Transient Model Scenarios of Climate Changes for New Zealand

Climate simulations from a group o f coupled atmosphere-ocean general circulation models are assessed against current climate observations for the south-west Pacific region. While the six models analysed simulated many of the broad climate features of this region satisfactorily, they all exhibited some shortcomings in their detailed validation against the current climate of the Australasian region and the observed El Nino—Southern Oscillation variability that affects this part of the globe. A simple physically-based downscaling is applied, that describes how New Zealand's orography causes the climate of a station to deviate from the large-scale background climate at the same latitude, under the influence o f west-east and south-north airflow variations. Downscaled projections o f New Zealand temperature and precipitation changes are then calculated from transient simulations o f the models, which are all forced by a very similar emissions scenario o f sulphate aerosol concentration and 1°/0 per annum compounding carbon dioxide concentration increase. Projected changes are presented for periods of 50 years ahead (1970-1999 to 2030-2049 change) and 100 years ahead (1970-1999 to 2070-2099). The models generally predict that mean temperatures will increase somewhat slower than for the Southern Hemisphere as a whole than for the globe, that precipitation will increase in the west and decrease in the east of the country, and that the prevailing westerly winds will intensify across New Zealand. However there is considerable variation between models. Most of the changes discussed are actual model projections for the transient simulation, relative to control run changes over the same period, rather than changes scaled per degree global warming as are sometimes given. These anthropogenic projections should be viewed within the context of natural decadal variability in circulation, that periodically could either augment or weaken the west-east rainfall gradient across New Zealand. Concentrations of greenhouse gases in the atmosphere are expected to continue to grow through the coming century, leading to changes in global, regional and local climate (1PCC, 1996). There is a widespread demand for predictions of these climate changes, for use i n studies o f likely impacts, negotiation o f agreements f o r constraining anthropogenic greenhouse gas emissions, and identification o f adaptation options. However, for the reasons described below, science cannot give exact predictions of how local and regional climates may change in the future. Corresponding author: Dr A. B. Mullan, NIWA, P.O. Box 14-901, Kilbirnie, Wellington, New Zealand. Emailb mullan@niwa.cri. nz 4 W e a t h e r and Climate 21 The anthropogenic greenhouse gas and aerosol emissionsl which will influence climate over the coming century respond to changes in population and economic growth, technology, energy availability and national and international policies. Even i f the emissions for the next century could be predicted precisely, there would still be scientific uncertainties in estimating the resulting greenhouse gas concentrations and global, regional and local climate changes. The 1995 assessment by the Intergovernmental Panel on Climate Change (IPCC) concluded that confidence is higher in hemispheric-tocontinental scale projections of climate change than in regional projections from climate models, where "confidence remains low" (IPCC, 1996). One solution to the dilemma of providing predictions in an uncertain world is the use o f scenarios. I n this approach, several greenhouse gas emission pathways are constructed f o r the future, based o n a range o f plausible social, economic and technological developments. For each of these scenarios, projections are then made for greenhouse gas concentrations and the resulting climate changes, based on scientific understanding and incorporating science-based uncertainty ranges. The result is a set of several different climate predictions, spanning the most l ikely future emissions pathways. Such projections are central to answering "whati f" questions about the vulnerability of environmental, economic and social systems and their ability to respond and adapt (NSSCCC, 1997). Scenarios of this type have been developed for the globe as a whole by the IPCC (e.g. Kattenberg et al., 1996). Regional scenarios are needed i n addition to these global scenarios, because changes in climate will not be geographically uniform (IPCC, 1996). The starting point for a regional scenario is usually a simulation o f climate across the whole globe with a computerised General Circulation Model (GCM, also referred to as Global Climate Model), which is driven by the radiative forcing f rom a particular greenhouse gas emission scenario, and simulates climate elements at a horizontal spacing of about 250 km. However this limited spatial resolution does not pick up the influences of topographical features (such as New Zealand's Southern Alps) which have a marked effect on local climate, and means that many smaller scale processes important to climate (such as the formation o f clouds and the resulting rainfall generation) have to be simulated in a simplified manner. Nested regional climate modelling (Kattenberg et al., 1996) is one approach to downscaling from the GCM scale to the regional or local scale. Output from a GCM drives a regional climate model (RegCM), w h i c h is run at a much higher spatial resolution (typically 40 k m horizontal resolution, bu t potentially 1 0 k m wi th a sufficiently powerful computer). The RegCM incorporates many o f the important topographical influences on climate, as well as simulating some of the important physical processes in more detail. A second approach is statistical downscaling (Kattenberg, op. cit.). This is a two-stage process, in which statistical relationships are first developed 1 Aerosols are small particles which reflect solar radiation and influence cloud properties. They derive largely from sulphate emissions and offset some of the warming caused by greenhouse gases. MuIlan, Wratt, Renwick. : Transient model scenarios o f climate changes 5 between local climate variables (such as temperature and rainfall) and larger-scale features (such as atmospheric pressure patterns and spatially averaged temperatures), using historical measurements and analyses. These relationships are then applied to the output of GCM predictions, to simulate local climate characteristics. Kidson and Thompson (1998) concluded that, for the models they studied, statistical downscaling and nested regional modelling exhibit similar levels o f skill in simulating current aspects o f the New Zealand climate. Statistical downscaling requires fewer computer resources than nested modelling, but assumes that current "downscaling" relationships will remain valid under a significantly perturbed climate. 1.1 New Zealand Climate Scenarios In 1988, the New Zealand Climate Committee o f the Royal Society o f New Zealand (RSNZ, 1988) made preliminary projections of New Zealand climate change, based on early GCM results and qualitative understanding o f relationships between regional atmospheric circulation and local climate. The only quantitative earlier scenarios (Pittock and Salinger, 1982; Salinger, 1982) were based on palaeoclimate reconstructions o r comparison of warm and cold years in the observed record. The first detailed climate change scenarios for New Zealand derived from GCM simulations were prepared by MuIlan and Renwick (1990), using a statistical downscaling approach. This work was updated over the next few years, leading to the scenarios reported in MuIlan (1994) and Whetton et al. (1996b), and was also the basis f o r early scenarios used i n the CLIMPACTS integrated climate impacts assessment model (Kenny et al, 1994). More recently, regional climate modelling has also been applied as a downscaling tool for simulating climate change over New Zealand (Renwick et al 1998, 1999). The work described in the previous paragraph was based on downscaling from equilibrium atmosphere GCM simulations, coupled to a simple slab ocean. The carbon dioxide in the model was doubled instantaneously, and the model was run until a climate equilibrium was reached under this new radiative forcing. Such GCMs generally predict a decrease in the meridional temperature gradient in the Southern Hemisphere, and a consequent decrease in the westerly f low across New Zealand. Cl imate modelling groups have now developed transient coupled atmosphere-ocean GCMs (A0GCMs) which simulate a gradual increase in greenhouse gas concentrations, and couple a more detailed simulation o f ocean circulation with their atmospheric simulations. These simulations generally predict an increase in Southern Hemisphere meridional temperature gradient (the equator warms more quickly than the southern oceans), and a consequent increase in westerlies across New Zealand (Whetton et al, 1996b). These differences arise because, in coupled models, heat is drawn down into the southern ocean, slowing the rate of surface warming. I f atmospheric greenhouse gas concentrations were to be stabilised, the coupled (transient) AOGCMs would ultimately approach an equilibrium solution that might not look all that different from the slab ocean result. While AOGCMs are more physically complete in terms of the processes they describe, there are still uncertainties 6 W e a t h e r and Climate 21 about the rate o f vertical heat transport in the ocean (e.g., England and Hirst, 1997; Power and Hirst, 1997), and in heat fluxes between atmosphere and ocean. These differences between the equilibrium and transient simulations have important consequences. The distribution of rainfall across New Zealand is strongly influenced by the interaction between airflows from a westerly quarter and the predominantly northeastsouthwest oriented mountain chains (Wratt et al., 1996). Initial work on statis