It took a very long time indeed for humans to determine that the earth is far from the stable home we think it is. Changes in the geometry of the oval orbit along which the earth circles the sun, changes in the earth's tilt over time, changes in the way in which the earth progresses along this orbit, and other, as yet unknown factors all contribute to what can only be described as a very variable climate, both in the short and long term.
For immense stretches of time, the earth has been a hothouse with no trace of ice anywhere, a colossal ball of ice, and every variation in between. Currently we consider ourselves to be in the middle of a pleasant, balmy period, but in actual fact, the earth is locked in the grip of an ice age, and it has been for the last 2 million years. Phrases like "since the last ice age ended" are a dime a dozen, but the truth of the matter is that we are merely in what is known as an interglacial, a warm period sandwiched between much colder, glacial periods. The earth actually first entered this cold phase about 40 million years ago, when the Antarctic ice sheet began to form, but it is only in the last 1.6 million years that the earth's climate has oscillated between long, cold periods (glacials) and short, warmer periods (interglacials).
Scientists have worked out that over the last 1.6 million years, there have been at least seven of these glacial-interglacial cycles, and possibly many more. How can scientists know what the climate was like hundreds of thousands of years ago, when it is still impossible to forecast, with 100 percent accuracy, the weather tomorrow? Every single day, an enduring record of the earth's climate is stored away on the seafloor or in ice sheets. The record deposited on the seafloor is not in words or numbers, but is codified in the remains of microscopic, planktonic organisms
Ice Ages—In 100,000-year cycles, the eccentricity of earth's orbit changes. Less solar radiation reaches the earth during the more eccentric orbit (outer orbit), a factor that is important in triggering ice ages. (Phil Miller)
Ice Ages—In 19,000- to 23,000-year cycles, the direction in which the earth tilts toward the sun switches. When the Northern Hemisphere is tilted away from the sun during the winter equinox (lower orbit), cold conditions prevail and ice ages take hold. The degree to which the earth tilts on its axis also switches, but in 41,000-year cycles. This is the third factor that contributes to the development of ice ages. (Phil Miller)
(foraminifera and coccoliths) that lived at or near the surface of the ocean. These tiny living things secrete a protective shell of calcium carbonate that is often very ornate, and like terrestrial plants, the coccoliths use photosynthesis to convert water and carbon dioxide into food. When these tiny organisms die, they sink to the seabed, leaving tiny shells that build up into sedimentary deposits on the seafloor. Experts who study these shells, micropaleontologists, can identify different species of foraminifera and coccoliths. In life, each species inhabited a narrow range of sea surface temperatures, and so scientists can analyze the layers to determine if ancient surface waters of the ocean were cooler or warmer than today at the same geographic location.
During the past two decades, analyses of ice cores from Antarctica have provided new information on climate variability during the last 800,000 years. As it falls, snow carries with it atmospheric gases such as carbon dioxide and methane in the same concentration as they appear in the air. Over time, in Antarctica, this snow is compacted from firn to ice, and the record of atmospheric composition is trapped in the bubbles in the ice. A long core of this compressed snow is a record of the earth's climate that stretches back for 800,000 years. These cores show that during a full glacial, the concentration of carbon dioxide averaged 180 parts per million (ppm); during the interglacials, the concentration of this gas averaged 280 ppm. However, human activities since the industrial revolution have been pumping carbon dioxide into the atmosphere in ever greater quantities, and in 2008, the concentration of carbon dioxide in the earth's atmosphere exceeded 380 ppm. As a result, the earth is now warmer than it would be without human activity.
These cores enable us to look back in time and to see how the earth's climate has changed over the eons. If we go back as far as 620,000 years, it seems that there have been seven glacial-interglacial cycles, each of which has lasted between 88,000 and 118,000 years. These cycles are dominated by the cold, glacial phases, as the warmer interglacials have only lasted for between 28,000 and 49,000 years.
As an average human life span is around 75 years, we have little appreciation of cycles that are played out over hundreds of thousands of years—all we can ever see are the aftereffects. The implications for life on earth of these continual oscillations between chilly and warm are huge. Land-living animals can migrate in the face of climatic change, but plants, with their roots fixed firmly in the ground, must simply allow their range to recede and expand with the changing conditions. The glacial periods are not only cold, but also dry, conditions that do not favor the growth of dense forests. During these cold periods, forest cover the world over dwindled, and grassland edged in to replace the trees. As the glacial phase ended, the situation reversed, and the forests moved back into their old range. Animals are free to move around, but specialist forest dwellers dwindled or disappeared altogether, while other animals, more suited to open habitats, thrived. These circumstances opened up new habitats every few thousand years, ideal for the evolution of new species that adapted to fill the new niches. Several of the animals in this book—great beasts like the woolly mammoth, mastodon, and woolly rhinoceros—were cold-adapted species that evolved to take advantage of the habitats created by the glacial-interglacial cycles.
So scientists have worked out that for the last 1.6 million years, earth and its organisms have endured cycles of numbing cold interspersed with warmer periods, but what causes these cycles? After lots of experiments and number crunching over many, many years, scientists now have a good idea of what causes these cycles. Beginning in 1930, Milutin Milankovitch, a Serbian geophysicist and astronomer, spent many long hours, days, and years working out a theory of climate change. He determined that the earth's orbit around the sun is not the simple, consistent, circular route it was always assumed to be. The orbit of the earth around the sun varies from an almost circular path to a very elliptical one (eccentricity) over a roughly 100,000-year cycle. Also, the earth is slightly tilted on its axis of spin, a planetary phenomenon that results in the seasons. This tilt varies between 20.4 degrees and 26.2 degrees over a 41,000-year cycle. One more peculiarity in the way that earth moves through space is that it wobbles on its axis (precession), in the same way as a spinning top set in motion on a flat surface will wobble. One complete cycle takes about 21,000 years, and it also changes the way that observers on the ground see the night sky. Today, the earth's north pole points at the pole star, Polaris, but 13,000 years from now, the pole star will be Vega because of precession. Depending on how these cycles overlap, less solar energy ends up reaching the Northern Hemisphere—the suggested trigger of the cold glacials. More snow falls during these cold spells, less thaws, and the ice sheets start to grow. As they grow, more and more solar energy is reflected back into space by the white snow and ice, and the cooling effect is exacerbated.
Geologically, the current epoch is known as the Holocene. The Holocene is actually a warm interglacial, and it has lasted, so far, for 10,000 years. It's a sobering thought that all recorded human history, at the very beginning of which agricultural civilizations began to replace hunter-gathering as a way of life, has been played out in a relatively warm interglacial. All the known civilizations, all the wars, all the technological advances have come to pass in a narrow, warm window. It would be very naïve for the human race to think that these balmy conditions are going to last forever, and the deep sea and ice cores have shown us that the transition from warm to cold can be astonishingly quick—a couple of decades. These rapid changes are recorded in ice cores from Greenland, and there is no reason to think that the next transition won't be similarly swift. Climate change is a hot topic at the moment; it appears everywhere in the news, and it seems that the earth's climate is beginning to destabilize, which some people think heralds a new climatic age. If this is the case, what are we heading into? The mass media have exhausted the term global warming, and it is highly likely that the observed increases in temperature are likely to prolong the present interglacial, the Holocene, but eventually, the natural variations in the earth's orbital elements will lead to another ice age. Over thousands of years, humans, as a species, have become adapted to the relatively easy time afforded by the Holocene. When the earth enters another ice age (and inevitably, it will), our current way of life will be impossible, and the human race will be pushed toward extinction like countless other species over time. In the unlikely event of the earth warming up by a few degrees and staying warm for the next few millennia, the human race would be similarly challenged, and our survival would balance on a knife's edge. In particular, melting of the glacier ice will raise global sea levels, inundating low-lying islands, deltas, and coastal plains. During the last interglacial, sea levels rose by about 6 m, probably due to the near-complete melting of the Greenland ice sheet. Responding to rises in sea level will present major challenges to many nations, including dealing with the displacement of many human populations.
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