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5.4. Conclusion

Throughout this chapter, some of the evidence for episodes of climate change during the course of Earth History has been reviewed, with particular emphasis on the last 2 million years. For each era, the time scale of climate change was explicitly defined and the relevant climatic forcing mechanisms evaluated.

The longest time scales of climate change (107 to 108 years) involve the shifting of the positions of continents and associated mountain building and ocean bathymetry variations. Such mechanisms can account for the supercycles of greenhouse and icehouse worlds during the Phanerozoic (last 550Ma), and the Cenozoic (last 65Ma) climatic deterioration.

Over time scales of 104 to 105 years, the so-called Milankovitch orbital periodicities, which vary the amounts of insolation at the Earth's surface, are invoked to explain the glacial-interglacial transitions that have been well-documented for the Pleistocene Epoch (2Ma to 10Ka). Such external forcing acts as a pacemaker to many non-linear perturbations within the climate system, including CO2 feedbacks, ocean circulation changes and ice-albedo effects, which are required to account for the magnitude of glacial-interglacial climate change.

In the last 10,000 years, climate has fluctuated over decadal to millennial time scales. Volcanism, solar variability and ocean circulation changes have all been suggested, to a varying degree of certainty, as causes of these shorter episodes of climate change.

The state of the climate at any one moment is determined by a combination of all these forcing factors that affect the radiation budget of the global climate system.