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FIGURE 12.1. The instrumental and paleorecord shows that "weather" and "climate" vary on all timescales, from hours and days to millions of years. Here we tabulate the mechanisms operating at different timescales. Greenhouse gases might also be added to the table: natural CO2 cycles occur on timescales up to 1k—10k y and longer and the human (100 y) timescale for CO2 is now important; methane changes can occur at 10 y timescales out to 10k y and beyond. It should also be noted that nonlinearities make the true separation of timescales impossible.

FIGURE 12.1. The instrumental and paleorecord shows that "weather" and "climate" vary on all timescales, from hours and days to millions of years. Here we tabulate the mechanisms operating at different timescales. Greenhouse gases might also be added to the table: natural CO2 cycles occur on timescales up to 1k—10k y and longer and the human (100 y) timescale for CO2 is now important; methane changes can occur at 10 y timescales out to 10k y and beyond. It should also be noted that nonlinearities make the true separation of timescales impossible.

During the last glacial period, 15k-60ky ago, dramatic discharges of large quantities of ice from the land ice sheets (Heinrich events) have occurred every 10ky or so. During the same period, abrupt warming events have occurred over Greenland and the northern North Atlantic every 1.5k y (Dansgaard-Oeschger oscillations), as will be discussed in Section 12.3.5. Each of these events caused a warming of some 10°C that occurred abruptly, within 20-50 y, lasted a few hundreds of years, and terminated abruptly again. Regional fluctuations on shorter timescales of order 1-2° C have also occurred, such as the Little Ice Age centered over Europe during the seventeenth century. A more recent example of climate fluctuations on timescales of decades and shorter is the dust bowl of the Great Plains in the 1930s. Looking back much further, 100My ago, ice was in all likelihood totally absent from the planet, and deep ocean temperatures were perhaps more than 10°C warmer than today. Scientists even speculate about whether, during periods in the distant past, Earth was totally frozen over in a "snowball." What is clear is that Earth's climate has always changed, and continues to do so, with the added complication that human activities are now also a contributing factor.

Clearly we must try and understand the nature of climate fluctuations on all timescales. This is a vast undertaking. There is no accepted general theory of climate, but many factors are implicated in the control of climate. The most important processes, and the timescales on which they act, are listed in Fig. 12.1. One important lesson from the paleoclimate record is that the climate can change more rapidly than a known forcing. For example, climate is capable of large changes over a short time, as in the massive reglaciation event known as the Younger Dryas, around 12ky ago (see Section 12.3.5). This lasted perhaps 1.5k y or so, but began and ended abruptly. The climate perhaps has preferred states between which it can flip in a discontinuous manner. The mechanisms behind such abrupt climate change are unclear, and are the subject of much current research. Finally, the interpretation of a particular proxy record in terms of climate variables is often uncertain, as is the extent to which it is indicative of regional or global change, issues that are crucial when one is trying to identify mechanisms. In this concluding chapter, then, we begin to explore some of the issues, making use of what we have learned about the observed state of the atmosphere and ocean and the underlying mechanisms. We place an emphasis on the role of the ocean in climate variability.

Heat, water, momentum, radiatively important gases (such as CO2), and many other substances cross the sea surface, making the ocean a central component of the climate system, particularly on the long timescales associated with ocean circulation. Much of the solar radiation reaching the Earth is absorbed by the ocean and land, where it is stored primarily near the surface. The ocean releases heat and water vapor to the atmosphere and its currents transport heat and salt around the globe. As discussed in Chapter 11, meridional heat transport by the ocean makes an important contribution to the maintenance of the pole-equator temperature gradient, and freshwater transport by the ocean is a central component of the global hydrological cycle. The oceanic heat and salt transports are not steady, and fluctuations in them are thought to be important players in climate variability on interan-nual to decadal to centennial timescales and upwards. For example, variability in ocean heat and salt transport may have played a role in ice-age dynamics and abrupt climate change, as will be discussed later in Section 12.3.

We discuss in Section 12.1 the buffering of atmospheric temperature changes by the ocean's enormous heat capacity. In Section 12.2 we discuss atmosphere-ocean coupling in the El Nino-Southern Oscillation phenomenon of the tropical Pacific. Finally, in Section 12.3, we briefly review what we know about the evolution of climate over Earth history: warm climates, cold climates, and glacial-interglacial cycles.

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