Understanding climate variables and scenarios used in the projections
This page has a definitions of the climate variables and climate change projections terms used within the data.
The climate projections are based on emissions scenarios for the future.
Scenarios are plausible stories about how the future may unfold, so we can begin to understand how climate change might impact:
- our lives
- our environment
- our financial systems
- our wellbeing.
Scenarios take into consideration our understanding of the climate, and how global policies and social or economic development might affect emissions. They can be used to develop projections of the impact climate change might have in the future, based on how quickly global emissions rise or fall or whether climate tipping points are reached.
Shared socio-economic pathway (SSP) scenarios are global emissions scenarios developed by the Intergovernmental Panel on Climate Change (IPCC). They include gross domestic product, population size, urbanisation, economic collaboration, and human and technological development.
See the climate scenarios toolkit for more information about scenarios.
Scenarios used in the climate change projections data
Three scenarios were used in these projections:
- The ‘Sustainability’ scenario, SSP1-2.6, assumes that the world shifts gradually toward a more sustainable path, emphasising more inclusive development that respects environmental boundaries. It assumes that warming stays below 2˚C, with net zero CO2 emissions reached by 2050.
- The ‘Middle of the road’ scenario, SSP2-4.5, assumes that the world follows a path in which social, economic, and technological trends do not shift markedly from historical patterns. It assumes that warming reaches 2.7˚C by 2100.
- The ‘Regional rivalry’ scenario, SSP3-7.0, assumes the world becomes more focused on national and regional security issues, and there is no additional climate policy. It assumes CO2 emissions approximately double from current levels by 2100 and warming reaches 3.6˚C by 2100.
Average daily air temperature (°C)
The average daily air temperature per year or season.
The daily minimum and maximum temperatures and the daily temperature range variables are also available.
Hot days
The number of days per year or season with a maximum daily air temperature over 25°C.
Very hot days
The number of days per year or season with a maximum daily air temperature over 30°C.
Frost days
The number of days per year or season with a minimum daily air temperature below 0°C.
Temperature on the hottest day of the year (°C)
The maximum daily air temperature per year.
Temperature on the coldest day of the year (°C)
The minimum daily air temperature per year.
Growing degree days (GDD)
The accumulated daily temperature above a base threshold (5°C and 10°C) per year.
Growing degree days [Stats NZ] indicate the amount of warmth available for plant growth.
Cooling degree days (CDD)
The annual accumulated daily temperature above 18°C - considered a ‘comfortable’ temperature - and so cooling of buildings may be required.
See the definitions in Annex VI of the IPCC AR6 WGI [IPCC] for further explanation.
Heating degree days (HDD)
The annual accumulated daily temperature below 18°C - considered a ‘comfortable’ temperature - and so heating of buildings may be required.
See the definitions in Annex VI of the IPCC AR6 WGI [IPCC] for further explanation.
Total rainfall (mm for baseline periods, per cent change for future periods)
The total amount of rainfall per year or season.
Rainy days
The number of days per year or season with 1 mm or more of rainfall.
Very rainy days
The number of days per year or season with more than 25 mm of rainfall.
Heavy rainfall (mm for baseline periods, per cent change for future periods)
The 99th percentile of all daily rainfall over each 20-year period.
Dry days
The number of days with less than 1 mm of rainfall.
Drought exposure (expressed as PED, mm)
Potential evapotranspiration deficit (PED) is a drought index. It is the gap between water demand and water availability, and it affects the moisture retained in soil and plant growth. An increase in PED indicates an increase in drought severity.
See Drought indicator charts [NIWA] for further explanation.
Average wind speed (m/s for baseline periods, per cent change for future periods)
The average daily wind speed at 10 metres above the ground.
Windy days
The number of days with maximum wind speed more than 10 metres per second.
Strong wind (m/s for baseline periods, per cent change for future periods)
The 99th percentile of all maximum daily wind speeds over each 20-year period.
Average relative humidity (%)
The average daily relative humidity per year or season.
Average solar radiation (W/m2)
The average daily incoming solar radiation per year or season.
Downscaling
Translating climate model data into finer resolution simulations. The 2024 Aotearoa New Zealand national climate projections have been downscaled to a 5 km scale for all of Aotearoa New Zealand. The data is the average of the values produced by each of the six climate models used by NIWA for the downscaling process (ensemble data showing the multi-model mean).
See Updated national climate projections for Aotearoa New Zealand [NIWA]
Coupled Model Intercomparison Project Phase 6 (CMIP6)
The Coupled Model Intercomparison Project (CMIP) is an international climate modelling project, part of the World Climate Research Programme (WCRP). This project coordinates global comparisons across different climate model simulations. Each phase of CMIP improves on the previous phase’s climate modelling. CMIP6 is the most recent phase, and six models from CMIP6 were used in the 2024 Aotearoa national climate change projections (CMIP5 was used for the 2018 climate change projections).
See Climate change projections for New Zealand
See Aotearoa New Zealand climate change projections guidance.
Future periods
Three future time periods (2021-2040, 2041-2060 and 2081-2100) are available in the dataset. These allow users to view climate projections for the near-term future, mid-century and end-of-century periods.
Global warming periods
Four global warming levels (1.5°C, 2°C, 3°C and 4°C) are available in the dataset, though these are not included in the mapping tool.
Rather than showing the projected climate changes at a given time period, these show the projected climate changes when a specific global temperature threshold is reached. The temperature thresholds represent the amount of average global warming above pre-industrial levels.
Historical baseline (base period)
Two historical baselines (1986-2005 and 1995-2014) are available in the dataset. The data for these baselines represent the average climate conditions in the recent past. The 1986-2005 baseline was used for the previous projections for Aotearoa New Zealand, based on CMIP5 models. The 1995-2014 baseline is consistent with the IPCC Sixth Assessment Report.
Intergovernmental Panel on Climate Change (IPCC)
IPCC is the United Nations body for assessing the science related to climate change. The IPCC’s Sixth Assessment Report produced updated climate scenarios and projections, which have been used in the development of New Zealand’s national climate projections.
Representative Concentration Pathways (RCPs)
Representative Concentration Pathways (RCPs) are global scenarios developed by the scientific community. They are focused on a range of potential future changes in greenhouse gas concentrations and do not explicitly account for socioeconomic developments. The 2024 Aotearoa national climate projections include a range of SSPs, whereas RCPs were used for the 2018 climate change projections.
Shared socio-economic pathways (SSPs)
Shared socio-economic pathways (SSPs) are global scenarios developed by the Intergovernmental Panel on Climate Change (IPCC). They are narratives of potential futures that consider social and economic factors. SSPs can be used to explore pathways the world could take and are useful when considering potential climate change impacts.
See Explainer: How ‘Shared Socioeconomic Pathways’ explore future climate change [Carbon Brief]. The 2024 Aotearoa national climate projections include a range of SSPs.