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interglacial climate we now enjoy. Obviously, the record is rather "noisy," but look in particular at the period between 20,000 and 35,000 years before the present (b.p.). During this time, the climate seems to switch periodically between a warm phase and a cool phase roughly seven or eight degrees cooler. This switching also occurs during the climb out of the ice age, beginning around 17,000 years b.p. The warming trend is clear, but twice, during the Older and Younger Dryas periods, the Earth plunged back into extended periods of cold. Only at about 8,000 years b.p. or so does the climate—represented in Figure 12.1 by the air temperature over the Greenland ice sheets—finally climb into the relatively steady and equable interglacial range.

This behavior is very reminiscent of a type of control known as ON-OFF regulation, actually the most common method for thermostatic control. ON-OFF control is most easily understood by comparing it with proportional control, which is exemplified by mammalian thermoregulation (Chapter 11). In proportional control, the rate of heat production by the organism varies proportionally with the magnitude of the error in tem-perature—that is, the difference between the set temperature and the actual temperature of the system. ON-OFF control operates by switching a heater either fully on or fully off: heat production rate is modulated by the duration of the ON-phase relative to the OFFphase, the so-called duty cycle.

If the oscillation of climate reflects the operation of a "climate switch," what then controls the switch? There is, in fact, a strong physical component in the switching mechanism. The actual energy required to throw the switch may be small, however, perhaps small enough that the biota might help trip it—and thereby regulate climate to a limited degree.

The Oceanic Heat Conveyor

The identity of the switch controlling the climate is not clear at present, but it is probably safe to conclude that changing patterns of ocean circulation are impor warm, dilute k v Indian-Pacific anticonveyor

cold, briny

Southern Ocean raceway (deep)

Figure 12.2 The major circulation patterns in the world's oceans. [After Broecker (1997)]

tant. Water in the oceans is a major conveyor of heat absorbed from the Sun. Because the distribution and movement of heat are what control weather, these patterns of oceanic circulation are probably the major determinants of the Earth's climate. The current thinking, so to speak, is that the climate switch is driven by the ocean switching between two very different patterns of circulation.

Presently, water circulates through the world's oceans along three major pathways (Fig. 12.2). One is in the Southern Ocean that surrounds Antarctica, the only ocean where the east-to-west movement of water is not blocked by continents. The Earth's rotation imparts a strong east-to-west movement to these waters, forming a current known as the Southern Ocean raceway. The Atlantic conveyor circulates water between the Southern Ocean and the north Atlantic, as far north as Greenland. Finally, the Indian-Pacific anti-conveyor circulates water between the Indian and Pacific oceans and the Southern Ocean. In the Pacific, the anticonveyor transports water up to the Bering Straits.

All three currents have strong vertical components to them, driven by gravity acting on variations in water density. In the Southern Ocean raceway, water along the ice shelves freezes, producing a dense, cold brine, which then sinks to the abyssal depths as a skirtlike vertical current surrounding Antarctica. The Atlantic conveyor is fed by the comparatively warm and

Southern Ocean

Southern Ocean

Southern Ocean raceway (deep)

Figure 12.2 The major circulation patterns in the world's oceans. [After Broecker (1997)]

buoyant surface waters of the Southern Ocean, and these are transported northward as a surface current. As this water moves north through the tropics and across the Equator, it loses water by evaporation, which increases its density. At the same time, it is also warmed, which makes it buoyant and partially compensates for the effects of evaporation. Once the conveyor has passed the tropics, though, it loses heat to the now cooler atmosphere, giving northern Europe the relatively benign climate it now enjoys. As these surface waters cool, the combined effects of evaporation and cold make them dense enough to sink. At the lower depth they feed the return leg of the Atlantic conveyor as a cold, salty current.

The Indian-Pacific anticonveyor also sends water north, but here it is the northward current that is cold and salty, fed from the deep waters of the Southern Ocean raceway. At its northern extreme near the Bering Straits, these waters are mixed with relatively brackish waters from the Arctic Ocean, which receive considerable quantities of freshwater runoff from Siberia and North America. Now the water in the anticonveyor is lighter, so it rises and then feeds a return surface current of relatively warm and dilute water. When this water joins the Southern Ocean raceway, heat may be transferred between the Indian and Pacific oceans and the Atlantic.

These patterns of circulation probably are common to interglacial periods. The connection is not hard to see: the more widely heat can be distributed across latitudes, the more temperate climates will be across the globe. The tropics and equatorial regions will be cooler than they otherwise would be, because heat is transported away from these regions to the poles. Similarly, the temperate and polar regions will be warmer than they otherwise would be, because their climates receive a subsidy of heat from the tropics.

Ice ages, on the other hand, probably correlated with different patterns of ocean circulation. Most likely, ice ages come about when the conveyor systems shut down. If that happens, heat absorbed by waters at tropical latitudes would tend to stay there, rather than being distributed to higher latitudes as it now is. Tropi

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