Part one - Current and future climate

This part covers:

  • projected climate change effects at national level and, where possible, at a regional level
  • possible changes in the intensity of extreme weather events such as floods and storms
  • current variability of New Zealand’s climate
  • significance of climate change versus natural climate variability.

Future climate

Predictions of future climate depend on projections of future concentrations of greenhouse gases and aerosols, as well as on model assessments of how the global climate system will respond to these changing concentrations. Future greenhouse gas concentrations depend on future emissions, which depend in turn on national and international policies, and changes in population, economic growth, technology and energy availability.

The maps in Figures 1–3 show projected patterns for temperature and precipitation derived from models assuming greenhouse gas emissions follow a course described in the source report as ‘middle-of-the-road’.

Because natural effects cause the New Zealand climate to vary from year to year (see Figure 4), the changes are specified in terms of the average change for the period 2030–2049 (referred to below as 2040), and for 2080–2099 (similarly referred to as 2090), relative to the climate of 1980–1999 (1990).

The source report on which this guide was based provides full technical detail about how the New Zealand projections were developed.

Changes in temperature

The mid-range projections are that New Zealand temperatures will increase by about 1°C by 2040, and 2°C by 2090, relative to 1990. However quite a wide range of future warming is projected for New Zealand when the results of all the models are analysed and all IPCC emissions scenarios are considered: 0.2–2.0°C by 2040 and 0.7–5.1°C by 2090.

Figure 1 shows that the annual-average pattern of warming is projected to be fairly uniform over the country, although slightly greater over the North Island than the South Island. In summer and autumn (not shown), the North Island and northwest of the South Island have the greatest warming, whereas in the winter season the South Island has the greatest warming. Spring has the least warming of all seasons.

Figure 1: Projected mid-range changes in annual mean temperature (in °C) relative to 1990.

The changes shown are an average of the results of 12 climate models for a mid-range IPCC emissions scenario. Note the different temperature scales for 2040 and 2090. These maps are intended to illustrate broad geographical patterns of climate change within New Zealand. They should not be used as definitive predictions of climate change for specific geographical locations. Projections for specific regions are provided in Tables 2 and 3.

 

Projected mid-range changes in annual mean temperature (in oC) relative to 1990

This figure shows two maps of New Zealand, giving projected changes over New Zealand in annual mean temperatures in degrees Celsius relative to 1990 for 2040 and 2090 from the twelve-model averages.

For 2040 the projected changes in mean annual temperature for New Zealand range from 0.8 degrees (South-Westland and Stewart Island) to 1.0 degrees (Canterbury-Kaikoura and most of the North Island). For 2090 the projected changes in temperature range from 1.8 degrees (Stewart Island) to 2.2 degrees (most of the north of the South Island, and most of the North Island).

a Based on an average over 12 climate models for a mid-range (A1B) emission scenario.

Copyright NIWA 2008
Projection: New Zealand Map Grid

Disclaimer: NIWA have prepared this map exercising all reasonable skill and care. Nevertheless, NIWA can give no warranty that the map is free from errors, omissions or other inaccuracies. Users of this map will release NIWA from all liability whether indirect or consequential, arising out of the provision of this map.

Projected changes in rainfall

Figure 2 shows that the projected mid-range change in the average annual rainfall has a pattern of increases in the west (up to 5 per cent by 2040 and 10 per cent by 2090) and decreases in the east and north (exceeding 5 per cent in places by 2090). This annual pattern of ‘wetter in the west and drier in the east’ results from the changes in the dominant seasons of winter and spring.

Figure 2: Projected mid-range changes in annual mean rainfall (in %) relative to 1990.

The changes shown are an average of the results of 12 climate models for a mid-range IPCC emissions scenario.

 

Projected mid-range changes in annual mean rainfall (in %) relative to 1990.

This figure shows two maps of New Zealand, giving projected changes over New Zealand in precipitation in percent relative to 1990 for 2040 and 2090 from the twelve-model averages.

For 2040 the projected changes in mean annual precipitation for New Zealand range from an increase of 7.5 percent (West Coast of the South Island) to a decrease of 5 percent (along a thin coastal strip from Kaikoura north to East Cape, and in Northland). For 2090 the projected changes in precipitation range from an increase of over 10 percent (West Coast of the South Island) to a decrease of 7.5 percent (in patches along the coastal strip from Kaikoura north to East Cape, and in Northland).

a Based on an average over 12 climate models for a mid-range (A1B) emission scenario.

Copyright NIWA 2008
Projection: New Zealand Map Grid

Disclaimer: NIWA have prepared this map exercising all reasonable skill and care. Nevertheless, NIWA can give no warranty that the map is free from errors, omissions or other inaccuracies. Users of this map will release NIWA from all liability whether indirect or consequential, arising out of the provision of this map.

Newly projected changes in seasonal mean rainfall (Figure 3) show a different and more marked seasonality than was evident in earlier projections used in the previous edition of this guide. The latest mid-range model results suggest that increased westerlies in winter and spring will bring more rainfall in the west of both islands and drier conditions in the east and north. Conversely, there will be a decreased frequency of westerly conditions in summer and autumn, with drier conditions in the west of the North Island and possible rainfall increases in Gisborne and Hawke’s Bay.

Figure 3: Projected mid-range changes in seasonal mean rainfall (in %) for 2090 relative to 1990.

The changes shown are an average of the results of 12 climate models for a mid-range IPCC emissions scenario.

 

Projected mid-range changes in seasonal mean rainfall (in %) for 2090 relative to 1990.

This figure shows four seasonal maps of projected changes in seasonal mean rainfall (in percentage) over New Zealand for 2090 relative to 1990 (twelve model average) for the A1B scenario.

For summer, the map shows increases in seasonal mean rainfall of up 10 percent in Hawkes Bay and parts of the East Cape. Smaller increases of up to 7.5 percent are projected in most parts of Marlborough and Canterbury, the Wairarapa and southern Bay of Plenty. Decreases of up to -5 percent are shown in the east of the North Island and Kapiti region, the northern West Coast and parts of Southland. Smaller decreases over Stewart Island, the Waikato Region and south Westland are projected.

For autumn, increases of up to 7.5 percent in seasonal mean rainfall are projected for Marlborough, Canterbury, Otago, Westland, Western Bay of Plenty, and from East Cape down through Hawkes Bay and the Wairarapa. Decreases of up to -5 percent are shown for Northland, Waikato, Wanganui,  most of the central North Island and the region around the northern end of the Southern Alps.

For winter, marked increases in seasonal mean rainfall are projected, particularly in the west, with the West Coast of the South Island, Southland, parts of Taranaki, Manawatu and the Waikato increasing by over 10 percent. Decreases in the east of up to -7.5 percent in Northland and from East Cape down through Hawkes Bay and the Wairarapa are expected, as well as down the east coast of the South Island as far as Canterbury. Smaller decreases in the Bay of Plenty and parts of the Waikato are shown.

For spring, marked increases in seasonal mean rainfall of over 10 percent are projected for the West Coast of the South Island and Southland. Smaller increases of up to 5 percent are expected for Otago, the north-west of the South Island and most of Taranaki and the Manawatu. Marked decreases of more than -7.5 percent are shown in Northland, Auckland, Coromandel, East Cape, Hawkes Bay and parts of the Wairarapa. Smaller decreases in the central North Island and Marlborough/Canterbury regions are expected.

a Based on an average over 12 climate models for a mid-range (A1B) emission scenario.

Copyright NIWA 2008
Projection: New Zealand Map Grid

Disclaimer: NIWA have prepared this map exercising all reasonable skill and care. Nevertheless, NIWA can give no warranty that the map is free from errors, omissions or other inaccuracies. Users of this map will release NIWA from all liability whether indirect or consequential, arising out of the provision of this map.

Extreme weather events

Natural variations in our climate will continue to impact on the New Zealand climate in the future and will be superimposed on human-induced, long-term climate change trends. Climate change is expected to shift the range of variability and, in some instances, to alter the patterns of variability. It will not remove this natural variability. Thus, the effects of climate change may be felt both through changes in long-term averages and in terms of the changed frequency and intensity of extreme events (such as heavy rainfall, storm surges, drought, or very high temperatures). It is this combination of underlying mean climate, appropriate climate change adjustments, and natural variations that will provide the extremes that future New Zealand society faces. A small shift in the average climate can cause significant changes in the occurrence of extremes: generally, it is the extreme events that cause damage.

An increase in the risk of heavy rainfall events could be particularly important for local government planning. However, because increases in rainfall intensity do not necessarily imply an increase in annual rainfall totals, heavy rainfall events may increase even in areas where the mean rainfall is projected to decrease, because the frequency of storm events may change.

Other climate changes

Changes in temperature and rainfall, along with other climate changes, are likely to lead to more floods as well as more droughts in some parts of the country.
Projections for other climate changes include:

  • decreased frost risk
  • increased frequency of high temperatures
  • increased frequency of extreme daily rainfalls
  • higher snow lines and possible reduced snow coverage
  • possible increase in strong winds
  • an increase in average sea level.

Table 1 qualitatively summarises the main features of the New Zealand climate change projections and contains the best current scientific estimate of the direction and magnitude of change.

Table 1: Main features of New Zealand climate change projections for 2040 and 2090

Climate variable Direction of change Magnitude of change Spatial and seasonal variation
Mean temperature Increase (****) All-scenario average 0.9°C by 2040, 2.1°C by 2090 (**) Least warming in spring season (*)
Daily temperature extremes (frosts, hot days) Fewer cold temperatures and frosts (****), more high-temperature episodes (****) Whole frequency distribution moves right (see 2.2.3 of the source report) See 2.2.3 of the source report
Mean rainfall Varies around country, and with season. Increases in annual mean expected for Tasman, West Coast, Otago, Southland and Chatham Islands; decreases in annual mean in Northland, Auckland, Gisborne and Hawke’s Bay (**) Substantial variation around the country and with season (see 2.2.2 of the source report) Tendency to increase in south and west in the winter and spring (**); tendency to decrease in the western North Island, and increase in Gisborne and Hawke’s Bay, in summer and autumn (*)
Extreme rainfall Heavier and/or more frequent extreme rainfalls (**), especially where mean rainfall increase predicted (***) No change through to halving of heavy rainfall return period by 2040; no change through to fourfold reduction in return period by 2090 (**) Increases in heavy rainfall most likely in areas where mean rainfall is projected to increase (***)
Snow Shortened duration of seasonal snow lying (***), rise in snowline (**), decrease in snowfall events (*)    
Glaciers Continuing long-term reduction in ice volume and glacier length (***)   Reductions delayed for glaciers exposed to increasing westerlies
Wind (average) Increase in the annual mean westerly component of windflow across New Zealand (**) About a 10% increase in annual mean westerly component of flow by 2040 and beyond (*) By 2090, increased mean westerly in winter (>50%) and spring (20%), and decreased westerly in summer and autumn (20%) (*)
Strong winds Increase in severe wind risk possible (**) Up to a 10% increase in the strong winds (>10m/s, top 1 percentile) by 2090 (*)  
Storms More storminess possible, but little information available for New Zealand (*)    
Sea level Increase (****) At least 18–59 cm rise (New Zealand average) between 1990 and 2100 (****) Refer to Coastal Hazards and Climate Change guidance manual: www.mfe.govt.nz/publications/climate/coastal-hazards-climate-change-guidance-manual
Waves Increased frequency of heavy swells in regions exposed to prevailing westerlies (**) Refer to Coastal Hazards and Climate Change guidance manual: www.mfe.govt.nz/publications/climate/coastal-hazards-climate-change-guidance-manual  
Storm surge Assume storm tide elevation will rise at the same rate as mean sea-level rise (**) Refer to Coastal Hazards and Climate Change guidance manual: www.mfe.govt.nz/publications/climate/coastal-hazards-climate-change-guidance-manual  
Ocean currents Various changes plausible, but little research or modelling yet done See 2.2.9 of the source report  
Ocean temperature Increase (****) Similar to increases in mean air temperature Patterns close to the coast will be affected by winds and upwelling and ocean current changes (**)

Note: The degree of confidence placed by the authors of the source report in the projections is indicated by the number of stars in brackets:

**** Very confident, at least 9 out of 10 chance of being correct. Very confident means that it is considered very unlikely that these estimates will be substantially revised as scientific knowledge progresses.
*** Confident.
** Moderate confidence, which means it is more likely than not to be correct in terms of indicated direction and approximate magnitude of the change.
* Low confidence, but the best estimate possible at present from the most recent information. Such estimates could be revised considerably in the future.

For planning purposes, councils should consider the full range of scenarios provided in the source report on which this guide is based. Changes in the return period of heavy rainfall events may vary between different parts of the country and will also depend on the rainfall duration being considered (see Section 2.2.4 of the source report for more detail).

Projected changes by region

Table 2 and Table 3 on the next two pages show the projected changes for temperature and rainfall for specific locations. The first figure in each table entry gives a mid-range ‘working value’ for what the change will be and the figures in brackets give the range across the model results within which the change could lie. The first value is in fact the average of the changes estimated by several computer models and for a variety of possible emissions scenarios, while the range gives the maximum and minimum values across these models and scenarios. Refer to the source report for further detail.

Table 2: Projected changes in seasonal and annual mean temperature (in ˚C) relative to 1990, by regional council area

The first number in each case is a mid-range estimate of what the change will be, the figures in brackets give the model range within which the change could lie.

Region Decade Summer Autumn Winter Spring Annual
Northland 2040 1.1  [ 0.3, 2.7] 1.0 [ 0.2, 2.9] 0.9 [ 0.1, 2.4] 0.8 [ 0.1, 2.2] 0.9 [ 0.2, 2.6]
2090 2.3 [ 0.8, 6.6] 2.1 [ 0.6, 6.0] 2.0 [ 0.5, 5.5] 1.9 [ 0.4, 5.5] 2.1 [ 0.6, 5.9]
Auckland 2040 1.1 [ 0.3, 2.6] 1.0 [ 0.2, 2.8] 0.9 [ 0.2, 2.4] 0.8 [ 0.1, 2.2] 0.9 [ 0.2, 2.5]
2090 2.3 [ 0.8, 6.5] 2.1 [ 0.6, 5.9] 2.0 [ 0.5, 5.5] 1.9 [ 0.4, 5.4] 2.1 [ 0.6, 5.8]
Waikato 2040 1.1 [ 0.2, 2.5] 1.0 [ 0.3, 2.7] 0.9 [ 0.2, 2.2] 0.8 [ 0.0, 2.0] 0.9 [ 0.2, 2.4]
2090 2.3 [ 0.9, 6.3] 2.2 [ 0.6, 5.6] 2.1 [ 0.5, 5.2] 1.8 [ 0.3, 5.1] 2.1 [ 0.6, 5.6]
Bay of Plenty 2040 1.0 [ 0.3, 2.5] 1.0 [ 0.3, 2.7] 0.9 [ 0.1, 2.2] 0.8 [ 0.0, 2.1] 0.9 [ 0.2, 2.4]
2090 2.2 [ 0.8, 6.2] 2.2 [ 0.6, 5.6] 2.0 [ 0.5, 5.2] 1.8 [ 0.3, 5.1] 2.1 [ 0.6, 5.5]
Taranaki 2040 1.1 [ 0.2, 2.4] 1.0 [ 0.2, 2.6] 0.9 [ 0.1, 2.2] 0.8 [ 0.0, 2.0] 0.9 [ 0.2, 2.3]
2090 2.3 [ 0.9, 6.1] 2.2 [ 0.6, 5.3] 2.1 [ 0.5, 5.1] 1.8 [ 0.3, 4.9] 2.1 [ 0.6, 5.3]
Manawatu-Wanganui 2040 1.1 [ 0.2, 2.3] 1.0 [ 0.2, 2.6] 0.9 [ 0.2, 2.2] 0.8 [ 0.0, 1.9] 0.9 [ 0.2, 2.2]
2090 2.3 [ 0.9, 6.0] 2.2 [ 0.6, 5.3] 2.1 [ 0.5, 5.0] 1.8 [ 0.3, 4.9] 2.1 [ 0.6, 5.3]
Hawke’s Bay 2040 1.0 [ 0.2, 2.5] 1.0 [ 0.3, 2.6] 0.9 [ 0.1, 2.2] 0.8 [ 0.0, 2.0] 0.9 [ 0.2, 2.3]
2090 2.1 [ 0.8, 6.0] 2.1 [ 0.6, 5.3] 2.1 [ 0.5, 5.1] 1.9 [ 0.3, 5.1] 2.1 [ 0.6, 5.4]
Gisborne 2040 1.0 [ 0.2, 2.6] 1.0 [ 0.3, 2.7] 0.9 [ 0.1, 2.2] 0.8 [ 0.0, 2.1] 0.9 [ 0.2, 2.4]
2090 2.2 [ 0.8, 6.2] 2.2 [ 0.6, 5.6] 2.0 [ 0.5, 5.2] 1.9 [ 0.3, 5.2] 2.1 [ 0.6, 5.5]
Wellington 2040 1.0 [ 0.2, 2.2] 1.0 [ 0.3, 2.5] 0.9 [ 0.2, 2.1] 0.8 [ 0.1, 1.9] 0.9 [ 0.3, 2.2]
2090 2.2 [ 0.9, 5.7] 2.1 [ 0.6, 5.1] 2.1 [ 0.6, 5.0] 1.8 [ 0.3, 4.8] 2.1 [ 0.6, 5.2]
Tasman-Nelson 2040 1.0 [ 0.2, 2.2] 1.0 [ 0.2, 2.3] 0.9 [ 0.2, 2.0] 0.7 [ 0.1, 1.8] 0.9 [ 0.2, 2.0]
2090 2.2 [ 0.9, 5.6] 2.1 [ 0.6, 5.1] 2.0 [ 0.5, 4.9] 1.7 [ 0.3, 4.6] 2.0 [ 0.6, 5.0]
Marlborough 2040 1.0 [ 0.2, 2.1] 1.0 [ 0.2, 2.4] 0.9 [ 0.2, 2.0] 0.8 [ 0.1, 1.8] 0.9 [ 0.2, 2.1]
2090 2.1 [ 0.9, 5.6] 2.1 [ 0.6, 5.0] 2.1 [ 0.6, 5.0] 1.8 [ 0.3, 4.8] 2.0 [ 0.6, 5.1]
West Coast 2040 1.0 [ 0.2, 2.4] 1.0 [ 0.2, 2.1] 0.9 [ 0.2, 1.8] 0.7 [ 0.1, 1.7] 0.9 [ 0.2, 1.8]
2090 2.2 [ 0.9, 5.3] 2.1 [ 0.7, 5.0] 2.1 [ 0.6, 4.9] 1.7 [ 0.4, 4.5] 2.0 [ 0.7, 4.9]
Canterbury 2040 0.9 [ 0.1, 2.2] 0.9 [ 0.2, 2.2] 1.0 [ 0.4, 2.0] 0.8 [ 0.2, 1.8] 0.9 [ 0.2, 1.9]
2090 2.1 [ 0.8, 5.2] 2.1 [ 0.7, 4.9] 2.2 [ 0.8, 5.1] 1.8 [ 0.4, 4.7] 2.0 [ 0.7, 5.0]
Otago 2040 0.9 [ 0.0, 2.4] 0.9 [ 0.1, 1.9] 1.0 [ 0.3, 2.1] 0.7 [ 0.0, 1.8] 0.9 [ 0.1, 1.9]
2090 2.0 [ 0.7, 4.8] 2.0 [ 0.8, 4.6] 2.2 [ 0.8, 4.8] 1.7 [ 0.5, 4.3] 2.0 [ 0.8, 4.6]
Southland 2040 0.9 [ 0.0, 2.4] 0.9 [ 0.1, 1.9] 0.9 [ 0.2, 2.0] 0.7 [–0.1, 1.7]  0.8 [ 0.1, 1.9]
2090 2.0 [ 0.7, 4.7] 2.0 [ 0.8, 4.6] 2.1 [ 0.8, 4.7] 1.6 [ 0.5, 4.1]  1.9 [ 0.8, 4.5]
Chatham Islands 2040 0.8 [ 0.2, 1.9] 0.9 [ 0.2, 2.0] 0.9 [ 0.1, 2.3] 0.7 [ 0.1, 1.8] 0.8 [ 0.2, 1.9]
2090 1.9 [ 0.8, 4.6] 2.1 [ 0.6, 4.9] 2.0 [ 0.3, 4.5] 1.8 [ 0.3, 4.6] 2.0 [ 0.5, 4.7]

 

Table 3: Projected changes in seasonal and annual precipitation (in %) relative to 1990 for selected stations within each regional council area.

The first number in each case is a mid-range estimate of what the change will be, the figures in brackets give the model range within which the change could lie.

Region Location Decade Summer Autumn Winter Spring Annual
Northland Kaitaia 2040 1 [−15, 20] −0 [−14, 16] −5 [−23, 1] −6 [–18, 4] −3 [−13, 5]
2090 −1 [−26, 21] −3 [−22, 11] −8 [−32, 2] −11 [−33, 8] −6 [−22, 5]
Whangarei 2040 1 [−14, 23] 1 [−15, 33] −9 [−38, −1] −9 [−25, 3] −4 [−16, 7]
2090 0 [−20, 19] 1 [−27, 26] −12 [−45, −0] −16 [−45, 1] −7 [−28, 2]
Auckland Warkworth 2040 1 [−16, 20] 1 [−13, 22] −4 [−22, 2] −6 [−18, 6] −3 [−13, 5]
2090 −2 [−31, 20] −1 [−20, 12] −4 [−24, 5] −12 [−33, 6] −5 [−19, 6]
Mangere 2040 1 [−17, 20] 1 [−14, 17] −1 [−10, 5] −5 [−15, 10] −1 [−10, 6]
2090 −1 [−33, 20] −2 [−21, 12] −1 [−12, 9] −9 [−30, 11] −3 [−13, 9]
Waikato Ruakura 2040 1 [−18, 19] 2 [−13, 10] 1 [−4, 8] −2 [−10, 13] 0 [−6, 6]
2090 −1 [−34, 18] −1 [−24, 10] 3 [−7, 15] −4 [−23, 16] −1 [−11, 11]
Taupo 2040 3 [−16, 28] 3 [−9, 16] 1 [−4, 7] −3 [−10, 12] 1 [−5, 8]
2090 4 [−19, 30] 1 [−16, 9] 3 [−8, 15] −5 [−23, 13] 1 [−7, 10]
Bay of Plenty Tauranga 2040 2 [−16, 25] 3 [−12, 25] −4 [−16, 2] −5 [−18, 7] −1 [−10, 8]
2090 2 [−20, 23] 2 [−15, 16] −3 [−16, 8] −9 [−32, 12] −2 [−12, 5]
Taranaki New Plymouth 2040 0 [−20, 18] 3 [−8, 13] 2 [−2, 9] 0 [−8, 16] 2 [−3, 9]
2090 −2 [−38, 15] 1 [−18, 15] 6 [−6, 20] −1 [−17, 21] 1 [−10, 11]
Manawatu-Wanganui Wanganui 2040 −1 [−21, 13] 3 [−8, 10] 5 [−3, 15] 1 [−10, 15] 2 [−3, 10]
2090 −3 [−42, 12] −1 [−20, 12] 8 [−5, 25] 0 [−16, 23] 1 [−11, 11]
Taumarunui 2040 0 [−19, 19] 2 [−10, 13] 7 [0, 17] 2 [−12, 19] 3 [0, 13]
2090 −1 [−36, 18] −2 [−25, 12] 13 [1, 36] 1 [−16, 26] 3 [−7, 15]
Hawke’s Bay Napier 2040 4 [−33, 38] 5 [−14, 42] −13 [−34, −1] −7 [−17, 3] −3 [−14, 14]
2090 9 [−46, 52] 5 [−14, 25] −16 [−45, −1] −13 [−38, 9] −4 [−20, 11]
Gisborne Gisborne 2040 3 [−26, 33] 4 [−18, 46] −11 [−30, −2] −9 [−21, 3] −4 [−15, 14]
2090 5 [−38, 41] 4 [−25, 27] −13 [−41, 1] −16 [−42, 7] −5 [−22, 8]
Wellington Masterton 2040 2 [−17, 25] 4 [−8, 32] −6 [−20, 4] −1 [−8, 10] −1 [−7, 9]
2090 4 [−28, 32] 3 [−7, 13] −7 [−28, 2] −4 [−20, 16] −2 [−15, 7]
Paraparaumu 2040 0 [−21, 13] 4 [−3, 14] 4 [−1, 13] 2 [−5, 14] 2 [−3, 10]
2090 −1 [−38, 16] 2 [−12, 14] 9 [0, 26] 2 [−15, 26] 3 [−7, 14]
Tasman-Nelson Nelson 2040 4 [−14, 27] 5 [−2, 19] 1 [−4, 9] 0 [−8, 9] 2 [−3, 9]
2090 6 [−13, 30] 5 [−4, 18] 6 [−2, 19] −1 [−20, 19] 4 [−3, 14]
Marlborough Blenheim 2040 3 [−16, 25] 4 [−4, 24] −1 [−10, 7] −1 [−7, 10] 1 [−5, 9]
2090 5 [−15, 28] 5 [−5, 16] 1 [−14, 9] −1 [−18, 20] 2 [−7, 13]
West Coast Hokitika 2040 0 [−22, 19] 3 [−11, 18] 11 [1, 24] 5 [−1, 18] 5 [−2, 20]
2090 −1 [−44, 32] 3 [−28, 26] 21 [5, 52] 8 [−11, 46] 8 [−5, 31]
Canterbury Christchurch 2040 2 [−15, 22] 5 [−10, 30] −8 [−30, 7] −1 [−8, 9] −1 [−10, 9]
2090 3 [−17, 25] 6 [−6, 20] −11 [−41, 10] −2 [−15, 25] −2 [−14, 16]
Hanmer 2040 2 [−16, 25] 4 [−5, 19] −7 [−26, 6] 0 [−6, 12] −1 [−8, 7]
2090 4 [−25, 32] 3 [−7, 15] −10 [−34, 6] −1 [−13, 29] −2 [−14, 15]
Tekapo 2040 1 [−16, 16] 2 [−12, 10] 8 [−1, 19] 6 [−3, 17] 4 [0, 13]
2090 2 [−30, 31] 0 [−16, 17] 18 [5, 41] 10 [−6, 47] 8 [0, 29]
Otago Dunedin 2040 1 [−11, 13] 2 [−9, 10] 3 [−10, 13] 2 [−5, 11] 2 [−4, 9]
2090 0 [−29, 19] 2 [−11, 16] 7 [−16, 24] 6 [−1, 32] 4 [−9, 23]
Queenstown 2040 1 [−16, 20] 2 [−15, 23] 16 [2, 38] 8 [−3, 21] 7 [1, 22]
2090 1 [−38, 37] 2 [−32, 20] 29 [ 7, 76] 15 [−5, 50] 12 [−2, 34]
Southland Invercargill 2040 −1 [−15, 22] 2 [−17, 22] 10 [2, 30] 7 [−3, 22] 4 [−2, 19]
2090 −2 [−44, 27] 2 [−31, 19] 18 [1, 51] 13 [0, 47] 7 [−12, 29]
Chatham Islands   2040 −2 [−10, 10] 4 [−7, 29] 4 [−10, 43] 3 [−8, 19] 3 [−5, 23]
2090 −3 [−20, 16] 4 [−14, 29] 8 [−16, 67] 6 [−14, 45] 4 [−11, 35]

Current climate variability

New Zealand’s climate varies naturally from year to year and from decade to decade. Human-induced long-term trends will be superimposed on these natural variations, and it is this combination that will provide the future climate extremes to which New Zealand society will be exposed. Much of the natural variation is apparently random; however there are some cyclical elements as shown for temperature (Figure 4 ).

Figure 4 shows that, in individual years, annual New Zealand-wide temperatures can deviate from the long-term average by up to 1°C (plus or minus). It also shows that, despite these fluctuations, there has been a long-term increase of about 0.9°C between 1908 and 2006. Similarly, annual rainfall can deviate from its long-term average, by about plus or minus 20 per cent.

Figure 4: New Zealand average temperature

Annual mean temperatures are shown by the red (positive) and blue (negative) bars with the long-term trend shown by the solid black rising line (shown as difference in °C from 1980–99 climatology).

 

Figure 4: New Zealand average temperature

This figure displays the year-to-year variability of the New Zealand national-average temperature for the period from 1908 to 2006. Data are shown as departures from the 1980-1999 climatological period.

Superimposed on these departures is a smoothed line to indicate the long-term trend. There is a steady increase in the New Zealand temperature series since 1908 of nearly one tenth of a degree Celsius per decade. However year-to-year changes in temperatures can be substantially larger than the 1908-2006 trend, with fluctuations up to plus or minus one degree Celsius about the long-term average.

Some of the shortest-term temperature fluctuations arise simply because of the natural variability in the weather and its random fluctuations or ‘chaos’. Another factor of New Zealand’s climate in the recent past is the effect of large volcanic eruptions in the tropics. Typically, they have lead to a cooling of 0.6–0.8°C for a year or more. For example, the impact of the May 1991 Mt Pinatubo eruption is clearly evident in the New Zealand temperature record during 1992 (Figure 4).

Other changes in our climate are associated with large-scale climate patterns over the Southern Hemisphere or the Pacific Ocean. There are a number of key natural processes that operate over timescales of seasons to decades, particularly the El Niño-Southern Oscillation (ENSO) and the Interdecadal Pacific Oscillation (IPO).

ENSO is a tropical, Pacific-wide oscillation that affects pressure, winds, sea-surface temperature and rainfall. In the El Niño phase, New Zealand usually experiences stronger than normal south-westerly airflow. This generally results in lower seasonal temperatures nationally with drier conditions in the north-east of the country. The La Niña phase is essentially the opposite, usually creating more north-easterly flows, higher temperatures, and wetter conditions in the north and east of the North Island. Atmospheric pressures in the La Niña phase tend to be higher than normal over the South Island which can lead to lower rainfall and, in turn, drought conditions in the north-east of the South Island. Drought can, therefore, occur in New Zealand in both El Niño and La Niña phases. Because the tropical Pacific sea-surface temperature anomalies persist for up to a year, there is substantial predictability in how ENSO oscillations affect New Zealand’s climate. The ENSO cycle varies between about three and seven years, with large variability in the intensity of individual oscillations. The Interdecadal Pacific Oscillation is another, recently identified, source of natural variability in climate. It has cycles that can last over several decades rather than across century timescales. Three phases of the IPO have been identified during the 20th century: a positive phase (1922–44), a negative phase (1946–77), and another positive phase (1978–98).

In a positive phase, sea-surface temperatures around New Zealand tended to be lower, and westerly or south-westerly winds stronger. Temperatures in all regions were lower. In the negative phase, airflows from the east and north-east were observed to increase, as did temperatures in all regions. Conditions became wetter in the north of the North Island, particularly in autumn, and drier in the south-east of the South Island, particularly in summer. The increase in New Zealand temperatures around 1950 (see Figure 4) coincided with the change from positive to negative phase IPO at that time. Sea levels around New Zealand are also affected by the IPO. Particularly large increases in sea level have been associated with IPO transitions from the positive to the negative phase.

It is yet uncertain to what extent the IPO can be accurately used to predict climate phases or patterns in future decades. More frequent natural climate variations, such as the ENSO, are also difficult to predict very far in advance. Hence it is not feasible to project natural variations over the next century. Analysis of the latest sea temperature data suggests that another negative IPO phase may currently prevail – meaning more La Niña (and less El Niño) activity could be expected, compared to the 1978–98 period, together with a period of higher temperatures. Weaker westerlies are also likely, which goes against the observed trend of increasing westerlies.

Significance of climate change versus natural climate variability

Historical records show the national-average temperature can vary by up to about 1°C from year to year, and more than this on a seasonal timescale. Thus, the warmest individual years in the current climate have temperatures lying near the upper end of the projected average (climatological) warming for the 2030s. What currently is an unusually warm year could be the norm in 30–50 years, while an unusually warm year in 30–50 years’ time is very likely to be warmer than anything we experience at present.

Projected temperatures for the mid-to-high range of the end of the century are well outside the values experienced by New Zealand in the 20th century. A hot year in 2090 will be more extreme still.

Similar comparisons can be made for rainfall, but with opposite trends expected for different parts of the country. Seasonal anomalies today seem comparable to the projected average ranges for 2030. Areas that currently have water management issues could see present extremes (eg, water shortages) become the norm by the 2030s – depending on the direction of projected rainfall change for their region, and which emissions scenario and model simulation turns out to be the closest to reality. This will be particularly relevant for drought management in eastern New Zealand.

Further details of the estimated changes in both the average and extremes for each major climate parameter are found in Chapter 2 of the source report.