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.
The last ice age
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.
The interglacial period
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.
Changes since the 1860s
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.
Methods of assessment
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.