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Chapter 2: Projections of Future New Zealand Climate Change
2.1 Introduction
This chapter outlines the changes in New Zealand's climate that are expected to result from global human-induced emissions of greenhouse gases and aerosols. These projections build on local work [Mullan et al (2001a) and the CLIMPACTS group (Mullan et al 2001b).] on downscaling results from Global Climate Models (GCMs) to New Zealand. The starting point is the expected global change in atmospheric composition and climate outlined in the Third Assessment Report of the IPCC. [Chapter 9, Cubasch et al 2001.] Appendix 2 provides technical details on how the New Zealand projections were developed, and Appendix 3 gives further regional information. Chapter 3 discusses climate changes that can occur as a result of natural variability.
If you are seeking projections of a particular climate element for use in an impacts assessment you may wish to start from Tables 5.1 and 5.2 of Chapter 5. Those tables provide guidance on values for use in scenario analyses, and refer users back to particular parts of the current chapter.
Climate involves a distribution of values, and not simply long-term averages. Indeed, it is recognised that the biggest impacts of climate change will probably be felt through changes in extremes. Changes in extremes cannot be derived directly from GCM outputs, owing to the limited spatial resolution of the models, and the enormous size of data sets on the daily (or even sub-daily) timescale. However, as illustrated in Box 2.1, small changes in average values (for example, in average annual temperature) can result in large changes in the frequency with which climate extremes occur (for example, for frosts and very high temperatures, and similarly for heavy rainfall, floods and drought). Projections of changes in the average value of a climate element can also help us estimate how the frequency of extremes may change, although this may require additional assumptions about the shape of the distribution.
2.1.1 Global climate scenarios
Predictions of future climate depend on projections of future concentrations of greenhouse gases and aerosols. These depend on projections of emissions, which depend on changes in population, economic growth, technology, energy availability and national and international policies. The IPCC developed 35 different future emissions pathways or 'scenarios' [Nakicenovic and Swart 2000.] as a basis for projecting future climate changes. These SRES scenarios formed the basis of much of the climate projection work done for the IPCC's Third Assessment. [Cubasch et al, op cit.] As explained in Appendix 2, complex atmosphere-ocean global climate models (AOGCMS) were run on supercomputers to simulate future global and regional climate for a sub-range of these scenarios. A much simpler globally-averaged model was then 'tuned' to these AOGCM runs, and applied to all 35 SRES scenarios, leading to the global temperature projections shown in Figure 2.1.
Figure 2.1 indicates a range of possible future global temperatures, which reflect the range of plausible emissions scenarios and the range of AOGCM predictions for given scenarios. [There are differences between the predictions from individual climate models.] All the SRES scenarios project ongoing increases in the atmospheric concentration of greenhouse gases over the coming century (even those scenarios where the emissions start to decrease at some point before 2100). The projected global temperature increases from all scenarios over the next 50 to 100 years are much larger than those that have occurred over the past 1000 years. 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 35 equally weighted sides.
Box 2.1: Small changes in average conditions can lead to large changes in the frequency with which extremes occur
Local impacts of climate are likely to depend more on changes in the frequency of extreme events (such as heavy rainfall, drought, or very high temperatures) than on changes in average conditions. However, these two aspects of climate - averages and extremes - are closely connected. The figure below is a simplified illustration of how a small change in average conditions can lead to a large change in the frequency with which extremes occur. (In the real world the curves will not be so smooth or symmetrical).
Suppose the dashed line represents the current frequency of hourly temperatures over a year, and the heavy line a possible future distribution. The shaded area under a curve represents how often temperatures occur above a particular threshold (orange, red) or below a threshold (blue).
Suppose that in this case the cold area (blue) represents hourly temperatures below freezing, the orange area represents temperatures above 30°C, the red area represents temperatures above 35°C, and the change in mean (average) temperature shown by the arrow is 3°C. So in this case, a modest change in average temperature is accompanied by a total cessation of frosts, occurrence of higher temperatures than hitherto experienced (red area) and a substantial increased frequency of temperatures above 30°C.
This has important implications for adaptation (as noted by Warrick 2002). For example a particular farming operation might already be well adapted to temperatures ranging between -2°C and 30°C, and able to cope occasionally with temperatures between 30°C and 35°C. While the changes in the mean temperature lie well within the 'autonomous' (easily coped with) adaptation region, changes into the red area are outside the 'coping' region and damage occurs. This example is overly simplistic (in the real world the shape, width and height of the curve may also change), but it serves to illustrate the importance of extremes.
The SRES scenarios do not account for possible explicit climate change policy actions to reduce greenhouse gas emissions, such as the Kyoto Protocol. However some scenarios assume a reduction in world population after a mid-century peak, and the rapid and widespread introduction of clean and resource-efficient technologies. The Third Assessment Report also contains projections for some 'stabilisation scenarios' resulting from a more aggressive reduction in carbon dioxide emissions. The SRES scenarios also do not account for any unexpected climate 'surprises', such as increased methane emissions from permafrost melting or undersea methane clathrates. [Clathrates, also call gas hydrates, are crystalline solids which look like ice, and which occur when water molecules form a cage-like structure around smaller "guest molecules" such as methane. Clathrates occur naturally in cold environments, such as the deep ocean.]
2.1.2 Downscaling to New Zealand
To identify likely future climate changes across New Zealand, projections of global and large-scale regional changes must be 'downscaled'. Appendix 2 explains the statistical downscaling techniques used to produce climate change projections for New Zealand, and the further work undertaken for this Guidance Manual to expand the downscaling work to cover the full range of SRES scenarios.
Using these techniques, climate projections have been prepared for changes from 1990 to 2020-2049, and from 1990 to 2070-2099. These future 30-year periods are centred around 2035 and 2085, and are referred to in this chapter as the '2030s' and '2080s' respectively. 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 35 SRES scenarios, but also the range of climate model predictions for individual emission scenarios. Like the IPCC, we are unable to indicate a most likely value from within the projection range. However 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 agreement between a number of models.
2.2 Projections for New Zealand
Table 2.1 qualitatively summarises the main features of these New Zealand climate projections. Quantitative estimates of changes for parameters relevant to local government functions and services, and advice on how to construct relevant scenarios to estimate the importance of those changes, are given in Chapter 5 (specifically Tables 5.1 and 5.2), along with Appendix 4.
All estimates in Table 2.1 represent the best current scientific estimate of the direction and magnitude of change. The degree of confidence [The confidence level applies to New Zealand projections and may therefore differ from the global confidence levels of Table 1 in the IPCC's 2001 Summary for Policymakers.] placed by NIWA scientists on the projections is indicated in brackets (VH = very high, H = high, M = medium, L = low). 'Very high' confidence means that it is considered very unlikely that these estimates will be substantially revised as scientific knowledge progresses, while 'low' confidence means it is possible that estimates will 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.
Most of the projections tabulated or mapped in this Chapter are for the 2030s or 2080s. We recognise councils will also be interested in other decades during the 21st century. As explained later in this guide (Table 5.2), initial projections for these non-tabulated decades can be obtained by interpolating between 1990, 2030s and 2080s values.
Table 2.1: Main features of New Zealand climate change projections for 2030s and 2080s
View main features of New Zealand climate change projections for 2030s and 2080s (large table).
The following sections, along with material in Appendix 3, provide more detail on the projected changes summarised in Table 2.1.
2.2.1 Mean temperature
As explained in Appendix 3, downscaled projections of mean temperature ["Mean" temperature is the average of daily minimum and maximum temperatures. Global climate models have too few grid-points over New Zealand to clearly identify differences in the projected trends of minimum and maximum temperature, so the mean temperature (denoted by "TMea" in the maps) has been focused on in this section.] corresponding to the full IPCC SRES range were prepared for 58 sites around New Zealand. Temperature changes for characteristic sites in each regional council region are summarised in Table 2.2 (for the 2030s) and Table 2.3 (for the 2080s). Averaging over all temperature sites gives a New Zealand-average warming of 0.2-1.3°C by the 2030s and 0.5-3.5°C by the 2080s. These projected New Zealand temperature changes are in all cases smaller than the globally averaged changes for the corresponding SRES scenarios (Table A2.1, Appendix 2).
Figure 2.2 maps the annual lowest, mid-IPCC and highest projected New Zealand temperature changes for the 2030s and 2080s. Figures 2.3 and 2.4 map projected seasonal mean changes for a mid-IPCC scenario. Note that the spread of the projections broadens considerably from the 2030s to the 2080s, as a consequence of scaling the local changes with respect to the global temperature projections (Figures 2.1 or A2.1). Further description and more seasonal details can be found in Appendix 3 (Section A3.2.1 and Figures A3.2 to A3.5).
Table 2.2: Projected changes for each regional council area in seasonal and annual mean temperature (in °C) from 1990 to the 2030s
Table 2.3: Projected changes for each regional council area in seasonal and annual mean temperature (in °C) from 1990 to the 2080s
Note on interpretation of maps of projected climate change - Figures 2.2-2.7, A3.2-A3.9:
These maps are intended to identify the range of changes possible at a specific location, by comparing the "lowest" and "highest" panels for a selected season and future period. The maps should be used with caution (and ideally after consulting a science provider) if it is necessary to infer changes in spatial gradients or changes in joint distributions, such as simultaneous rainfall and temperature changes. For example, the IPCC low emission scenarios, which are associated with the smallest temperature increases, correspond generally to the smallest rainfall increases in the southwest of the South Island (the "lowest" map) and the smallest rainfall decreases in the northeast of the North Island (the "highest" map). Also, the range of changes shown in the seasonal maps combined, will always be larger than the range shown in the annual map (because of the cancellation effects when summing over the year).
Figure 2.3: Projected changes in seasonal mean temperature (in °C), for the 2030s relative to 1990: middle of IPCC range
Figure 2.4: Projected changes in seasonal mean temperature (in °C), for the 2080s relative to 1990: middle of IPCC range
2.2.2 Rainfall patterns
Downscaled rainfall projections corresponding to the full SRES ranges were prepared for the 92 New Zealand sites shown in Figure A3.1 (see Appendix 3). There are systematic variations in the projected rainfall within regions (for example, wetter in the west and drier in the east of Canterbury). So it is not very useful to tabulate averages for each region as was done for temperature. Instead rainfall projections have been tabulated for specific places. Councils may need to carefully examine these regional gradients in rainfall changes, when considering issues related to river levels. For example, in coastal Canterbury rainfall is projected to decrease, but large alpine-fed rivers could have increased flows because of greater rainfall in the headwaters.
Tables 2.4 and 2.5 give the 'lowest to highest' range for selected sites within each region. Two sites are included in the tables whenever there is a marked spatial variation across a region. Figure 2.5 maps the projected ranges in annual mean precipitation change (lowest, mid-IPCC and highest) for the 2030s and 2080s. Figures 2.6 and 2.7 show the mid-IPCC seasonal projections.
Table 2.4: Projected changes for selected stations within each regional council area in seasonal and annual precipitation (in %) for the 2030s
The lowest and highest rainfall projection patterns show a strong southwest to northeast gradient: the rainfall changes in the southwest of the country vary from no change (or a slight decrease) to a large increase in annual mean, whereas northeastern areas vary from a large decrease to no change. There is a lot of variability between models, and for many locations even the sign of the rainfall change cannot be stated with any confidence (Tables 2.3, 2.4). However, in the 2080s annual mean (Figure 2.5 and Table 2.5), the regions of Taranaki, Manawatu-Wanganui, West Coast, Otago and Southland tend to show increased rainfall for all scenarios, compared to Hawkes Bay and Gisborne which show rainfall decreases for all scenarios. Seasonally, the largest decreases occur in the north and east of the North Island in spring.
Table 2.5: Projected changes for selected stations within each regional council area in seasonal and annual precipitation (in %) for the 2080s
Figure 2.6: Projected changes in seasonal precipitation (in %), for the 2030s relative to 1990: middle of IPCC range
Figure 2.7: Projected changes in seasonal precipitation (in %), for the 2080s relative to 1990: middle of IPCC range
Figure 2.8: Projected changes between 1990 and 2100 in the number of days below freezing (upper two panels), and the number of days above 25oC (lower two panels)
2.2.3 Daily temperature extremes
Daily temperature extremes (overnight minimum and daily maximum) will also vary with regional warming, in addition to changes in mean temperature (Section 2.2.1). Box 2.1 illustrates that small changes in the mean (average) temperature value can potentially have a large effect on the frequency with which a specified high temperature is exceeded, or with which temperatures below a low value (such as 0°C) occur.
Figure 2.8 is an example of how the frequency of frosts and of hot days above 25°C could change for two of the SRES scenarios. These results (which are for 2100 not the 2080s) were produced with the CLIMPACTS system [Mullan et al (2001b).] that used global temperature projections for the SRES B1 low sensitivity scenario to scale a New Zealand pattern of temperature change derived from the Hadley General Circulation Model. The downscaled projections of mean temperature were fed into a weather generator that simulates daily sequences of maximum and minimum temperature at individual sites, and assumed a shift in mean temperature but no change in standard deviation. [While global climate model simulations indicate significant changes in mean temperature, they are generally unclear or show only small changes in standard deviations of temperature.] Figure 2.8 indicates large decreases in the number of frost days in the lower North Island and the South Island. [Because the far north of New Zealand already receives very few frosts, the frost frequency there cannot decrease substantially.] A substantial increase is indicated in the number of days above 25°C, particularly at already warm northern sites.
2.2.4 Heavy rainfall
A warmer atmosphere can hold more moisture (about 8% more for every 1°C increase in temperature), so the potential for heavier extreme rainfall certainly exists. The IPCC in its third assessment declared that more intense rainfall events are "very likely over many areas". [Table 1, Summary for Policy Makers, IPCC 2001.] However there is limited information available for deciding on which areas of New Zealand this might apply to, and the magnitude of the expected change.
A reduction in the return period of heavy rainfall events has been predicted in a study [Whetton et al (1996).] which suggested that by 2030 for New Zealand there would be "no change through to a halving of the return period of heavy rainfall events" and by 2070 "no change through to a fourfold reduction in the return period". A return period can be thought of as the average number of years between events with rainfall exceeding some specified high value. Return periods can be translated into annual exceedance probabilities (AEP) that specify the likelihood a rainfall amount is exceeded in any given year. For example, a 100-year return period event has an annual exceedance probability of 0.01, a 20-year return period event has an annual exceedance probability of 0.05, and so on.
The statement quoted in the previous paragraph was generic scenario guidance published in 1996 for the whole of Australia and New Zealand. It was based on analyses of daily rainfall time series from a regional climate model driven by an early CSIRO equilibrium [Equilibrium climate models look at the change from current conditions for an atmosphere in which the carbon dioxide concentration is held steady at some higher amount (typically double the current level). They have now been superseded by 'transient' or 'coupled' models, in which the greenhouse gas concentrations gradually increase from current values, and which better simulate ocean - atmosphere interactions.] global climate model. Actual changes in return period may differ across the country, and are also likely to vary with the rainfall duration being considered. Section 5.2 (including Table 5.2) provides guidance on estimating changes in heavy rainfall intensity for a particular location, for a range of average recurrence intervals (ARIs - see glossary) and rainfall durations. Table 5.2 is also based on data from the modelling studies analysed for the 1996 publication, and should only be used for initial "screening" studies.
More recent coupled climate model simulations confirm the likelihood that heavy rainfall events will become more frequent. A recent study [Semenov and Bengtsson (2002).] of changes to rainfall distributions under global warming can be applied to estimate a range of possible changes in extreme rainfall for a particular site. For example (see Appendix 3 for details), for Auckland the worst case (most severe) end of the range for 2100 indicates that a rainfall amount with a return period of 50 years (AEP=0.02) under the current climate would have a return period of less than 10 years (AEP>0.10) by 2100. The same approach could be applied to other New Zealand sites with long rainfall records.
The likely effect of increases of temperature and westerly wind speed on rain falling over New Zealand mountains has been modelled. [Gray and Larsen (2003).] For the particular storm modelled, it was suggested that a 2°C change in temperature would lead to a 6-7% increase in both maximum and catchment averaged rainfall. Similar increases were predicted for a 10% increase in wind speed. Increasing both wind speed and temperature together led to a 16% increase in rainfall.
In summary: various modelling studies suggest that heavy rainfall events will occur more frequently in New Zealand over the coming century, but the likely size of this change is uncertain. Initial estimates of future changes in heavy rainfall return periods, which can be used in screening studies to ascertain the relevance of rainfall changes for local government services such as stormwater drainage and flood protection works, are given in Table 5.2, with a worked example in Appendix 4.
2.2.5 Snowfall and snowline
It is physically plausible that snow cover will decrease and snowlines rise as the climate warms, but there are confounding issues. Warmer air holds more moisture and during winter this moisture could be precipitated as snow at high elevations, so warming does not rule out increased winter snowfall, although the duration of seasonal snow could be shortened.
2.2.6 Sea level
The rise of relative [The word 'relative' is used to exclude vertical land movement (uplift or subsidence of land or coastal seabed) that may occur, for example, as a consequence of earthquake activity.] sea level around New Zealand is likely to be similar to the global projections of sea-level rise by the IPCC Third 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 not be a 'temporary' aberration, but 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 between 9 and 88 cm between 1990 and 2100, for the full range of SRES scenarios. More information about sea level change, and its likely impacts and response options, is provided in the companion Coastal Hazards Guidance Manual.
2.2.7 Wind patterns
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 (2001c).] As shown in Box 2.2, this 'mean westerly' is built up from conditions when the actual east-west wind component is sometimes westerly (positive) and sometimes easterly (negative). The average southerly component is built up similarly from the north-south wind component.
Box 2.2: Westerly component of the wind across New Zealand
See figure at its full size (including text description).
The solid arrows represent individual wind speed and direction values. The dashed horizontal arrows are the "westerly components" of these winds - positive when they point to the right and negative when they point to the left. The "mean westerly component" is the average of these individual westerly components. It is 1 m/s for the three periods shown above.
Table 2.6 provides projected changes in the seasonal and annual average westerly and southerly components of the flow across New Zealand downscaled from the full range of IPCC SRES scenarios (see also Appendix A3.2.6). The future scenarios lean strongly towards increasing westerly flow, particularly in the annual mean. The mid-range projection for the 2080s is a 60% increase in the annual mean westerly component of the flow. Projected changes in the north-south wind component are less clear.
Table 2.6: Projected changes in seasonal and annual westerly and southerly wind components (in m/sec)
Strong [A "strong" wind is defined on the Beaufort Wind Scale as Level 6 (in the range 22-27 knots, or 11-14 m/sec), one level above "fresh" and one level below "near gale". However, we use the term here in a non-technical sense to cover the entire range above, say, about 10 m/sec.] winds are associated with intense convection (expected to increase in a warmer climate) and with intense low-pressure systems, which might also become more common (see extra-tropical cyclones below), so an increase in severe wind risk could occur. [Knippertz et al (2000) identified an increasing number of strong wind events over the North Atlantic in their climate model simulation, which they relate to the increasing number of intense cyclones.]
Following the concepts explained in Box 2.1, a first estimate of the change in frequency of strong winds can be made by assuming the shape of the statistical distribution of wind speeds remains similar to the present, but the distribution is displaced a little towards high speed values. As a result, for mid-range temperature change scenarios, the highest wind speed expected to occur once per year could increase by about 3% by 2080. Over the sea or flat land the annual frequency of occurrence of winds of 30 m/s or above might increase by about 40% by 2030 and 100% by 2080.
Uncertainties in these projected changes in extreme wind speeds are considerable. The IPCC in its Third Assessment Report says little about strong winds, but states that for some extreme phenomena there is currently insufficient information to assess recent trends, and climate models currently lack the spatial detail required to make confident projections. The overall variability of winds has been assumed to be constant for the above estimates. It is very likely that changes to the variability will occur in the future and may be occurring at present. How extreme winds affect different parts of New Zealand is also likely to change in the future. The regional distribution of wind extremes is probably dependent on the stability of the air approaching the land from the ocean, which may change.
Nevertheless, strong winds do cause damage to structures, forests and crops in New Zealand. The above estimates are based on the best current understanding for New Zealand, but are likely to be revised as further research and model results become available.
2.2.8 Ex-tropical cyclones, and mid-latitude storms
The IPCC Third Assessment Report indicates that by the end of the 21st century, it is likely that in some regions the peak wind intensities in tropical cyclones will increase by 5-10% and peak rainfall intensities by 20-30%. Tropical cyclones have changed their characteristics by the time they reach New Zealand, and tend to affect mainly the northern and eastern regions of the North Island, although occasionally they track further south. During El Niño periods, tropical cyclones tend to track further east in the South Pacific, and many climate models show an El Niño-like change in the mean state of the tropical Pacific over the next 50 years. However, whether or not this decreases the likelihood of ex-tropical cyclones reaching central New Zealand is not yet clear.
Appendix 3 outlines knowledge about causes of mid-latitude storms, and how these may change in future. 'Storminess' is likely to increase in the Southern Hemisphere this century, but it is not yet possible to say whether this would mean more intense storms, or a higher frequency of passing cold fronts, or some combination of these. Moreover, a general increase in the Southern Hemisphere does not necessarily equate with an increase locally in the small sector of the hemisphere that New Zealand occupies. Regional changes vary considerably between models.
In summary, current knowledge suggests the most likely future outcomes 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, but the level of confidence is low.
2.2.9 Ocean currents and wave patterns
The coupled atmosphere-ocean global climate models do not include enough detail to show the narrow ocean currents that flow around New Zealand, and very little analysis has been done of future wave patterns. This means that no quantitative statements about these climatic features can be made at this stage.
Changes in the winds do hold clues as to what might be expected in the ocean. [Mullan et al (2001c).] The warm currents flowing down the eastern coast of the North Island are driven primarily by the 'twisting' action of the winds over the subtropical South Pacific. These wind patterns show little change in the model runs, suggesting no major modifications in the warm-water currents. However, the projected increased westerlies (especially south of the country) are likely to accelerate the cold Antarctic Circumpolar Current. Such a change would further isolate waters on the Campbell Plateau, [Carter (2001).] and increase the inflow of cold water to the Chatham Rise, which could modify cloud cover in the region. In addition, increased westerly winds could increase upwelling of cooler subsurface waters along the New Zealand coast. For a straight westerly, the coasts affected are the northward-facing coasts (i.e. the West Coast of the South Island, and the northeast coast of the North Island); but changes in the north-south wind component could substantially modify which regions are affected.
Increased westerlies would also influence the ocean wave climate that impacts on New Zealand. In particular, coastal regions exposed to the prevailing winds would be subject to an increase in the frequency of heavy swells that would add to effects of higher sea levels.
2.2.10 Carbon dioxide and other atmospheric changes
Emissions of carbon dioxide (CO2) due to fossil fuel burning are virtually certain to be the dominant influence on the trends in atmospheric CO2 during the 21st century, according to the IPCC (2001). By 2100, carbon cycle models project atmospheric CO2 concentrations of 540 to 970 parts per million (ppm) across the SRES scenarios. This is from 90% to 250% above the nominal concentration of 280 ppm in 1750, and from 45% to 160% above the present concentration of 366 ppm. IPCC also notes that uncertainties in feedbacks from the terrestrial biosphere could broaden the CO2 range even further above that used in the SRES scenarios, to 490 to 1260 ppm by 2100. Projected concentrations of non-CO2 greenhouse gases vary considerably across the SRES scenarios, and are summarised in Table 2.7.
Anthropogenic aerosols, such as sulphur aerosols and black or organic carbon aerosols, could show either increases or decreases according to the SRES scenarios. Natural aerosols (sea salt and dust) are projected to increase as a result of changes in climate (increased evaporation, potentially stronger surface winds, and drying of continental regions).
Future changes in levels of ultraviolet radiation depend on changes in ozone (more ozone, less UV, and vice versa), and are still the subject of active research. Ozone levels in the troposphere are expected to increase (Table 2.7), but stratospheric ozone changes are unclear because of interactions with other greenhouse gases that involve complex feedbacks between radiation, chemistry, transport, and biogeochemical cycles. [McKenzie et al (2003).] CFCs are now controlled by the Montreal Protocol, and consequently their effect on stratospheric ozone levels is expected to disappear by around 2050. Some models predict a faster recovery of ozone levels, some predict a slower recovery, and some even predict more of an ozone/UV problem than at present because of the other feedbacks that could occur.
Table 2.7: Changes in greenhouse gases by 2100, under the IPCC SRES scenarios
| Greenhouse gas | Current concentration | Change by 2100 |
|---|---|---|
| Carbon dioxide (CO2) | 366 ppm | +170 to +600 ppm |
| Methane (CH4) | 1760 ppb | -190 to +1970 ppb |
| Nitrous oxide (N2O) | 316 ppb | -38 to +144 ppb |
| Tropospheric Ozone (O3) | variable | -12 to +62% |
| Other (HFCs, SF6) | depends on gas | wide range |
Note: Current concentrations are also shown, in units of parts per million (ppm), or parts per billion (ppb), as appropriate.
2.3 Summary
This chapter has summarised the current knowledge of human-induced climate change expected for New Zealand through this century. Projected changes at 2020-2049 and 2070-2099 (relative to 1990) have been highlighted. These changes, derived from climate model simulations, have been scaled to the full range of the IPCC Third Assessment Report, which suggested a global temperature increase between 1.4 and 5.8°C by 2100.
The broad pattern of change expected involves:
- increased temperatures (with greater increases in the winter season, and in the north of New Zealand)
- decreased frost risk but increased risk of very high temperatures
- stronger west-east rainfall gradient (wetter in the west and drier in the east)
- increased frequency of extreme daily rainfalls
- increased sea level
- increased westerly winds
- a number of other changes, with a lower degree of confidence (Table 2.1).
The New Zealand changes cover a very wide range, reflecting the diverse emissions scenarios of the IPCC and also climate model uncertainties. Mid-range projections in annual average temperature and precipitation are:
- temperature increase of 0.6 to 0.7°C from 1990 to 2030s (45-year change), and 1.6 to 2.0°C from 1990 to 2080s (95-year change)
- precipitation change between about -5 to +5% from 1990 to 2030s, and about -10 to +15% from 1990 to 2080s (in most places).
Chapter 5 provides advice on typical changes that local government should select in assessing risk. Natural climate fluctuations also occur, and these are discussed in Chapter 3.
