12.1. Antarctic ice cores show large CO2 drops during the last 2,000 years compared to the long-term rising trend (shaded).
full-glacial and full-interglacial climates. During some of these oscillations, huge numbers of icebergs broke away from the coastal margins of the ice sheets, floated out into the Atlantic, and dropped enormous quantities of rock and mineral grains into the ocean sediments.
These relatively brief oscillations are superimposed on top of the slower, longer-term climatic changes at orbital cycles, and they appear to be the result of an entirely different phenomenon that is not yet well understood. Consider a simple analogy to something familiar. The daily (diurnal) heating cycle is driven directly by the Sun, with maximum warmth in late afternoon, and maximum cooling in the hours just before the Sun rises. These changes occur day after day in a predictable cycle. But on some days, especially in summer, storms build up in the afternoon, hide the Sun, drench the Earth with rain, and bring much cooler temperatures for an hour or two. In much the same way that these unpredictable storms briefly cool the predictable heat of the afternoon Sun, these shorter-term climatic oscillations over time scales of decades to millennia have ridden on the back of the longer and more predictable changes that occur over orbital time scales.
The reason these shorter oscillations are much smaller during times like today when the North American and Eurasian ice sheets are absent or reduced in size is not understood. The most recent large oscillation in climate occurred nearly 12,000 years ago, when the ice sheets were still large, followed by a smaller one
8,200 years ago, when the ice was almost gone. Since then, during the time humans created primitive and then advanced civilizations, these oscillations have been much smaller.
The largest interruption in this interval of relatively stable climate was the cooling during the Middle Ages called the Little Ice Age. This interval is variously considered to have lasted for as long as AD 1250—1900 to as short an interval as 1550—1850. The Little Ice Age is thought to have been preceded by a slightly warmer interval called the medieval climate optimum, with maximum warmth occurring around 900-1200. The Little Ice Age came to an end with the industrial-era warming of the 1900s that continues today.
In some places, the "Little" Ice Age was a very big deal indeed. Most such regions lie near the modern-day limits of Arctic snow and ice, where changes in climate are greatly amplified. For those alpine villagers who saw glaciers descend some 100 meters (more than 300 feet) down the side of mountains and grind their farms and villages to rubble, the Little Ice Age was certainly a big deal. These dramatic disruptions had been the basis for Louis Agassiz's realization that continent-sized ice sheets had once existed (chapter 4). Numerous etchings and some early photographs record the last of these events in the late 1800s. As the ice advanced, the upper tree line also descended. Intervening glacier retreats in prior centuries showed that the Little Ice Age was not a time of unrelieved cold, although on average it was cooler than today.
The coldest decades of the Little Ice Age were also a big deal for those farmers trying to grow frost-sensitive crops like corn and grapes at high latitudes and altitudes along the limits where such crops were barely possible even at the best of times. During some years and decades, the crops were destroyed by unseasonable freezes, or the harvest dates were delayed by summers with persistent cold or rain. Vineyards that had been started in Britain during the climatic optimum were progressively abandoned as the Little Ice Age intensified, and the northern limit for growing grapes in France and Germany retreated some 500 kilometers to the south. Cereals could no longer be cultivated in the hills of northern and western Britain.
Another place where the cold of Little Ice Age winters made a big difference was Iceland, a country that has long been highly dependent on its cod and other fisheries not just for commerce but also for basic food and survival. For many winter days in the Little Ice Age, the sea-ice pack surrounded the northern seaports and the fleet had to stay in port. Homebound Icelanders with little else to do kept reliable records of the number of days per year when the impassable barrier of ice blocked their fleets from reaching the sea (fig. 12.2). Sea ice was rare in AD 1000-1200, increasingly abundant between 1200 and 1800-1900, and rare again in the late 1900s.
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