It became almost plausible in the hands of the Serbian engineer Milutin Milankovitch. Working in the s and s, he not only improved the tedious calculations of the varying distances and angles of the Sun's radiation, but also applied an important new idea. Suppose there was a particular season when the sunlight falling in a given hemisphere was so weak, even in the summer, that the snow that fell in high latitudes in winter did not all melt away?
It would build up, year after year. As others had pointed out, a covering of snow would reflect away enough sunlight to help keep a region cold, giving an amplifying feedback. Under such circumstances, a snowfield could grow over the centuries into a continental ice sheet. Milankovitch ground through calculations for the slow variations of the angle that sunlight fell in that particular zone, especially in summer. By the s, some climate textbooks were teaching that Milankovitch's theory gave a plausible solution to the problem of timing the ice ages. Supporting evidence came from "varves," a Swedish word for the pairs of layers seen in the mud covering the bottom of northern lakes.
Each year the spring runoff laid down a thin layer of silt followed by a settling of finer particles. From bogs and outcrops where the beds of fossil lakes were exposed, or cores of slick clay drilled out of living lakes, researchers painstakingly counted and measured the layers. Some reported finding a 21,year cycle of changes. That approximately matched the timing for a wobbling of the Earth's axis which Milankovitch had calculated as a crucial element namely, the cycle of seasons determined by the 26,year precession of the equinoxes combined with a longer precession of the entire elliptical orbit.
Most geologists, however, dismissed the astronomical theory. A generation of geologists had laboriously constructed this sequence from studies around the world of surface features, such as the gravel deposits moraines that marked where glaciers had halted.
The Milankovitch theory, wrote one authority condescendingly in , had served a useful function as "a dogma of faith" that had stimulated research, but compared with the actual glacial record, the orbital chronology "must be stamped as illusory. Since the astronomical theory relied upon an increase in the sunlight falling on one hemisphere along with a decrease on the other hemisphere, many experts considered the world-wide pattern of ice ages a devastating refutation.
The computed variations in the angle and intensity of incoming sunlight were only tiny changes, "insufficient to explain the periods of glaciation. That same year, a leading American planetary scientist wrote a European colleague to ask how the astronomical theory stood over there, remarking that "People I have consulted in this country They are of the opinion that the theory cannot account for past changes. The effects are too small and the chronology of the occurrence of glaciation is so uncertain that any correspondence That was still anybody's guess.
The tool that would unlock the secret was constructed in the s, although it took scientists a decade to make full use of it.
How Glaciers Work
This tool was radiocarbon dating. It could tell with surprising precision the age of features like a glacial moraine. You only needed to dig out fragments of trees or other organic material that had been buried thousands of years ago, and measure the fraction of the radioactive isotope carbon in them.
Of course researchers had to devise and test a number of laboratory techniques before they could get trustworthy results. Once that was done, they could assign a timescale to the climate fluctuations that had previously been sketched out by various traditional means. The best of these means, in the s, was pollen science. The study of ancient climates had turned out to be invaluable for identifying strata as an aid to oil exploration, and that had paid for specialists who brought the technique to a high degree of refinement.
But other carbon dates seemed altogether out of step with the Milankovitch timetable. The swift postwar development of nuclear science meanwhile fostered another highly promising technique. In , the nuclear chemist Harold Urey discovered a way to measure ancient temperatures.
The key was in the oxygen built into fossil sea shells. Emiliani measured the oxygen isotopes in the microscopic shells of foraminifera, a kind of ocean plankton. Tracking the shells layer by layer in long cores of clay extracted from the seabed, he found a record of temperature variations. Emiliani's paper, a landmark of paleoclimatology, provided the world's first high-quality record of ice age temperatures.
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Historians usually treat techniques as a stodgy foundation, unseen beneath the more exciting story of scientific ideas. Yet techniques are often crucial, and controversial. The stories of two especially important cases are explored in short essays on Uses of Radiocarbon Dating and Temperatures from Fossil Shells.
Emiliani tentatively identified the rises and dips of temperatures with the geologists' traditional chronology of the past three ice ages. His efforts were motivated largely by a desire to learn something about the evolution of the human race, which had surely been powerfully influenced by the climate shocks of the ice ages.
But his results turned out to tell less about the causes of human evolution than about the causes of climate change. To get a timescale connecting the temperature changes with depth down the core, he made carbon measurements covering the top few tens of thousands of years farther back there was too little of the isotope to measure. That gave him an estimate for how fast sediments accumulated on the seabed at that point. Emiliani now found a rough correlation with the varying amount of sunlight that, according to Milankovitch's astronomical calculations, struck high northern latitudes in summer.
To get the match he had to figure in a lag of about five thousand years. That seemed reasonable, considering how long it would take a mass of ice to react. The chemist Hans Suess, another graduate of Urey's lab, took the lead in improving the carbon chronology. He reported, among other things, that the last ice age had come to a surprisingly abrupt end, starting sometime around 15, years ago. Looking farther back, Suess found hints of a roughly 40,year cycle, which sounded like the 41,year cycle that Milankovitch had computed for slight variations in the inclination of the Earth's axis.
To resolve the issue, Emiliani began urging colleagues to launch a major program and pull up truly long cores, a hundred-meter record covering many hundreds of thousands of years. But for a long time the drillers' crude techniques were incapable of extracting long, undisturbed cores from the slimy ooze.
As one of them remarked ruefully, "one does not make wood carvings with a butcher's knife. In the early s suggestive new evidence was dug up literally by the geochemist Wallace Broecker and collaborators. Ancient coral reefs were perched at various elevations above the present sea level on islands that geological forces were gradually uplifting.
The fossil reefs gave witness to how sea level had risen and fallen as ice sheets built up on the continents and melted away. The coral could be dated by hacking out samples and measuring their uranium and other radioactive isotopes. These isotopes decayed over millennia on a timescale that had been accurately measured in nuclear laboratories.
Unlike carbon, the decay was slow enough so there was still enough left to measure after hundreds of thousands of years. As a check, the sea level changes could be set alongside the oxygen-isotope temperature changes measured in deep-sea cores. Again the orbital cycles emerged, plainer than ever. At a conference on climate change held in Boulder, Colorado in , Broecker announced that "The Milankovitch hypothesis can no longer be considered just an interesting curiosity.
That could happen if the climate system were so delicately balanced that a small push could prompt it to switch between different states. Meanwhile oceanographers managed to extract a fine set of cores that reached back more than , years. Analyzing the cores, Emiliani announced he could not make the data fit the traditional ice ages timetable at all. He rejected the entire scheme, painstakingly worked out around the end of the 19th century in Europe and accepted by generations of geologists, of a Pleistocene epoch comprising four major glacial advances alternating with long and equable interglacial periods.
Emiliani said the interglacials had been briefer, and had been complicated by irregular rises and falls of temperature, making dozens of ice ages. Most significantly, he believed the sequence correlated rather well with the complex Milankovitch curve of summer sunlight at high northern latitude. Calculating how the cycle should continue in the future, in Emiliani predicted that "a new glaciation will begin within a few thousand years. Seldom was such work straightforward. Geologists defended their traditional chronology passionately and skillfully. For a few years they held their ground, for it turned out that Emiliani's data on oxygen isotopes taken up in plankton shells did not directly measure ocean temperatures after all.
Emiliani fiercely defended his position, but other workers in the late s convinced the scientific community that he was mistaken. When water was withdrawn from the oceans to form continental ice sheets, the heavier and lighter isotopes evaporated and fell as rain or snow in different proportions. The way plankton absorbed oxygen at a given temperature mattered less than what proportion of each isotope was available in the sea water as ice sheets came and went. Yet in a deeper sense Emiliani was vindicated. Whatever the forces that changed the isotope ratio, its rise and fall did represent the coming and going of ice ages.
These changes did turn out to correlate with ocean surface temperatures. New evidence for that came from scientists who took a census of the particular species of foraminifera, recognizing that the assemblage of different species varied with the temperature of the water where the animals had lived. The data confirmed that there had been dozens of major glaciations during the past couple of million years, not the four or so enshrined in textbooks.
Corroborating evidence came from a wholly different type of record. In a brick-clay quarry in Czechoslovakia, George Kukla noticed how wind-blown dust had built up into deep layers of soil what geologists call "loess". Although Kukla could not get dates that matched Emiliani's, the multiple repetitions of advance and retreat of ice sheets were immediately visible in the colored bands of different types of loess.
It was one of the few cases in this story where traditional field geology, tramping around with your eyes open, paid a big dividend. In , still more complete and convincing evidence came from an expedition that Broecker and a few others took to Barbados. Terraces of ancient coral covered much of the island, rising to hundreds of meters above the present sea level. The dates for when the coral reefs had been living ,, ,, and 82, years ago closely matched dates from Milankovitch cycles for times when the ice sheets should have been melted and the seas at their highest ,, ,, and 82, years ago.
The dating matched, that is, so long as one looked for the times when the maximum amount of sunlight struck a particular band of mid-northern latitudes during the summer. Since the Milankovitch cycles could be computed directly from celestial mechanics, one could project them forward in time, as Emiliani had done in In , presenting more Caribbean cores, he again advised that "the present episode of amiable climate is coming to an end. However, he added, greenhouse effect warming caused by human emissions might overwhelm the orbital shifts, so we might instead face "a runaway deglaciation.
The Coming and Going of an Ice Age
Some other scientists agreed that the current interglacial warm period had peaked 6, or so years ago, and should be approaching its natural end. A prominent example was Kukla, continuing his study of loess layers in Czechoslovakia. He could now date the layers thanks to a new technique provided by other scientists.
Geological and oceanographic studies had shown that over the course of millions of years, from time to time the Earth's entire magnetic field flipped: These reverses were recorded where layers of sediment or volcanic lava had entombed the direction of the magnetic field at the time. Geologists had worked out a chronology in lava flows, dated by the faint radioactivity of an isotope of potassium that decayed very slowly. When the loess layers were dated in this fashion, Milankovitch cycles turned up.
Extrapolating the cycles into the future, Kukla thought the next shift to an ice age "is due very soon. If the climate experts of the time seem to have been a bit preoccupied with ice ages, that fitted their training and interests. For a hundred years their field had concerned itself above all with the ice ages. Their techniques, from pollen studies to sea floor drilling, were devoted to measuring the swings between interglacial and glacial periods.
Home at their desks, they occupied themselves with figuring how ice age climates had differed from the present, and attacking the grand challenge of explaining what might cause the swings. Now that they were beginning to turn their attention from the past to the future, the most natural meaning to attach to "climate change" was the next swing into cold.
In , a group of leading ice-age experts met at Brown University to discuss how and when the present warm interglacial period might end. A large majority agreed that "the natural end of our warm epoch is undoubtedly near. Unless there were impacts from future human activity, they thought that serious cooling "must be expected within the next few millennia or even centuries. They hesitated to accept the Milankovitch theory at all unless they could get definitive proof from some entirely different kind of evidence.
The Greenland ice sheet is a daunting sight. Most investigators first come to it by air, past colossal bare cliffs where unimaginable quantities of ice pour down to the sea in a slow-motion flood. Beyond that the landscape rises and rises, over entire mountain ranges hidden under ice, to a limitless plain of gently undulating white. Greenland had played an important role in the 19th-century controversy over the ice ages. A few geologists had dared to postulate the existence, in the distant past, of seas of solid ice kilometers thick.
Then astonished explorers of Greenland found just such a thing beneath their skis. In the late s scientists came back to Greenland, hoping to find the key to the history of climate change. Recent work suggests that the K year cycle dominates due to increased southern-pole sea-ice increasing total solar reflectivity. The "traditional" Milankovitch explanation struggles to explain the dominance of the ,year cycle over the last 8 cycles. Muller , Gordon J. MacDonald ,    and others have pointed out that those calculations are for a two-dimensional orbit of Earth but the three-dimensional orbit also has a ,year cycle of orbital inclination.
They proposed that these variations in orbital inclination lead to variations in insolation, as the Earth moves in and out of known dust bands in the solar system. Although this is a different mechanism to the traditional view, the "predicted" periods over the last , years are nearly the same. Another worker, William Ruddiman , has suggested a model that explains the ,year cycle by the modulating effect of eccentricity weak ,year cycle on precession 26,year cycle combined with greenhouse gas feedbacks in the 41, and 26,year cycles. Yet another theory has been advanced by Peter Huybers who argued that the 41,year cycle has always been dominant, but that the Earth has entered a mode of climate behavior where only the second or third cycle triggers an ice age.
This would imply that the ,year periodicity is really an illusion created by averaging together cycles lasting 80, and , years. The jumps are induced by the orbital forcing, while in the early Pleistocene the 41,year glacial cycles resulted from jumps between only two climate states. A dynamical model explaining this behavior was proposed by Peter Ditlevsen. At times during the paleoclimate, carbon dioxide levels were two or three times greater than today.
Volcanoes and movements in continental plates contributed to high amounts of CO 2 in the atmosphere. Carbon dioxide from volcanoes probably contributed to periods with highest overall temperatures. The current ice age, called the Quaternary glaciation , which began 2. The interglacials lasted about 10,—20, years. The major glacial stages of the current ice age in North America are the Illinoian , Eemian and Wisconsin glaciation. The use of the Nebraskan, Afton, Kansan, and Yarmouthian stages to subdivide the ice age in North America has been discontinued by Quaternary geologists and geomorphologists.
These stages have all been merged into the Pre-Illinoian in the s. During the most recent North American glaciation, during the latter part of the Last Glacial Maximum 26, to 13, years ago , ice sheets extended to about 45th parallel north. These sheets were 3 to 4 kilometres 1. This Wisconsin glaciation left widespread impacts on the North American landscape. The Great Lakes and the Finger Lakes were carved by ice deepening old valleys.
Most of the lakes in Minnesota and Wisconsin were gouged out by glaciers and later filled with glacial meltwaters. The old Teays River drainage system was radically altered and largely reshaped into the Ohio River drainage system. Other rivers were dammed and diverted to new channels, such as Niagara Falls , which formed a dramatic waterfall and gorge, when the waterflow encountered a limestone escarpment. The area from Long Island to Nantucket, Massachusetts was formed from glacial till , and the plethora of lakes on the Canadian Shield in northern Canada can be almost entirely attributed to the action of the ice.
As the ice retreated and the rock dust dried, winds carried the material hundreds of miles, forming beds of loess many dozens of feet thick in the Missouri Valley. Post-glacial rebound continues to reshape the Great Lakes and other areas formerly under the weight of the ice sheets. The Driftless Area , a portion of western and southwestern Wisconsin along with parts of adjacent Minnesota , Iowa , and Illinois , was not covered by glaciers.
A specially interesting climatic change during glacial times has taken place in the semi-arid Andes. Beside the expected cooling down in comparison with the current climate, a significant precipitation change happened here. From this follows that—beside of an annual depression of temperature about c. Accordingly, at glacial times the humid climatic belt that today is situated several latitude degrees further to the S, was shifted much further to the N.
Although the last glacial period ended more than 8, years ago, its effects can still be felt today. The erratic boulders , till , drumlins , eskers , fjords , kettle lakes , moraines , cirques , horns , etc.
The weight of the ice sheets was so great that they deformed the Earth's crust and mantle. After the ice sheets melted, the ice-covered land rebounded. During glaciation, water was taken from the oceans to form the ice at high latitudes, thus global sea level dropped by about meters, exposing the continental shelves and forming land-bridges between land-masses for animals to migrate. During deglaciation , the melted ice-water returned to the oceans, causing sea level to rise. This process can cause sudden shifts in coastlines and hydration systems resulting in newly submerged lands, emerging lands, collapsed ice dams resulting in salination of lakes, new ice dams creating vast areas of freshwater, and a general alteration in regional weather patterns on a large but temporary scale.
It can even cause temporary reglaciation. This type of chaotic pattern of rapidly changing land, ice, saltwater and freshwater has been proposed as the likely model for the Baltic and Scandinavian regions, as well as much of central North America at the end of the last glacial maximum, with the present-day coastlines only being achieved in the last few millennia of prehistory. Also, the effect of elevation on Scandinavia submerged a vast continental plain that had existed under much of what is now the North Sea, connecting the British Isles to Continental Europe.
The redistribution of ice-water on the surface of the Earth and the flow of mantle rocks causes changes in the gravitational field as well as changes to the distribution of the moment of inertia of the Earth. These changes to the moment of inertia result in a change in the angular velocity , axis , and wobble of the Earth's rotation. The weight of the redistributed surface mass loaded the lithosphere , caused it to flex and also induced stress within the Earth.
The presence of the glaciers generally suppressed the movement of faults below. Earthquakes triggered near the ice margin may in turn accelerate ice calving and may account for the Heinrich events. In Europe, glacial erosion and isostatic sinking from weight of ice made the Baltic Sea , which before the Ice Age was all land drained by the Eridanos River.
From Wikipedia, the free encyclopedia. This article is about a generic geological period of temperature reduction. For the most recent glacial period commonly referred to as the Ice Age, see Last glacial period and Pleistocene. For other uses, see Ice age disambiguation. Period of long-term reduction in temperature of Earth's surface and atmosphere. Human timeline and Nature timeline. Glacial period and Interglacial. Minimum and maximum glaciation.
Minimum interglacial, black and maximum glacial, grey glaciation of the northern hemisphere. Minimum interglacial, black and maximum glacial, grey glaciation of the southern hemisphere. This section does not cite any sources. Please help improve this section by adding citations to reliable sources. Unsourced material may be challenged and removed. January Learn how and when to remove this template message. This section needs expansion with: Recent glacial and interglacial phases in other areas outside North America.
You can help by adding to it. Glacial history of Minnesota. Carbon Dioxide, Climate and the Greenhouse Effect. Comptes Rendus Geoscience in French. The annals of Mont Blanc.
Global Warming Cycles: Ice Ages and Glacial Retreat - Julie Kerr Casper - Google Книги
Discovering the Ice Ages. The Earth in Decay. A History of British Geomorphology — Regulator of Global Climates?: A climate disaster triggered by the evolution of oxygenic photosynthesis" PDF. Retrieved 8 August Life through deep time. Archived from the original on 15 June Archived from the original PDF on June 27, An exceptionally long interglacial ahead?
Ice Sheets and Landforms. Retrieved 10 August Archived from the original PDF on Newswise, Retrieved on December 17, Development in Quaternary Science: Extent and Chronology Vol. Encyclopedia of Snow, Ice and Glaciers. Retrieved 25 April After about 11, years ago, humans began to cultivate food, domesticate animals, and build cities in the continuously stable climate.
Enter a new interglacial period, and with it, a new epoch: This, as they say, is our time. However, William Ruddiman, a climatologist at the University of Virginia, is one scientist that suggests that the transition to the next glaciation should have already begun. Ruddiman's research says that early, pre-industrial societies generated enough greenhouse gases to actually stop a current-day ice age from happening altogether. But whether nature or humans avoided an ice age recently, we still have 20, years or more to wait before the next one.
But nobody has 20,year planning in their blood. American Institute of Physics: The Discovery of Global Warming http: Profile on Mutin Milankovitch http: Earth and Sky scientist profile: Watch a Glacier Melt Interactive: What is an Ice Core? The Coming and Going of an Ice Age main content. The Coming and Going of an Ice Age. Glaciers can deposit large boulders, called erratics, hundreds of kilometers from their source.