Climate and climate variability
12.1. The ocean as a buffer of temperature change 12.1.1. Nonseasonal changes in SST
12.2. El Niño and the Southern Oscillation
12.2.1. Interannual variability
12.2.2. ''Normal'' conditions—equatorial upwelling and the Walker circulation
12.2.4. Other modes of variability
12.3.1. Climate over Earth history
12.3.2. Paleotemperatures over the past 70 million years: The ô18O record
12.3.3. Greenhouse climates
12.3.4. Cold climates
12.3.5. Glacial-interglacial cycles
12.3.6. Global warming
12.4. Further reading
Climate is frequently defined as the average weather, with an averaging period long enough to smooth out the variability of synoptic systems. Our emphasis in this book has been on understanding the clima-tological state of the atmosphere and ocean in which the averaging period is over many years. Climate "norms," such as those studied in Chapters 5 and 9, are typically based on instrumental data averaged over several decades. However, these norms themselves change, and we know from the paleorecord that climate fluctuates on all timescales. These timescales cannot all be associated with one component of the climate system, but rather must reflect the interaction of its component parts, the atmosphere, ocean, land-surface, indeed all the elements set out in Fig. 12.1.
There have been ice ages during which the temperature in middle latitudes has dropped by 5°C or more, the ice caps have tripled in volume and even more so in surface area, to cover large tracts of North America and Europe. Ice ages have returned roughly every 100k y for the last 800k y or so.1 Indeed during 90% of the last 800ky, Earth has been in a glacial climate and only 10% of the time in interglacial conditions similar to those of today.
'We denote 1000 years by 1ky, 1 million years by 1My, and 1 billion years by 1By.
Timescale thousands of millions of days years years years w
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