Models can be used to represent changes or potential variations in regional climate in response to broader scale (national or global) climate change scenarios. Based on these models the local environmental impacts of climate change can then be assessed. However a challenge arises, in that the spatial resolution of General Circulation Models (GCMs) is too coarse to represent regional climate variations at the scales required for environmental impact assessments in New Zealand.
To resolve this challenge of scale, when information from global climate models is utilised which could otherwise constrain regional climate, downscaling of climate change impact analyses is necessary. This is particularly important in the New Zealand context, because the recognised maritime, topographic and convective climate processes, which are reflected in the local to regional scale variability, are not always well represented by the broader global-scale features simulated by the models (GCMs).
Three techniques are available to generate climate change information that can be used as input for environmental models: i) downscaling to the VCSN grid (Ministry of Environment, 2008); ii) semi-empirical (statistical) downscaling (SDS) of GCM outputs; and iii) dynamic regional climate models (RCMs) nested within a GCM.
A NIWA study compared the downstream impacts using statistical downscaling and RCMs dynamical downscaling for three different emissions scenarios. The emissions scenarios included those characterised in the IPCC Fourth Assessment (B1 – low emissions, A1B – middle of the road emissions, and A2 – high emissions) and two of the 12 GCM models used in New Zealand (UKMO_HADCM3 and MPI_ECHAM5). The study focused on surface water hydrological responses (i.e., discharge, infiltration, evaporation, snow storage) for a number of river basins across the North and South Island of New Zealand. The analysis compared the current situation (1980-1999) with two future time periods (2030-2049 and 2080-2099) for a specific river basin (Kaipara Harbour) and recommendations were drawn regarding the uncertainty of climate change impacts.
NIWA contacts:
Christian Zammit, Shailesh Singh, Daniel Collins