State of New Zealand’s climate

Temperature

New Zealand’s annual average land-surface temperature (measured 1.3 metres above the ground) has increased 1 degree Celsius since 1909, when measurements first began. This is consistent with the lower end of the global average increase of land temperature over a similar period (1–1.2 degrees Celsius from 1880 to 2012) due to the moderating influence of our oceanic location.

From 1909 to 2016, our annual average temperature was 12.3 degrees Celsius.

Our five warmest years occurred in the last 20 years, with 2016 the warmest. Globally, 19 of the 20 warmest years occurred within the last 20 years. The more variable and more moderate recent warming over New Zealand reflects the moderating influence of our oceanic location (NIWA, nd-b) (see figure 8).

Figure 8

Note: The unusual drop in temperature from 1992 to 1993 is a result of the volcanic eruption at Mount Pinatubo in the Philippines.

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We can expect temperatures to continue increasing, with warming unabated to 2100 and beyond unless the world follows a low-emissions path that includes removing some carbon dioxide presently in the atmosphere (Ministry for the Environment, 2016) (see New Zealand’s future climate and climate risks and Appendix: Climate change projections for New Zealand).

Frost and warm days

Frost days are days when the minimum air temperature is 0 degrees Celsius or lower, while warm days are days when the maximum air temperature is higher than 25 degrees Celsius. Climate models project fewer cold and more warm extremes in the future (see Appendix: Climate change projections for New Zealand).

Over the 45 years between 1972 and 2016, the number of frost days decreased at 10 of the 30 measured sites and increased at 1 site, while no trend was apparent at the other 19 measured sites around New Zealand. The number of warm days increased at 8 and decreased at 1 of the 30 measured sites. No trend in warm days was apparent at 21 sites (figure 9).

Growing degree days

Growing degree days measure heat accumulation, which can be used to predict plant, and subsequently, animal growth. For example, they can be used to predict when certain flowers will bloom or insects will emerge from dormancy. Growing degree days count the total number of degrees Celsius the average temperature each day is above a base temperature, commonly a threshold of 10 degrees Celsius.

Plant and animal cycles are interdependent across the food chain. Small changes, for example, in the timing of insect reproduction, can mean that species further up the food chain miss out on a crucial food source. The greater the change in timings, the more pressure species further up the food chain experience. In extreme cases, some species are threatened with extinction.

Figure 9

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Rainfall

The impacts of high or low rainfall can be positive or negative. Rainfall is a valuable source of water for crops and gardens, as well as a generator of hydroelectricity. However, rainfall can restrict some recreational activities and has serious impacts when it leads to flooding, or when a lack of it leads to drought.

Annual and seasonal rainfall

Annual and seasonal rainfall are highly variable and depend on short-term weather patterns and long-term climate oscillations such as El Niño Southern Oscillation, the Interdecadal Pacific Oscillation, and the Southern Annular Mode.

Geographically, annual average rainfall varies from less than 400 millimetres in places like Central Otago to over 4,000 millimetres in mountainous areas such as the Tararua range, and even more in the Southern Alps (an average of over 6,000 millimetres has been recorded at Milford Sound).

Rainfall also varies seasonally, with more rain in winter than in summer for most of the country except the southern half of the South Island, where most rain falls in summer. Despite no clear trend in annual rainfall, seasonal rainfall in some locations showed trends (see figure 11). For example, between 1960 and 2016, winter rainfall in Whangarei, Wellington, and New Plymouth decreased while summer rainfall increased in Dunedin and Kerikeri.

Climate models project that rainfall is very likely to increase on average over winter and spring in the south of the South Island and west of both the North and South islands (see Appendix: Climate change projections for New Zealand). Meanwhile, drier average conditions are expected in the east and north. In summer, wetter conditions are likely in the north and east of both islands (Ministry for the Environment, 2016).

Intense rainfall events

Changes in the frequency, intensity, spatial extent, duration, and timing of weather and climate extremes are expected to result from a changing climate (IPCC, 2012). Some reasons for this are well understood. For example, the natural global water cycle is intensifying as the atmosphere warms. This leads to increased evaporation, which may worsen droughts but may also increase the
frequency of intense rainfall events because
a warmer atmosphere can hold more water.

Figure 11

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For most of New Zealand, there is no clear evidence that intense rainfall events have changed between 1960 and 2016. However, there were trends at some locations (see figure 12):

• the proportion of annual rainfall occurring in intense events (in the 95th percentile) decreased at 4 of 30 locations (Auckland, New Plymouth, Rotorua, and Taupō) and increased at two (Napier and Timaru)
• the annual maximum one-day rainfall amounts decreased at 4 of 30 locations (Auckland, Hamilton, Taupō, and New Plymouth) and increased at two (Timaru and Dunedin).

As the climate changes, the number of intense rainfall events is expected to increase over most of the country (except for Northland and Hawke’s Bay), with up to a 20 percent increase of events possible in the south of the South Island (Ministry for the Environment, 2016) (see Appendix: Climate change projections for New Zealand).

The extent to which a specific individual event is influenced by increasing greenhouse gas concentrations is difficult to determine. Many factors combine to produce a specific event, and because such events are rare, there are usually only a few examples of past events for any given location (Herring et al, 2016).

So far, published studies have identified increasing greenhouse gas concentrations contributing to two recent New Zealand flooding events: 2011 – a flood in Golden Bay (Dean et al, 2013); and 2014 – a flood in Northland (Rosier et al, 2015). These studies indicate that, while such events might have happened in the absence of high greenhouse gas emissions, they were more extreme than they would have been without the warming caused by additional greenhouse gases in the atmosphere.

Wind

Wind is a valuable source of renewable energy, but strong gusts can damage property and trees and cause havoc for transport, communications, and power.

Average annual maximum wind gusts using data between 1972 and 2016 showed Wellington, Invercargill, and Gore were the windiest populated centres in the country, while Reefton, Gisborne, and Queenstown were the least windy.

In this report, we analysed extreme wind using two statistics: the number of days each year with a maximum wind gust in the 99th percentile and the annual highest maximum gust.

Between 1972 and 2016, the frequency and magnitude of extreme wind decreased at about one-third of sites across New Zealand (see figure 12). Over this period, the number of days a year with a gust that is extreme for that location decreased at 10 of 30 sites (Whangarei, Hamilton, Tauranga, Rotorua, Napier, Wellington, Blenheim, Christchurch, Timaru, and Dunedin) and increased at two sites (New Plymouth and Queenstown). New Plymouth also experienced an increase in highest maximum wind gusts.

Projections indicate climate change may alter the occurrence of extreme wind events, with the strength of extreme winds expected to increase over the southern half of the North Island and the South Island, especially east of the Southern Alps (Ministry for the Environment, 2016) (see Appendix: Climate change projections for New Zealand).

Figure 12

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