The actual timing of the outset of climate change is controlled by heat exchange with the ocean, which has a heat capacity order of magnitude larger than the atmosphere. The chapter discusses the physical processes involved in this transient response. Transient tracers produced by the bomb tests of the late fifties and early sixties provide one source of information on downward pathways from the ocean surface to deeper layers. The downward movement of these tracers offers an interesting analog to the penetration of a heat anomaly originating at the surface, with one important difference. The tracers are neutrally buoyant, while a heat anomaly will change the ocean circulation as it is carried downward in the main thermocline. It is crucially important to identify all the significant processes in the transient response of climate to increasing CO2. For this reason, the chapter uses models of the ocean and atmosphere that are quite complete physically, but other aspects such as the geometry of continents and oceans are deliberately kept as simple as possible.
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