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Chapter 5: Developing the Scenarios
5.1 Introduction
A scenario is a plausible description of how the future may develop, based on a coherent and internally consistent set of assumptions about key drivers. Climate, social and economic scenarios which span the likely range of future conditions can be formulated. They can be used together with expert knowledge and models of the sensitivity of natural or managed systems to climate (the information outlined in Chapter 4) to deduce a range of possible climate impacts on selected council activities and services.
Scenario analysis is an appropriate tool for effects assessment because it is not feasible to make a definitive single quantitative prediction of exactly how much a particular climatic element (e.g. heavy rainfall intensity) will change over coming decades. This is because rates of climate change will depend on future global emissions of greenhouse gases, which in turn depend on global social, economic and environmental policies and development. Incomplete scientific knowledge about some of the processes governing the climate, and natural year-to-year variability, also contribute to future uncertainty. It is therefore necessary to consider a range of possible futures when assessing climate impacts and developing adaptation strategies.
As already outlined in Chapters 1 and 4, we recommend a staged approach to assessing climate effects. For a particular council function or service this begins with a straightforward initial screening assessment using simple estimates of how climate factors relevant to this function may change. A more detailed effects study is only justified if this initial analysis indicates material climate change impacts or opportunities are likely, at least for the upper end of the scale of potential future climate changes. The current chapter provides guidance both on simplified scenarios for initial screening assessments, and on scenarios for use in more detailed studies when these are justified.
In developing scenarios, factors other than those related to climate variables need to be considered. Climate change will occur along with many other changes (social, economic, environmental) that are expected to occur in coming decades, many of which also carry uncertainties. It must also be remembered that climate change is an underlying long-term trend on which future variability in climate will be superimposed.
Deciding on the scenarios to use is an important step because the ones chosen will determine the bands of uncertainty that will be quantified, which will feed through to decision-making processes in terms of what to do and not to do about climate change, and over what timeframe.
It is recommended that three broad categories of scenario are considered:
- social
- economic
- physical/environmental.
Each of these is described briefly in this chapter, and examples provided to show how scenarios can be developed and applied, drawing in part from the climate change information provided in Chapters 2 and 3. When developing all scenarios it is good practice to consider the range of uncertainty, which may encompass the upper and lower end of projected climate change, high and low population projections, or different scenarios for economic development.
Box 5.1: Screening assessment
In this Guidance Manual we recommend an initial screening assessment for a particular council function, activity or service, to decide whether a more detailed climate change effects assessment and formal risk analysis is warranted. A screening assessment can be done for a particular function or service (Figure 1.1) or across all council activities (Figure 1.2).
The first step of a screening assessment is to identify whether a particular function or service is important to your council and might be sensitive to climate change. This can be done by simply ticking the 'yes', 'maybe' or 'no' for questions in the screening assessment table provided below, taking into account the context of evolving risk over time and the life of the project. More detail on the characteristics to be evaluated is given in Chapter 7.2. If you answer "yes" or "maybe" to Question 1 or 2, as well as to any of Questions 3-6, you should then probably answer "yes" to question 7 and undertake a scenario-based initial screening analysis using climate scenario guidance from the screening assessment column of Table 5.1. This analysis may well be all that is needed for the circumstance. However, if the results of this analysis lead you answer 'yes' or 'maybe' to Question 8, a more detailed risk assessment is appropriate, using more detailed scenario guidance from the right hand column of Table 5.1.
| Characteristic | Question | Yes | Maybe | No |
|---|---|---|---|---|
| Current driver | 1. Is there an existing problem that may be exacerbated by climate change? (e.g. recurrent inundation) | |||
| Future driver | 2. Is there a foreseeable problem that may be caused or exacerbated by climate change? | |||
| Complexity | 3. Is this a complex issue? (Locating a new suburb vs. one house) | |||
| Location | 4. Is the location sensitive to climate change? (Flood plain vs. bedrock hillside) | |||
| Duration | 5. Is it a permanent long-term change? (Locating a new suburb vs. permitting a campsite | |||
| Extent | 6. Does it involve a lot of infrastructure and services provided? (Urban area vs. remote rural) | |||
| Total | ||||
| 7. Is an initial screening analysis using a screening scenario from Table 5.1 justified? | ||||
| 8. Does this scenario-based initial screening analysis indicate material climate change impacts might occur? (A screening analysis must be performed in order to answer this question.) | ||||
| 9. Should a full risk assessment be done for this issue? | ||||
Making use of this chapter
This chapter identifies the key aspects for consideration in developing scenarios for a climate change impact assessment, and provides guidance to help develop a scenario study (or impact or effects assessment). Key things to consider are.
1) Climate change will not occur in isolation from other changes, so scenarios need to be developed for more than just climate (refer to 5.2 below, and the examples provided).
2) A staged approach to developing and applying scenarios of climate change is recommended (refer to Table 5.1 for examples, and sources of information/expertise for more in-depth scenario studies).
3) The range of uncertainty (see the example in Figure 5.2).
A note on timeframes
Climate has effects over a range of timeframes. On an interannual basis variations in climate can be affected in some years by El Niño or La Niña events. On a decadal basis there are fluctuations associated with, for example, the IPO (see Chapter 3). Climate change, in the context of this Guidance Manual, relates to changes that are only now becoming apparent as underlying trends and will increasingly become apparent over the next hundred years.
Local government planning can also occur over a range of timeframes. Infrastructure investments (such as flood protection) generally consider 50-100 year timeframes, which are consistent with climate change. Other planning activities (e.g., biosecurity) are not usually linked to climate timeframes.
Marrying decision-making timeframes with climate science timeframes needs to be an implicit part of a climate change assessment.
5.2 Developing scenarios
Social scenarios
The most obvious social scenarios relate to demographic changes, usually changes in population size and distribution. Future population changes are likely to have a significant influence on the demand and supply of local government functions and services, and consequently on the natural resources that are managed by local government.
Economic scenarios
Regional and district councils are increasingly focusing on economic development goals and how these relate to their functions and services. Future trends in activities such as agriculture, industrial development, and tourism will all have consequences for local government and the resources they manage. In predominantly rural regions, changes in land use could have significant effects on resource demand and supply, for example a trend towards land use intensification could lead to increased demand for water. Similarly, growth in industry and tourism will have significant effects in various regions.
Physical/environmental scenarios
The predominant physical or environmental scenarios that need to be developed relate to possible future changes in climate. As described in Box 5.1, we recommend a staged approach to impact assessment, which involves a preliminary screening assessment followed by a more detailed analysis if justified by the screening process. This requires two levels of scenario development: simple initial scenarios, and more detailed scenarios for in depth analysis.
The approaches summarised in Table 4.3 for assessing the effects of changes in climate generally require specific climate values (or statistics based on these) as input. These statistics, or methods for deriving them, are not always immediately apparent from Chapter 2. Further advice on obtaining appropriate climate parameters is provided in Table 5.1.
1) Scenarios for initial screening assessment. The second column of Table 5.1 outlines how to obtain region-specific values of climate parameters for use in these initial scenario studies, based largely on numbers available from this Guidance Manual. The emphasis is on mid-range climate projections. If use of these mid-range values in a screening assessment indicates material climate change impacts or opportunities are plausible, then a more detailed analysis is recommended (Figure 1.1). If the mid-range scenario does not lead to any significant impacts it is good practice to also examine the effects resulting from a scenario near the upper bound of possible future climate changes. This initial screening analysis is essentially a climate sensitivity study, and it may also be useful to examine historical data. This could involve a statistical analysis (as used in the North Shore City example in 5.5 below) or use of past events (e.g. floods, droughts, warmer years) as analogues for the future.
2) Scenarios for more detailed studies. If the initial screening assessment suggests material climate impacts are plausible, then some more detailed scenarios are often needed for the following detailed study. These may rely on a more complex physical or statistical modelling approach, draw on detailed analyses of current climate statistics in a location, and cover the 'high' and 'low' ends of the downscaled (to New Zealand) IPCC SRES scenario bands. They should be considered across the timeframes that are relevant to the particular function or natural resource that is being focussed on. Guidance on developing these more detailed scenarios is provided in the third column of Table 5.1.
Table 5.1: Values or sources of climate parameters suggested for use in scenario analysis
View values or sources of climate parameters suggested for use in scenario analysis (large table)
As heavy rainfall is a key variable in infrastructure planning and design, a further table (Table 5.2) is provided. This shows recommended percentage adjustments per degree of warming to apply to extreme rainfalls, for various average recurrence intervals (ARIs) and for rainfall durations from less than 10 minutes up to 72 hours, for screening assessment scenarios. As indicated in Table 5.1, current extreme rainfall rates for selected locations, durations and ARIs can be obtained from analysis of historical rainfall data sets from particular sites, or from the HIRDS CD. For temperature, use the projected changes in annual mean temperature from the right hand columns of Tables 2.2 and 2.3, or from Figure 2.2. At least two screening calculations should be undertaken- for low and high temperature change scenarios. A worked example of the application of this information is provided in Appendix 4. In carrying out such site-specific analyses, remember the uncertainties in return period estimates for the present climate. In many places rainfall records only cover a past period of a few decades, so that design rainfall estimates for 50 or 100 year ARIs contain statistical assumptions and data-based uncertainties.
Table 5.2: Factors for use in deriving extreme rainfall information for preliminary scenario studies (screening assessments)
Applications of climate change scenarios for preliminary (screening) and for more detailed studies are shown in Figures 5.1 and 5.2. In these examples, changes in area of suitability for kiwifruit have been calculated. A preliminary scenario study, using a mid-range scenario, indicated that the Bay of Plenty climate could become unsuitable for kiwifruit by the end of this century (Figure 5.1). A more in-depth study was then carried out (Figure 5.2) to evaluate incremental changes over the next 100 years and identify the range of uncertainty associated with these changes. [Kenny et al, 2000.] This latter result gives more in-depth information as to when climate change could become critical for the kiwifruit industry in the Bay of Plenty.
Figure 5.1: An example of a screening assessment of kiwifruit suitability, using a mid-range climate scenario for the year 2095
Figure 5.2: An example of a detailed scenario analysis showing changes in the area suitable for kiwifruit in the Bay of Plenty, using a range of scenarios over a 100-year timeframe
The following examples describe the developed scenarios for some climate change studies that have been undertaken recently.
5.3 Example 1: Southland water resources
In a recent 'State of the Environment' report for water, Environment Southland (2000) identified three main drivers that would impact on Southland's fresh water environment in future: environmental drivers (principally climate); population changes; and economic development. Trends were identified in all three drivers:
- Environmental: The greatest change has been an increase in the average minimum temperature in Southland over the last 40 years. The daily temperature range is decreasing at a greater rate than elsewhere in New Zealand.
- Population: Both urban and rural areas in Southland have been experiencing population declines since the late 1970s.
- Economic: Agriculture is a major contributor to the Southland economy, accounting for 82% of the total land area in the region that is not conservation land. Agricultural activities are the largest contributor of nutrients, microbiological and other contaminants to fresh water resources. Changes in land use can have a major effect on resultant environmental pressures. Over the last decade there has been a rapid expansion of dairy farming and associated industry infrastructure. Tourism is another economic activity that could lead to increased pressure on fresh water resources.
So if Environment Southland intended to study the possible effects of climate change on the region's fresh water environment, it would need to consider changes in the above key drivers over the next 30 to 100 years, which would require some consideration of alternative scenarios for each driver (see Table 5.3 for examples).
In this example, some linkage has been made between the different climate change scenarios and different population and economic scenarios. The climate change information presented here is drawn from the information provided in Chapter 2. It is important to reiterate that these are presented as plausible futures only and that there will be sub-regional variation in climate change parameters.
Table 5.3: Examples of possible alternative scenarios for key drivers affecting Southland fresh water resources
| Environment | Population | Economic | |
|---|---|---|---|
| Scenario 1 | Low-case scenario of climate change:
|
Downward trend in population stabilises, with low growth over the next 50-100 years | Moderate land use changes with slightly warmer and drier average conditions |
| Scenario 2 | High case scenario of climate change:
|
Downward trend in population stabilises, with more rapid growth over the next 50-100 years due to more favourable climate (particularly for the agricultural sector, such as dairy farming) | Greater intensification of land use with warmer, wetter conditions |
Note: Climate change scenarios for the 2080s only are used here, and are provided here in summary form.
5.4 Example 2: Water resource changes in three river catchments
The Ministry of Agriculture and Forestry commissioned Lincoln Environmental and NIWA "to quantify the potential change in agricultural water usage and availability due to climate change, and assess the implication of these changes on the potential pressures on water sources and water allocation issues" (Lincoln Environmental 2001).
Changes in three river catchments were studied: Rangitata in South Canterbury, Motueka in Nelson, and Tukituki in Hawke's Bay.
Scenarios were developed for two of the three categories identified above - environmental (climate and river flow changes) and economic (land use changes). However, as explained below, the land use changes themselves were generated principally from projected climate changes.
Climate and river flow changes
The main steps in developing these scenarios were:
- gathering of historical climate and river flow data for the period 1971-95 for selected sites in each catchment
- generation of two climate change scenarios for 2050, and site changes (monthly) for precipitation, maximum temperature, minimum temperature, dew point temperature, and wind run (two different GCMs were used for these scenarios, with the same greenhouse gas emissions scenario used for both).
- use of a weather generator to synthesise 30 years of daily climate data for 2050 for key sites, based on the monthly changes in the parameters listed above
- river flow scenarios, generated using the NIWA 'Topnet' model.
These scenarios included some important assumptions:
- They provided mean changes only for 2050, with no allowance for changes in the interannual variability of climate, e.g. as a result of ENSO events and IPO shifts
- Two key assumptions were made with the use of weather generators - the manner in which weather elements were simulated and that monthly mean values only were changed, with no change in other properties (e.g. no change in the typical variability and relative intensity of rainfall and temperature extremes)
- Two key assumptions were made with the river flow model - "no hydrologically significant changes in vegetation, no new diversion or abstraction of water, nor any new extraction of groundwater that sustains river flow".
Land use changes
The determination of land use changes in each of the three catchments was made through the calculation of changes in mean monthly degree-days in combination with the knowledge of local experts.
In determining these changes it is assumed that present economic trends for different crops and farming systems would hold for 2050. In general the pattern presented was one of more intensive land use.
To complete this study, the scenarios of climate, river flow, and land use change were brought together to quantify possible changes in water demand and supply, using an irrigation scheme simulation model.
5.5 Example 3: Stormwater and wastewater effects in North Shore City
North Shore City Council recently commissioned a major study (as part of Project CARE) on its wastewater system (Meritec Ltd and Australian Water Technologies 2003). As part of this study it was decided, for a relatively small incremental cost, to examine the possible effects of climate change on future wet weather overflows.
The approach taken by the consultants for North Shore City is summarised in Figure 5.3, and in the accompanying text. In brief, existing system performance has been translated into expected future performance based on changing rainfall (extreme events) using a statistically established relationship between existing rainfall patterns and existing system performance.
Figure 5.3: Approach used to generate and apply future scenarios for the North Shore City Council study on future wet weather overflows
Several key points, relating principally to the steps taken in developing climate change scenarios, are presented below. These are quoted directly from the executive summary of the North Shore City study:
- The existing condition of the receiving environment was determined by hydrologic, hydraulic and water quality simulation of the system based on 17-year historical rainfall record.
- NIWA carried out a study to determine the rainfall changes due to global warming and phase changes in the Interdecadal Pacific Oscillation (IPO) for the planning scenarios. Global mean temperature is expected to increase by 1oC by year 2050 and this may be accompanied by more rainfall. The NIWA study indicates that there will be heavier, longer duration extremes in IPO negative phase and heavier, shorter duration extremes in IPO positive phase resulting in more rainfall.
- A statistical approach was considered to evaluate the system performance due to IPO phase change and global warming, as the long term hydrologic and hydraulic simulation using future rainfall time series is considered to be time consuming, costly and may only provide a similar confidence level.
- Expected future storm characteristics (year 2050) were estimated from the historical storm characteristics and historical and predicted future rainfall IFD tables. Future storm characteristics were estimated separately for both positive and negative IPO phases, expected to be experienced by 2050.
- Although climate change is well accepted by professionals worldwide, the analysis adopted in this study is based on a number of simplified assumptions with inherent uncertainties associated with modelling the effects of global warming. The results therefore, should be used to assess trends more than provide absolute values, and their interpretation should be carried out by suitably qualified and experienced professionals.
5.6 Best practice guidance
It is recommended that:
- A risk assessment process begins with a simple screening study unless you already know that climate change could have significant impacts on the issue at hand.
- To evaluate climate change risk, use at least a mid-range scenario (values suggested in Table 5.1). If a mid-range scenario indicates the potential for noticeable negative impacts, evaluate the sensitivity of this result by assuming both high and low-end scenarios and checking the change in the resulting impacts.
- If a mid-range (or higher) scenario indicates potentially significant negative impacts, and the cost or social/cultural relevance of the asset or service is substantial, then evaluate the robustness of your results with a more complex procedure (see "detailed study scenarios" in Table 5.1).
- Relevant non-climate scenarios are included in your evaluation of impacts (such as population and land-use change and resulting pressures on environmental resources or exposure to hazards).
