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The climate has swung between icehouse climates, characterized by permanent polar ice sheets, and hothouse climates, during which the north and south poles were ice free.The climate varied by 20 degrees C or 36 degrees F.Weather models assume a horizontal grid size of a few kilometers (or miles) covering the Earth surface while climate models have a horizonal grid size of a few hundred kilometers. Figure 3 shows some of the physical phenomena which are calculated at each grid point.
In both cases, scientists do many runs or an ensemble and average the results. Scientists can estimate the annual local climate based on the thickness of tree ring. The only way to hindcast weather is if somebody observed it and wrote it down.Scientists simulate past climates but they cannot simulate past weather.We know and use the statistics but cannot model the process each carbon atom undergoes.Figure 1, shows the Earth climate going back 540 million years spanning the entire Phanerozoic Era.
Let’s deal with the computational intensity, first.