More about analogues and GCMs
Analogues
Climatic analogues are developed by choosing warmer-than-average years from the historical record and identifying what conditions were like, to get an idea of the likely effects of global warming. The limitation of this method is that our relatively short period of climatic data does not represent conditions that may come with future climate change, for example, unprecedented long-term increases in temperature, particularly in wintertime.
GCMs
The GCMs simulate atmospheric circulation, the energy exchanges, and other important land/ocean/atmosphere interactions. However, they cannot model well other small-scale processes, such as biological processes, precipitation and cloud cover. These processes have significant effects on the water resource.
GCMs usually project climate over several decades to more than a century ("climate" being based on average weather conditions over 10 years or more), but give only large-scale predictions, not the regional ones needed for planning. This is because grid spacing in most GCMs is between 2 and 5 degrees of longitude or latitude and it takes several grid distances to simulate a physical feature.
The predictions are complicated by difficulty in simulating the lags in the meteorological system, especially in the oceans; the effects of low cloud versus high cloud, and amounts of each; and temporary effects due to solar variability and airborne ash from volcanic eruptions, such as that of Mount Pinatubo.
Also difficult to predict are the results of the interactions of cloud cover, soil moisture, vegetation, and ice. How any one of these factors might react to a warmer climate is difficult to predict; when reactions from several such factors might interact, the uncertainty is magnified.
To improve predictions, we need computers with more computing capability. We also need more data on and understanding of the processes affecting climate. These would help us to build more accurate models and to validate model results against observed or measured processes.
To sum up, analogues and GCMs do not accurately predict actual future climate; but, they can give us some basis for preparing for the future.
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