The earth’s climate is the result of complex interactions between many processes in the atmosphere, ocean, cryosphere (snow, ice and permafrost) and on land. Predictions about how the climate is likely to respond to increased greenhouse gases must therefore consider a number of variables. For example, the oceans hold heat and transfer it around the globe, so it is essential to consider the effects of this along with the atmospheric processes.
Global climate models
Our present understanding of the climate system would be impossible without global climate models (GCMs). These are powerful computer programs that simulate climate systems in three spatial dimensions and in time. Climate modelling gauges interactions between the land, ocean and cryosphere. Comparisons between different models and a wide range of data allow scientists to usefully predict changes in climate for future decades or centuries.
Scenarios in New Zealand
To accurately predict human-induced changes in New Zealand’s climate, scientists need to know the global extent of greenhouse gas emissions, future changes in carbon dioxide concentrations, and the influence of New Zealand’s topography on local climate. Each of these factors comes with uncertainties. For example, gauging future emissions relies on anticipating human behaviour, including the success of constraints negotiated under the United Nations Framework Convention on Climate Change. Our understanding of the carbon cycle and of sources of non-carbon dioxide greenhouse gases is also incomplete.
Forecasting regional climate changes in New Zealand from global projections requires complex adjustment, since the global average does not necessarily apply to a given location in New Zealand. A variety of approaches are used to do this, although they all combine global model projections with higher-resolution local climate information.