Climate change scenarios for New Zealand
On this page
- What is a climate change scenario?
- Downscaling to New Zealand
- New Zealand regional climate change scenarios
- Changes in extremes
- Earlier New Zealand regional scenarios
- Sea level
Click on the links below for a summary report detailing climate change projections for New Zealand, and a summary of the likely impacts. This material, unless otherwise stated, is based upon the IPCC 4th Assessment report.
Click change: Projections for New Zealand (PDF 705 KB)
IPCC Fifth Assessment - Impacts: New Zealand (PDF 1090 KB)
What is a climate change scenario?
Predicting human-induced ("anthropogenic") changes in climate, over the next 100 years, for a particular part of New Zealand requires:
- A prediction of global greenhouse gas and aerosol emissions for the next century
- A global carbon cycle model to convert these emissions into changes in carbon dioxide concentrations (and similar models for calculating concentrations of other greenhouse gases and aerosols)
- A coupled atmosphere-ocean global circulation model (AOGCM) which uses the greenhouse gas and aerosol concentration information to predict climate variations forward in time.
- Downscaling of the AOGCM results through a procedure which takes account of the influence of New Zealand’s topography on local climate. This can be done either statistically or with a high resolution regional climate model.
Given our current knowledge and modelling technology, there are uncertainties in each of these steps. For example, emission predictions depend on the difficult task of predicting human behaviour, such as changes in population, economic growth, technology, energy availability and national and international policies, including predicting the results of international negotiations on constraining greenhouse gas emissions. Our understanding of the carbon cycle and of sources and sinks of non-carbon dioxide greenhouse gases is still incomplete. As discussed in NIWA’s climate modelling web page, there are significant uncertainties in current global climate model predictions – particularly at the regional level.
The climate change scenario approach recognises these uncertainties. A scenario is a scientifically – based projection of one plausible future climate for a region. For guidance on regional impacts of climate change, a range of scenarios is desirable. These can span credible estimates of future greenhouse gas emissions, and the uncertainty range in climate model predictions.
The Intergovernmental Panel on Climate Change developed 40 different emissions pathways or ‘scenarios’ in its Special Report on Emission Scenarios (Nakicenovic & Swart, 2000). These SRES scenarios cover a range of demographic, societal, economic, and technical-change "storylines" and formed the basis for much of the climate projection work done for the IPCC’s Third and Fourth Assessments. The SRES scenarios cover the key greenhouse gases (carbon dioxide, methane, nitrous oxide and CFCs) and the sulphate aerosols. They do not include specific initiatives to control greenhouse gas emissions, such as the Kyoto Protocol, but some of them (e.g. the B1 scenario) assume a reduction in world population after a mid-century peak, and the rapid and widespread introduction of clean and resource-efficient technologies. The SRES scenarios also do not account for any unexpected climate ‘surprises’, such as increased methane emissions from permafrost melting or undersea methane clathrates. Full AOGCMS were run for only some of these scenarios. A simpler globally-averaged model was then "tuned" to these AOGCM runs and applied to all 40 SRES scenarios.
Figure 1 indicates a range of possible future global temperatures, which reflect the range of plausible emissions scenarios and the range of GCM predictions for given scenarios. The IPCC does not contend that any one SRES scenario is more likely than any other – it is as if they have provided a dice for predicting future conditions with 40 equally weighted sides. However, the scenario labelled “A1B”, which gives an intermediate level of warming by the end of the century, has more GCM output data available than any other scenario, and is therefore the scenario used to derive most of the projections for New Zealand. New Zealand projections from the A1B scenario were rescaled to cover the full spread across all the IPCC emission scenarios (the vertical grey bars in Fig. 1).
Note: Solid lines are multi-model global averages of surface warming (relative to 1980-1999) for scenarios B1, A1B and A2, shown as continuations of the 20th century simulations. The coloured shading denotes the ±1 standard deviation range of individual model annual averages. The grey bars at right indicate the best estimate (solid horizontal line within each grey bar) and the ‘likely range’ for all 6 SRES illustrative scenarios. [Source: Figure SPM-5, IPCC Summary for Policymakers, IPCC 2007].
The IPCC made subtle changes between the Third and Fourth Assessments in the way they expressed the climate projections. The Third Assessment stated: “The globally averaged surface temperature is projected to increase by 1.4 to 5.8°C over the period 1990 to 2100.” These results are for the full range of 40 SRES scenarios, based on a number of climate models. In the Fourth Assessment, the projections were expressed as changes between 1980-1999 and 2080-2099, and projections were given separately for 6 illustrative scenarios, which spanned the range of the full 40 SRES scenarios. For each of the 6 scenarios, a best estimate was provided, as well as the likely range. The full range in global temperature increase over the 6 illustrative scenarios of the Fourth Assessment was 1.1 to 6.4°C.
To identify likely future climate changes across New Zealand, projected changes from global climate models are statistically downscaled. This is a method for building in local spatial detail from information at the much coarser-scale available from GCMs. Historical observations are used to develop regression equations that relate local climate fluctuations to changes at the larger-scale. These historical observations are then replaced by the model changes in the regression equations to produce the fine-scale projections. Downscaled changes were prepared on a 0.05° grid (approximately 5km by 4km) covering New Zealand.
The New Zealand downscaled projections follow the IPCC Fourth Assessment approach. That is, changes are relative to 1980-1999, which we abbreviate as “1990” for convenience. Changes are calculated for the two future periods 2030-2049 (“2040” for short) and 2080-2099 (“2090”). Thus, the New Zealand projections are 50 and 100 year changes, between the baseline climate (centred on 1990) and future periods centred on 2040 and 2090. Figure 2 provides a schematic for the time horizons of the climate projections.
Note: Curve (blue line) shows smoothly varying climate parameter such as temperature or sea level, relative to a base level defined as the average over 1980-1999 (first horizontal red line), but denoted as “1990”. Future 20-year averages indicated by the other red lines at 2040 (2030-2049 average) and 2090 (2080-2099). Dotted orange lines show projection horizons used in the previous Guidance Manual (MfE, 2004), identified as the 2030s (2020-2049 average) and the 2080s (2070-2099 average). [Source: Figure 2.2, 2008 Guidance Manual, MfE, 2008]
Downscaling is applied to the projections of 12 global climate models, all forced by the A1B middle-of-the-road emissions scenario (Fig. 1). A range of possible values for each climate variable (temperature, rainfall, etc) is provided. This reflects not only the range of greenhouse gas futures represented by the 6 SRES illustrative scenarios, but also the range of climate model predictions for individual emission scenarios. The other 5 SRES emissions scenarios are catered for by re-scaling the A1B results for New Zealand according to the ratio of global temperature increases, as documented in the IPCC Fourth Assessment.
NIWA scientists followed the downscaling approach used above to prepare New Zealand climate change scenarios for the 2040 and 2090, for the "Guidance Manual" for local government organizations. This has been published on the web by the Ministry for the Environment and is an update of the 2004 "Guidance Manual" (Ministry for the Environment, 2008). Like the IPCC, we are unable to indicate whether any one emission scenario is more likely than another, but do provide the average across all models and all emission scenarios. The extreme ends of the ranges may be slightly less likely than the central values, since they generally result from the one climate model which gives the most extreme projection, rather than reflecting the consensus from a number of models. Eliminating the most extreme models as outliers causes little change to the average over the remaining models, but can on occasion greatly reduce the range of the projected changes.
Readers of this web page are referred to the Guidance Manual for full details, including tables showing projected ranges for temperature and rainfall changes in various parts of the country and for different seasons. Table 1 qualitatively summarises the main features of these New Zealand climate projections. All estimates in Table 1 represent the best current scientific estimate of the direction and magnitude of change. The degree of confidence placed by NIWA scientists on 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.
Hence, a higher degree of caution should be employed where investment decisions are based on the ‘low’ confidence estimates.
Table 1: Main features of New Zealand climate change projections for 2040 and 2090 (Ministry for the Environment, 2008).
|Climate variable||Direction of change||Magnituded 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 Ministry for the Environment, 2008)||See Ministry for the Environment, 2008|
|Mean rainfall||Varies around country, and with season. Increases in annual mean expected for Tasman, West Coast, Otago, Southland and Chathams; decreases in annual mean in Northland, Auckland, Gisborne and Hawke’s Bay (**)||Substantial variation around the country and with season (see Ministry for the Environment, 2008)||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 (**) [See note 2]||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 (****)||See Coastal Guidance Manual|
|Waves||Increased frequency of heavy swells in regions exposed to prevailing westerlies (**)||See Coastal Guidance Manual|
|Storm surge||Assume storm tide elevation will rise at the same rate as mean sea-level rise (**)||See Coastal Guidance Manual|
|Ocean currents||Various changes plausible, but little research or modelling yet done||See Ministry for the Environment, 2008|
|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 2: 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.
Downscaled projections of mean temperature changes over New Zealand are shown in Figure 3 (annual-average changes).
Averaging over all models and all 6 illustrative emissions scenarios gives a New Zealand-average warming of 0.2–2.0°C by 2040 and 0.7–5.1°C by 2090. For just the A1B scenario alone, the projected warming is 0.3–1.4°C by 2040 and 1.1–3.4°C by 2090, with a 12-model average (or “best estimate”) of 0.9°C and 2.1°C for 2040 and 2090 respectively. For comparison, the IPCC quotes a best estimate of 2.8°C for the global temperature increase by 2090 under the A1B scenario, with a likely range of 1.7–4.4°C. The projected New Zealand temperature changes are in all cases smaller than the globally averaged changes for the corresponding SRES scenarios.
The annual-average pattern of warming shown in Figure 3 is fairly uniform over the country, although slightly greater over the North Island than the South. Also, the warming accelerates with time under this emissions scenario: i.e., the 2090 warming is more than twice the 2040 warming. In the summer and autumn seasons, the North Island and northwest of the South Island show the greatest warming, whereas in the winter season the South Island has the greatest warming. Spring shows the least warming of all seasons. For more information please refer to the Local Government Guidance Manual (Ministry for the Environment, 2008).
Some of the most significant environmental, economic and social effects of climate change might be caused by changes in climate extremes (for example floods, droughts, frosts, strong winds, tropical cyclones and storm surges), rather than just changes in mean climate conditions. Unfortunately, estimates of likely changes in regional climate extremes are generally even less certain than projections of likely changes in mean conditions. We have summarised current understanding below: The reader is referred to the Local Government Guidance Manual (Ministry for the Environment, 2008) below for more details.
Daily Temperature Extremes, Frosts
In addition to changes in mean temperature, daily temperature extremes will also vary with regional warming. A large decrease in the number of frost days is projected for the central North Island and in the South Island as the 21st Century progresses. (Because the far north of New Zealand already receives few frosts, the frost frequency there cannot decrease substantially). An increase in the number of days above 25°C is also expected, particularly at already warm northern locations.
A warmer atmosphere can hold more moisture (about 8% more for every 1°C increase in temperature), so the potential for heavier rainfall certainly exists. The IPCC in its Fourth Assessment declared that more intense rainfall events are "very likely over most areas". The mountainous nature of New Zealand, with its starkly contrasting rainfall climates, makes it difficult to be sure if this situation is universally applicable across the country. Any change in the mix of circulation patterns will have a major impact on the spatial distribution of precipitation.
Various recent model simulations confirm the likelihood that heavy rainfall events will occur more frequently in New Zealand over the coming century, but the likely size of this change remains uncertain. In broad terms, what is an extreme rainfall in the current climate might occur about twice as often by the end of the 21st Century under a mid-range climate change scenario. For the high temperature change scenario, an extreme rainfall in the current climate might occur 3 to 4 times as often by the end of the 21st Century.
Preliminary analysis based on the NIWA regional model runs under the B2 and A2 emission scenarios has shown that for extremes with return periods of 30 years and longer, the increase in rainfall depth was approximately 8% per degree of local warming, when averaged over the entire country. This figure (8% per °C) matches the increased moisture content of the atmosphere with warming, and is the value widely accepted as a reasonable upper limit for heavy rainfall changes, provided the circulation patterns remain essentially the same. However, changes in extreme rainfall were not geographically uniform and therefore regional comparisons cannot be made. It may be possible in future to have regionally varying changes, but this will require ongoing investigation and comprehensive modelling studies.
Increased rainfall intensity has obvious implications for increased flooding. The IPCC Fourth Assessment summarises a number of international studies that analyse the increased risk of floods in a future warmer climate.
After the Local Government Guidance Manual had been completed, NIWA undertook some specific research on how the frequency of drought might change over the coming Century. A resulting report for the Ministry for the Environment, and the Ministry of Agriculture and Forestry is available on the web (Mullan et al, 2005). The study developed drought risk projections for a range of climate change scenarios, corresponding to approximately the middle 75% of the IPCC global temperature change projection range. Under both the "low-medium" and the "medium-high" scenarios (which bracketed this 75% range), the drought risk was projected to increase in frequency during the coming century for all areas that are currently drought prone.
Changes in drought risk with climate change (PDF 2.3 MB)
Under the "low-medium" scenario, by the 2080s severe droughts (defined for the study as the current one-in-twenty year drought) are projected to occur at least twice as often as currently in the following areas: inland and northern parts of Otago; eastern parts of Canterbury and Marlborough; parts of Hawkes Bay; parts of the Bay of Plenty; and parts of Northland. Under the "medium-high" scenario severe droughts are projected to occur more than four times as often by the 2080s in the following regions: eastern parts of North Otago, Canterbury and Marlborough; much or the Wairarapa, Bay of Plenty and Coromandel; most of Gisborne; much of Northland.
Snowfall and Snowline
It is physically plausible that snow cover will decrease and snowlines rise as the climate warms. However, there are confounding issues. Warmer air holds more moisture, and during winter this moisture could be precipitated as snow at high altitudes. There could also be increased winter snowfall to low elevations in some cases on an event by event basis, for the same reason. However, with the expected increase in temperatures, any snow cover will melt more quickly, and thus the duration of seasonal snow lying on the ground should be shortened.
Global climate models suggest that for mid-range temperature change projections the mean westerly wind component across New Zealand will increase by approximately 10% of its current value in the next 50 years (Mullan et al, 2001 b). The implications of this for strong winds are uncertain – one suggestion (Wratt et al., 2003) is that over the sea or flat land the frequency of occurrence of winds of 30 m/s or above might double by 2080.
A strong seasonality is apparent in the wind changes from the IPCC Fourth Assessment models, with increased westerly in the winter and spring seasons and decreased westerly in summer and autumn. In spring the mean westerly flow increases by about 10% by 2040 and 20% by 2090. Winter westerlies increase even more, but there are projected decreases of 5 to 20% in the summer and autumn westerlies, in the average over all models and scenarios. There is clearly still a substantial uncertainty about the projected future wind changes, as evidenced by the wide range across the climate models. Only in winter do all the models project increasing westerly flow across New Zealand.
Ex-Tropical Cyclones and Mid-Latitude Storms
The IPCC Fourth Assessment Report concludes that it is likely that future tropical cyclones will become more intense, with larger peak wind speeds and more heavy precipitation. This is another area of considerable uncertainty. Current knowledge suggests the most likely future outcomes over the coming century for New Zealand are that ex-tropical cyclones might be slightly less likely to reach New Zealand, but if they do their impact might be greater. The intensity or frequency of mid-latitude storms might also increase somewhat in our region. However, our level of confidence in these projections is low.
The rise of sea level around New Zealand is likely to be similar to the global projections of sea-level rise by the IPCC Fourth Assessment Report. This is based on similarities between the New Zealand average and the global average over last century of around 1.8 mm/year. Sea-level rise will continue for several centuries even if greenhouse gas emissions are reduced.
Using the same approach as for global temperature change, IPCC projects that mean sea level will rise by at least 18 to 59 cm between 1990 (1980-1999 average) and the 2090s (2090-2099 average), taking the full range of SRES scenarios into account. A further 10 to 20 cm rise would occur if melt rates of Greenland and Antarctica were to increase above current levels with future temperature increases. The IPCC notes that even larger sea level rises cannot be excluded, but no consensus was possible because of limited understanding of the processes involved. The projected increase in westerlies may also influence the ocean wave climate that impacts on New Zealand. In particular, coastal regions exposed to the prevailing winds may be subject to an increase in the frequency of heavy swells that would add to effects of higher sea levels. More information about sea level change, and its likely impacts and response options, is provided in the companion Coastal Hazards Guidance Manual (Ministry for the Environment (2004b)).
In considering future climate changes and their impacts it is important to remember that we will continue to experience natural year to year variations in climate and in the frequency of extremes. Over the coming century New Zealand will have to deal with the combination of underlying mean climate, changes due to global warming, and natural variations. More information, including advice, climate change scenario maps and GIS layers, and some simulated daily climate data sets future climate change scenarios suitable for use in models (e.g. grass growth models, stormwater drainage system models), is available from the National Climate Centre.
IPCC. 2007. Summary for Policymakers. In: Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Solomon, S., Qin, D., Manning, M., Chen, Z. Marquis, M., Averyt, K.B., Tignor, M. and Miller, H.L. (Eds.), Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA. Available at http://www.ipcc.ch/ipccreports/ar4-wg1.htm
Ministry for the Environment (2004b). Coastal Hazards and Climate Change: a guidance manual for local government in New Zealand. Prepared by R. Bell, T. Hume, D. King, D. Ray, D. Lyon, S. Taylor, D. Papps, A. Linzey, N.Beattie, D. Todd and S. Marx. MfE Publication ME512. May 2004. www.mfe.govt.nz/publications/climate/coastal-hazards-climate-change-guidance-manual/
Ministry for the Environment (2008). Climate Change Effects and Impacts Assessment. A Guidance Manual for Local Government in New Zealand. 2nd Edition. Prepared by Mullan, B; Wratt, D; Dean, S; Hollis, M. (NIWA); Allan, S; Williams, T. (MWH NZ Ltd), and Kenny, G. (Earthwise Consulting Ltd), in consultation with Ministry for the Environment. NIWA Client Report WLG2007/62, February 2008, 156p.
Mullan, A.B.; Wratt, D.S.; Renwick, J.A. (2001). Transient model scenarios of climate changes for New Zealand. Weather and Climate 21: 3-43.
Mullan, A.B.; Bowen, M.; Chiswell, S. (2001b). The crystal ball: Model predictions of future climate. Water and Atmosphere 9, 10-11.
Mullan, A.B.; Porteous, A.; Wratt, D.S.; Hollis, M. (2005). Changes in Drought Risk with Climate Change. NIWA Client Report WLG2005-23, prepared for the Climate Change Office, Ministry for the Environment, and the Ministry of Agriculture and Forestry. 56p. www.mfe.govt.nz/publications/climate/coastal-hazards-climate-change-guidance-manual/
Nakicenovic, N.; Swart, R.; (2000). Emissions scenarios. Special report of the Intergovernmental Panel on Climate Change. Cambridge University Press. 570 p.
Renwick, J.A.; Katzfey, J.J.; Nguyen, K.C.; McGregor, J.L. (1998). Regional model simulations of New Zealand climate. Journal of Geophysical Research 103: 5973-5982.
Renwick, J.A.; Katzfey, J.J.; McGregor, J.L.; Nguyen, K.C. (1999). On regional model simulations of climate change over New Zealand. Weather and Climate 19: 3-14.
Wratt, D.S, Ridley, R.N.; Sinclair, M.R.; Larsen, H.; Thompson, S.M.; Henderson, R.; Austin, G.L.; Bradley, S.G.; Auer, A.; Sturman, A.P.; Owens, I.; Fitzharris, B.; Ryan, B.F.; Geyet, J.F. (1996). The New Zealand Southern Alps experiment. Bulletin of the American. Meteorological Society 77: 683-692.
Prepared by David Wratt & Brett Mullan