The Greenland ice core samples tell us that the Holocene period [the current geological period] has had alternating periods of cooling and warmth within a relatively stable range. The Holocene, at onset, stumbled at first. The retreat from the last ice age was interrupted by a brief but pronounced cooling called the Younger-Dyas period, which lasted about 1,200 years starting around 12,000 B.B. Homo sapiens was already established as a species and had been using tools since before the previous ice age. Their numbers and range had been increasing since the glaciers retreated. The Younger-Dyas episode suspended the retreat of the glaciers temporarily and might have provoked the transition from hunter-gatherer culture to agricultural “takeoff.” Cooling in the region now called the Middle East would have transformed forested terrain into grassland. Agriculture began as the domestication of cereal grasses. This learning process appears to have taken a few thousand years to complete, during which the climate warmed again and remained unusually benign, The glaciers resumed their retreat. . . .
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Climatic shifts in more recent times account for historical trends with implications for our present situation. A cooling followed the height of the Roman Empire, bringing, for instance, more rain to England, affecting adversely the health of livestock, and making marginal farming lands submarginal. The Dark Ages were also cool ages. A medieval warming occurred between the ninth and fourteenth centuries; this accounts for historical events such as the settlement of Greenland by the Vikings and the cultivation of vineyards in England. During this period, the human population bloomed with the feudal method of social organization. Toward the end of the medieval warming, the Black Death depleted the European population by as much as one-third and created labor shortages that put an end to feudal social organization. The Renaissance began in Italy as a slight cooling occurred; labor shortages following the Black Death enhanced the value of the individual. A more pronounced dip in temperature produced the Little Ice Age in Europe from the 1500s into the mid-1800s – as chronicled, for instance, by the Dutch landscape painters who show people skating on the frozen canals of Holland, which in the modern era no longer freeze. The average temperature differential between these two periods was only a few degrees, yet the effects were marked. The cooling of the Little Ice Age produced the deforestation of England and the increased use of coal, and therefore led to inventions for improving coal extraction, namely the steam-powered pump for removing water from coal mines, which soon led to steam-powered railroads and the whole industrial explosion, in which more versatile oil and gas came to replace coal. The brief fossil fuel interval of the last two hundred years has accompanied another warming period, perhaps even stimulated it. And, of course, the amenity of oil has permitted a twentieth-century human population boom like nothing ever seen before. These historical temperature fluctuations, however, may have been minor compared to what we are facing now, especially in light of our depleting oil and gas supplies.
Stepping back to view the larger-scale picture is sobering. The Greenland ice core record shows that the past 100,000 years have been a climatic roller coaster. It is clear that once climate change begins, it can occur very erratically, a kind of “speed wobble” that ends in a crash. The climax of the last ice age, for instance, was 21,000 years ago, when glaciers extended as far south as what is now Connecticut. The transition from the last ice age into the present Holocene was intensely wobbly, including the Younger-Dyas episode. As Elizabeth Kolbert reports: "The temperatures did not rise slowly or even steadily; instead the climate flipped several times from temperate conditions back to those of the ice age, and back again. Around fifteen thousand years ago, Greenland abruptly warmed by sixteen degrees in fifteen years or less. In one particularly traumatic episode some twelve thousand years ago, the mean temperature in Greenland shot up by fifteen degrees in a single decade."
It appears that the earth has gone in and out of ice ages on a fairly regular cyclical basis for at least one million years, though the individual cycles show idiosyncrasies of their own. The past interglacial warm period that seems most to resemble the present Holocene is the Eemian, running approximately 130,000 to 110,000 years ago (the point at which the Greenland ice record ends at bedrock). The complete transition from the Eemian warm period to the ice age that followed took no more than 400 years. As the cold grew more severe, the earth’s climate also became drier. Water evaporated less effectively from the ocean at colder temperatures and rainfall on land decreased, though ice accumulated from the poles downward. Forests all over the world gave way to drier grasslands and deserts. A slight warming occurred about 60,000 years ago, and then at 30,000 years another cycle of intense cooling and glaciation occurred, which peaked about 21,000 years ago. Around 14,000 years ago there was a rapid global warming and moistening, perhaps occurring within the space of only a few years or decades. The planet was then well on the way to the present Holocene interglacial period – if that’s what it is.
We may now be entering a climate speed wobble, which is being aggravated by mankind’s release of heat-trapping carbon dioxide. Carbon dioxide (CO2) is only one of several so-called greenhouse gases that tend to trap heat in the earth’s atmosphere, or prevent it from radiating into space, and it is the least effective by far of the three main culprits. The most effective heat-trapping agent is water vapor. The next one is methane, approximately twenty times more effective as an earth insulator than CO2. Methane is a by-product of agriculture (especially from rice paddies or released in the excrement of domestic animals) or produced by decay in swamps (methane is also known as “swamp gas”) or by the thawing of organic matter in the warming tundra. That said, however, carbon dioxide is certainly an effective greenhouse gas, and the amount of carbon dioxide as a percentage of the atmosphere today has not been so high since the days of the dinosaurs. By putting large quantities of greenhouse gases into the atmosphere, humans are exerting pressure on an inherently unstable climate system that might produce a drastic change without much prior warning.