 parts per million (ppm) to around 410 ppm, an increase of almost 50%. Methane and nitrous oxide concentrations have also increased because of human activity.
Increasing greenhouse gas concentrations are already influencing the global climate, and these impacts are expected to increase over the coming century and beyond. The amount of water vapour in the atmosphere increases with rising temperatures, amplifying the effect of other greenhouse gases. The average temperature has already risen by about 1°C over the past century, and the face of New Zealand could change remarkably if temperatures rise by several degrees.
Other phenomena can also influence the global climate, although these usually have a temporary effect or happen much more slowly than warming due to increasing greenhouse gases.
For example, large volcanic eruptions in the tropics can deposit gases and dust particles in the stratosphere. These reflect some of the incoming solar energy and lead to worldwide cooling. Following such eruptions, temperatures in New Zealand have dropped by a few tenths of a degree Celsius for up to three years.
The amount and distribution of solar radiation reaching the earth is very nearly constant, but the small variation occurs 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) varies by less than 0.1% over the approximately 11-year sunspot cycle. In contrast, reconstructions of past variations suggest that changes twice as large have occurred over the last 400 years, producing noticeable changes in the earth’s mean temperature.
Over tens of thousands of years, the well-documented glacial cycles are triggered by systematic and predictable variations in the earth’s orbit that alter the distribution of solar radiation. This effect alone is too small to cause the observed temperature changes, which are amplified by the release of greenhouse gases from oceans and land masses.
Ozone is a form of oxygen molecule produced by reactions between ultraviolet (UV) sunlight and ordinary diatomic oxygen. This occurs most efficiently in the stratosphere (at an altitude of 15–50 kilometres above the earth’s surface), creating a permanent layer of stratospheric ozone. Ozone strongly absorbs both solar UV and the earth’s infrared emissions, acting like the roof of a greenhouse and warming the stratosphere. The resulting temperature profile of the atmosphere traps most turbulence, and almost all of the water vapour, in the troposphere, the layer below an altitude of 10–15 km.
Stratospheric ozone has diminished since the 1970s as a result of the annual Antarctic ozone hole, but it is expected to recover over the coming decades thanks to concerted international action.
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 12,000 years.
When the last glacial period peaked 20,000 years ago, average temperatures were about 6°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 northern half of the present North Island, while half of the present 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 northern 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 BCE New Zealand’s present climate was established, with its characteristic strong westerly and south-westerly winds.
Between 850 CE 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 the start of the official New Zealand seven-station temperature series in 1909, there was an average temperature increase of 1.1°C to 2019. Five of the ten warmest years in the last century ago occurred in the seven years up until 2019. Warming in New Zealand surface waters has occurred at a rate of up to 0.3°C per decade over the last 40 years.
Distinct changes in rainfall have occurred since 1930. Between 1951 and 1975 there were increased easterly and north-easterly airflows, with the result that the north of New Zealand became wetter, and the south-east drier. Between 1976 and 1994 there were several strong El Niño weather events, resulting in decreased rainfall in the North Island and an increase over 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 across 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 160 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 ago. 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 Fifth Assessment Report in 2014 identified sufficient evidence to blame global warming on human activities. A strong conclusion was drawn: ‘Warming of the climate system is unequivocal, and since the 1950s, many of the observed changes are unprecedented over decades to millennia’. 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 sufficient to cause the observed changes.
New Zealand, together with many other developed countries, is taking action to minimise greenhouse gas emissions and reduce the effects of climate change. The Paris Agreement is an international agreement to reduce climate change. New Zealand has ratified the 2015 Paris Agreement. The government passed the Climate Change Response (Zero Carbon) Amendment Act in 2019, and many local and regional councils are developing their own plans to reduce emissions and adapt to the impact of a changing climate.
The prediction by scientists in the 1970s of an impending ice age is often raised by sceptics to discredit global warming. However, the cooling was expected to occur gradually, reaching glacial conditions in 20,000 years’ time. This long-term cooling would be due to natural variations in the earth’s orbit around 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, including sheep, cattle, deer, and goats. Nitrous oxide is produced in the soil by bacterial breakdown of animal excreta and nitrogenous fertilisers. Forty-one per cent of emissions come from carbon dioxide produced by the energy sector (mainly transport and electricity generation). Industrial processes and waste account for 11%. On the upside, New Zealand's abundant forests absorb carbon dioxide from the atmosphere. However, New Zealand's total emission of greenhouse gases is estimated to have increased by about 23% between 1990 and 2017. This figure is even higher when the volume of timber harvested from New Zealand’s plantation forests is taken into account.
The Paris Agreement is the current global agreement on climate change. It was adopted at the United Nations Framework Convention on Climate Change (UNFCCC) in 2015 and commits all signatories to take action on climate change. New Zealand is one of nearly 200 countries that have ratified the agreement, which aims to limit the increase in global average temperature to well below 2°C above pre-industrial levels by reducing greenhouse gas emissions globally. The Paris Agreement takes effect from 2020 and New Zealand’s commitment (to reduce greenhouse gas emissions by 30% below 2005 levels by 2030) will apply from 2021.
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 and 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 over 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 the climate in future decades and even centuries.
To accurately predict human-induced changes in New Zealand’s climate, scientists need to know the global extent of greenhouse gas emissions, likely 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, combining global model projections with higher-resolution local climate information.
New Zealand’s climate varies from year to year thanks to 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 change 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 has reported its findings in 1990, 1995, 2001, 2007 and 2014. The Sixth Assessment Report from the IPCC will be published in 2021.
Projections developed by the Intergovernmental Panel on Climate Change (IPCC) suggest that if our emissions continue to grow at the current rate, the average surface temperature around the world will increase by 3.7–4.8°C between 1990 and 2100. This rate of warming is probably without precedent during at least the last 10,000 years. The same period will also see a rise in global mean sea level of between 61 and 110 centimetres, the continued widespread retreat of glaciers, and significant increases or decreases in annual rainfall depending on location. These changes will bring a range of adverse and some beneficial effects to environmental and socio-economic systems.
The range of predicted changes is broad, for two reasons:
There are a number of potential changes for New Zealand’s climate. In a 2018 study completed by the National Institute of Water and Atmospheric Research for the Ministry for the Environment, the mid-range estimate for projected New Zealand temperature change was an expected increase of 1.4°C by 2090 relative to the period 1986–2005. Other likely changes included a rise in sea level above 1990s levels of between 46 and 105 centimetres by 2100, increased rainfall in the south and west of the country coupled with a decrease in the north and east, a long-term reduction in glacier length and ice thickness, and an increased westerly windflow across New Zealand.
With a changing climate, some crops may no longer be grown in some areas. Health risks could change. Local authorities may alter their regulations controlling building development and the use of water resources. For pastoral farming, increased droughts expected in the east of both islands and in Central Otago could lead to reduced 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 Hayward kiwifruit production in Bay of Plenty after 2050.
Acknowledgements to David Wratt