The fate of so many human societies has been so contingent upon atmospheric and oceanic conditions that it has become difficult to overstate climate's importance in shaping History as we know it. Indeed, it is widely recognized that the relative climate stability of the Holocene (roughly, the past 10,000 years of the Earth's history) has been the necessary condition for the growth of human populations, their sedentarization, the domestication of crops and large mammals, the development of large-scale food production and technology, and their spreading among different parts of the globe along similar biomes, chiefly defined by their climate [Diamond, 1999]. The backdrop for what we term Civilization, its origins, evolution and expansion across continents and overseas, seems to have been largely conditioned by climate.
Yet, within the seemingly quiescent Holocene, intense droughts catalyzed the demise of highly organized human societies (Akkadian Empire, Tiwanaku and Classic Mayan civilizations), often in a matter of a few years [deMenocal, 2001; Haug et al., 2003]. It is the Trade winds and their northeasterly direction that pushed Columbus to the Caribbean to find "Indians". The people of Japan partly owe the idiosyncrasy of their culture and language to a Mongol invasion reduced to nothingness by a tropical cyclone in 1281, thereafter named typhoon, or "divine wind" [Emanuel, 2005]. Closer to us, former commerce secretary William Daley estimates that "at least $1 trillion of [the U.S.] economy is weather-sensitive"*. Still, it would be excessive to claim that weather and climate are the sole determinants of human history and economics - to quote Gordon Manley, "the fall of Rome should not be attributed to a joggle of the
barometer". Nonetheless, it is now clear that they have had a pervasive influence on human affairs, at times a critical one [Davis, 2001].
This growing awareness, and the necessity to better understand the dynamics of the Earth's climate, have underpinned a tremendous research effort in the past 50 years. Prominent in such a development has been the increasing recognition that anthropogenic emissions of greenhouse gases (carbon dioxide and methane, chiefly), have been warming the global mean temperature by about 0.8°C since 1860 (http: // data.giss.nasa.gov/gistemp/2005/), a sizable change only expected to worsen as emissions grow [Houghton, 2001], and to which many human and biological systems will have trouble adapting [McCarthy, 2001]. It then becomes critical to understand how the climate of our planet functions and how it could change under external forcing.
Why low frequencies?
Just as history informs us about the nature of human beings and dynamics of the complex social entities they form, so do past climates inform us about the dynamics of Earth's climate system. By teaching us about the vastness of its parameter space, they provide a crucial testbed for the numerical models used to forecast its future evolution. As Edward Gibbon put it, "we know of no way of judging the future but by the past". One of the past's most salient messages, coming from all manner of geological and instrumental sources, is the demonstration that climate displays most of its variability on the longest timescales - the lowest frequencies - as epitomized by the Ice Ages of the Quaternary. Vexingly enough, we still don't know precisely why the latter occurred. Other sources of low-frequency variability remain at the forefront of modern climate research and forecasting. What do we mean by low-frequency? Here we face a problem of definition, as the lowest resolvable frequency a dataset may offer is inversely proportional to its length. In the 1970s, the term "low-frequency" described oceanic phenomena with a time-scale longer than a few seasons, and atmospheric scales longer than a few days. In the three subsequent decades, an ever-expanding stream of data has been gathered, showing almost ubiquitously that a climate record exhibits significant (if not dominant) variability on the longest timescale it resolves. As timeseries grew longer, spectra went redder. In our case, the lowest frequency we shall consider is determined by the availability of reliable data allowing a quantitative assessment of climate variability. This criterion is undoubtedly subjective, and here we have chosen to restrict our focus to timescales of 10 to 105 years: decades to Ice Ages.
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