During the past two million years, huge ice sheets have advanced across the Northern Hemisphere and retreated again more than twenty times, tte great, extended ice sheets last roughly 100,000 years, keeping the climate cold and the sea level low. tte warm periods that punctuate the cold spells have been exceptional, each lasting roughly 10,000 years, tteir scientific name, the interglacials, reflects their unusual nature.
tte most recent advance, called the Wisconsin, started about 120,000 years ago in Canada, Scandinavia, and Siberia. By the time the ice had spread to its maximum southern extent, most of northern Europe, New England and the Midwestern United States were buried under ice a kilometer thick, tte sea level had fallen to about 100 meters lower than it is today, and it would have been possible to walk from England to France, from Siberia to Alaska, and from New Guinea to Australia.
tte present interglacial, called the Holocene, began about 10,000 years ago, when the world became warmer and wetter, about 5-kelvin warmer on average, tte ice sheets melted and shrank back to their present-day configurations, leaving only the glacial ice in Greenland and parts of arctic Canada, as well as the massive ice sheets of Antarctica, and the sea level rose rapidly around the world.
Ms warm spell enabled civilization to flourish. Agriculture, and technology were developed. And during all that time populations grew throughout the world.
tte ponderous ebb and flow of the great continental glaciers are affected by three astronomical rhythms that slowly alter the distances and angles at which sunlight strikes the Earth (Fig. 9.13). ttey are sometimes called the Milankovitch cycles, after the Serbian engineer Milutin Milankovitch (1879-1959) who described how variations in the planet's orbit, wobble and tilt could influence the pattern of incoming solar radiation at different locations on the globe. Joseph Alphonse Adhemar (1797-1862), a French mathematician, previously suggested in 1842 that the ice ages might be due to variations in the way the Earth moves around the Sun, and James Croll (1821-1890), a self-taught Scotsman, took up the idea in greater detail in 1864, showing how long periodic variations in the Earth's distance from the Sun might change the terrestrial climate. But the theory received its fullest mathematical development from 1920 to 1941 by Milankovitch.
tte shortest astronomical rhythm is a periodic wobble, or precession, in the Earth's rotation axis that is repeated in periods of 23,000 years. It determines whether the seasons in a given hemisphere are enhanced or weakened by orbital variations. A longer periodic variation, of the Earth's axial tilt from 21.5 degrees to 24.5 degrees and
Thousands of Years Ago FIG. 9.13 Astronomical cycles cause the ice ages The advance and retreat of glaciers are controlled by changes in the Earth's orbital shape or eccentricity, and variations in its axial tilt and wobble. They alter the angles and distances from which solar radiation reaches Earth, and therefore change the amount and distribution of sunlight on our planet. The global ebb and flow of ice is inferred from the presence of lighter and heavier forms of oxygen, called isotopes, in the fossilized shells of tiny marine animals found in deep-sea sediments. During glaciations, the shells are enriched with oxygen-18 because oxygen-16, a lighter form, is trapped in glacial ice. The relative abundance of oxygen-18 and oxygen-16 (top) is compared with periodic 41,000-year variations in the tilt of the Earth's axis (middle) and in the shape, or eccentricity - longer 100,000-year variation, and wobble, or precession, of the Earth's rotation axis - shorter 23,000-year variation (bottom).
back again, occurs every 41,000 years. It is currently 23.5 degrees, and accounts for our yearly seasons, tte greater the tilt is, the more intense the seasons in both hemispheres, with hotter summers and colder winters.
tte third and longest cycle is due to a slow periodic change in the shape of the Earth's orbit every 100,000 years. As the orbit becomes more elongated, the Earth's distance from the Sun varies more during the year, intensifying the seasons in one hemisphere and moderating them in the other.
tte astronomical theory for the recurring ice ages was not strongly supported until 1976 when American climate scientists James D. Hays (1933- ) and John Imbrie (1925- ) and English geophysicist Nicholas J. "Nick" Shackleton (1937- ) published a key paper on the subject, entitled "Variations in the Earth's Orbit: Pacemaker of the Ice Ages" on 10 December 1976 in Science, ttey used an analysis of different types, or isotopes, of oxygen atoms in deep-sea sediments to infer the proportion of the world's water frozen within the glacial ice sheets at different times, revealing all three astronomical rhythms, with a dominant 100,000-year one. Analysis of ice cores drilled in Antarctica and Greenland between 1985 and 2005 confirmed that the major ice ages are initiated every 100,000 years by orbital-induced changes in the intensity and distribution of sunlight arriving at Earth.
It was somewhat surprising that the glaciers have advanced and retreated in synchronism with this longer rhythmic stretching of the Earth's orbit, tte shorter cycles have a greater, direct effect on the seasonal change in sunlight, but apparently produce smaller changes in ice volume than the longer one that has a weaker seasonal effect. By itself the 100,000-year cycle does not appear strong enough to bring about direct alterations of the terrestrial climate, so it must be leveraged by some other factor.
Successive layers of frozen atmosphere have been laid down in Greenland and Antarctica, providing a natural archive of the Earth's climate over the past 420,000 years. Bubbles of air trapped in falling snowflakes and entombed in ice are deposited every year, building up on top of each other like layers of sediment. When extracted in deep ice cores, the air bubbles trapped in the ancient ice indicate that the Antarctic air temperatures and atmospheric heat-trapping gases rose and fell in tandem as the glaciers came and went and came again.
tte temperatures go up whenever the levels of carbon dioxide and methane do, and they decrease together as well (Fig. 9.14). Scientists cannot yet agree whether an increase in greenhouse gases preceded or followed the rising temperatures, but the heat-trapping-gas increase does answer the riddle of why the largest climate variations occur every 100,000 years. Although the orbital variation in the intensity of incident solar radiation is far too small to directly create the observed temperature changes, the build up of greenhouse gases apparently amplifies effects triggered and timed by the rhythmic 100,000-year orbital change in the distance between the Earth and the Sun.
Since the present period of interglacial warmth has lasted about as long as any other interglacial, another ice age might soon be on its way. In fact, humans were most likely contributing significant quantities of greenhouse gases for thousands of years before the industrial revolution, offsetting the natural cooling trend of an approaching ice age. Clearing forests to increase tillable land would have produced carbon dioxide, and methane must have been released by irrigating extensive rice fields in Asia, tte extra gases probably offset declines associated with variations in the Earth's orbit, and
FIG. 9.14 Ice age temperatures and greenhouse gases Ice-core data indicate that changes in the atmospheric temperature over Antarctica closely parallel variations in the atmospheric concentrations of two greenhouse gases, carbon dioxide and methane, for the past 160,000 years. When the temperature rises, so does the amount of these two greenhouse gases, and vice versa. The strong correlation has been extended by a deeper Vostok ice core, to 3,623 meters in depth and the past 420,000 years. The carbon dioxide (parts per million per volume) and methane (parts per billion per volume) increases may have contributed to the glacial-interglacial changes by amplifying orbital forcing of climate change. The ice-core data does not include the past 200 years, shown as dashed and broken lines at the right. They indicate that the present-day levels of carbon dioxide and methane are unprecedented during the past four 150,000-year glacial-interglacial cycles. (Adapted from Claude Lorius, EOS, Vol. 69, No. 26, 1988.)
the associated distribution of sunlight on the globe, which would have otherwise made the planet colder and perhaps triggered an ice age.
Because the current level of greenhouse gases, recently deposited in our atmosphere by humans, far surpasses any natural fluctuation of these substances recorded during past ice ages (Fig. 9.14), we are not sure if the next ice age will dampen future global warming or whether recent global warming is delaying the coming of the ice. But when we completely exhaust oil supplies, in 100 years or less, and when our sources of coal and natural gas are also depleted, perhaps in a few hundred years, the ice should be on its way.
It is very hard to predict the distant future over much longer intervals of millions of years. We now live at times when polar ice caps exist, and the recurrent ice ages occur. But on a time scale of 100 million years, drifting continents can remove land from the polar regions, and permanent glaciers will not form there. A hundred million years ago, when the dinosaurs roamed the Earth, the climate was some 15-kelvin warmer everywhere than it is today, and there were no polar ice caps. Indeed, over most of its 4.6 billion years the Earth has probably been free of ice, even at its poles.
During the past 200 million years, powerful internal forces have reshaped the Earth, producing steady, irreversible changes in our global climate. Drifting continents collided to create towering mountain ranges, or split open to make way for new oceans, altering the flow of air or sea, which influence the climate, and paving the way for the growth of continental ice sheets. All of these forces continue today.
So, we really don't know if the distant future is one of fire or ice. I would side with the American poet Robert Frost (1874-1963) who wrote:
Some say the world will end in fire. Some say in ice.
From what I've tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough ofhate
To say that for destruction ice
Is also great
And would suffice.46
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