1

The Ice Ages

NOAA Paleoclimatology slide set

John T. Andrews INSTAAR and Department of Geological Sciences, University of Colorado,Bould

NOAA Paleoclimatology slide set: “The Ice Ages”http://www.ncdc.noaa.gov/paleo/slides/slideset/#ice1. Glacier in East Coast Mountains of Baffin Island, Nunavut, Canada.Tucked amongst the jagged peaks of the Canadian Arctic lies a glacier, a slow moving river of ice that flows downhill through the valley at speeds of several meters per year. Glacial ice is formed by the accumulation, compression, and re-crystallization of many years' snowfall at high altitudes or latitudes where temperatures are not sufficient to melt all the winter's snow accumulation. Glaciers come in all shapes and sizes, including this small mountain glacier found in the East Coast Mountains of Baffin Island in north-central Canada. Photo Credits:John T. Andrews INSTAAR and Department of Geological Sciences, University of Colorado,Boulder.

2

Mark McCaffrey NGDC/NOAA

2. Northern hemisphere glacial and estimated sea ice coverage.Medium-sized ice caps covering thousands of square kilometers in sub-polar and Polar Regions are also considered to be glaciers. The world's largest glaciers include the huge ice shelves that stretch out hundreds of kilometers over the oceans and seas of the Arctic and Antarctic. In recent geologic times there have also been enormous ice sheets as thick as four kilometers that blanketed most of Antarctica and Greenland.Scientists have been working to understand the mysteries surrounding the world's glaciers for over a century and a half, By examining the physical and chemical properties and geologic record of glaciers, and creating advanced computer models to simulate glacial growth, flow and melting mechanisms , our understanding of ice on earth has vastly improved, inspiring new questions and necessitating more research into the power and dynamic qualities of glacier.This image compares modern day glacier and ice coverage of the northern hemisphere with the same region 18,000 years before present. Note that the modern sea ice coverage is during the summer months; winter coverage is more extensive.Photo Credits:Mark McCaffrey NGDC/NOAA

3

Mark McCaffrey--Paleoclimate Program/NOAA

3. Southern hemisphere glacial and estimated sea ice coverageGlaciers presently cover some 15.8 million square kilometers of the earth's surface, which is about 10% of the earth's land area (roughly the size of South America). In this image of the surface topography of the Southern Hemisphere, ice covered regions are white, and the rainbow colors depict different elevations.15.8 million square kilometers may sound like a lot of ice but in the past glaciers occupied even more of our planet. Eighteen-thousand years ago, at the peak of the last ice age, scientists estimate that nearly 32% of the earth's land area was covered with ice, including much of Canada, Scandinavia, and the British Isles. In what is now the United States, huge sheets of ice even stretched as far south as southern Wisconsin and Long Island. In Europe, ice covered northern Germany, Poland, and the northern reaches of the former Soviet Union. These glaciers developed because the earth was in the midst of an ice age. What caused the earth's climate to change so dramatically? Why did ice ages begin, why did they end, and what can we expect the future to hold?This image compares 18,000 years before present and modern day glacial and estimated sea ice coverage of the southern hemisphere. Photo Credits:Mark McCaffrey--Paleoclimate Program/NOAA

4

Mark McCaffrey--Paleoclimate Program/NOAA

4. Erratic, Baffin Island, Nunavut, Canada.In 1837 Louis Agassiz presented his theory that ice had covered much of the northern hemisphere. He based his theory on an abundance of observations made in Scotland, northern England, Scandinavia, Switzerland, France, Italy and northern North America, but the idea was not widely accepted. One observation that Agassizattempted to explain with his ice-age theory was the presence huge boulders scattered across the landscape. Often these boulders rested atop bedrock of a very different composition; metamorphic quartzite on top of sedimentary limestone, for example. Such boulders, called erratics, were often found several kilometers from any other bedrock of their type. Clearly, something had lifted these enormous boulders and carried them tremendous distances. Agassiz theorized that these boulders had been trapped in enormous, moving ice sheets. When the ice melted, the boulders were dropped many miles from their origin. Photo Credits:John T. Andrews INSTAAR and Department of Geological Sciences, University of Colorado, Boulder.

5

John T. Andrews INSTAAR and Department of Geological Sciences, University of Colorado, Bou lder.

5. U-shaped valleys were another observation that could be explained by the ice-age theory. Scientists knew that rivers carved V-shaped valleys, but not U-shaped valleys, which are found only in alpine regions or high latitudes where glacial ice is formed. Photo Credits:John T. Andrews INSTAAR and Department of Geological Sciences, University of Colorado, Boulder.

6

John T. Andrews INSTAAR and Department of Geological Sciences, University of Colorado, Bou lder.

6. Striations on rock, Baffin Island, Nunavut, Canada.There were still other observations besides the erratic formations and U-shaped valleys. In the same regions where erratics occurred, naturalists noted exposed bedrock that sometimes bore parallel scratch marks (such as these grooves oriented vertically on the right side of this photograph), termed striations. The ice-age theory states that striations form when boulders trapped at the base of ice sheets scratch across bedrock, leaving deep scratch marks.When geologists began to take note of striations, erratics, and U-shaped valleys in the eighteenth and early nineteenth centuries, they were puzzled by these phenomena. In an age when science and religion were closely connected, most attributed these features to the great flood described in the Bible. But by the 1840s, a growing number of geologists were abandoning The Diluvian Theory in favor of a hypothesis put forth most stridently by Louis Agassiz.By the middle of the 19th century, most geologists were convinced by Agassiz's theory that past periods of more intense glaciation explained the presence of features like striations and erratics in regions far removed from present-day glaciers. With the existence of ice ages no longer in doubt, scientists began to wonder what caused such striking climate changes.. Some theorized that the amount of energy produced by the sun had varied in the past causing climatic oscillations. Others speculated that ash hurled into the atmosphere by volcanic eruptions could have blocked the sun's radiation and thrust the globe into a protracted cold spell. British geologist Charles Lyell proposed that ice ages occurred because of an increase in the elevation of the earth's crust. On the other hand, the French astronomer J. A. Adhamar suggested that variations in the earth's orbit around the sun might account for climatic fluctuations.Many of these theories, however, were soon disproved. If increased volcanic activity had directly caused the ice ages, then sediments deposited during ice ages should have contained sizable ash layers. But the geologic record failed to support the volcanic theory, just as it failed to substantiate the notion that variations in solar output dictated climatic oscillations or Lyell's scheme of increased crust elevation. Adhamar had also made a crucial mistake, arguing that the Northern Hemisphere was heating up while the Southern Hemisphere was cooling down, and vice versa. It was easily demonstrated that each hemisphere received the same exact amount of heat as the other over the course of the year. Photo Credits:John T. Andrews INSTAAR and Department of Geological Sciences, University of Colorado, Boulder.

7

Courtesy of John Imbrie—Department of Geological SciencesBrown University

7. Photograph of James Croll from J. C. Irons (1896)Adhamar's theory was seriously flawed, but it nevertheless contained the seed of a theory developed in the 1860's and '70s by Scottish scientist James Croll, shown here. There were three major points to Croll's theory. First, Croll argued that while the total amount of insolation received at a given latitude did not vary from year to year, the amount received in a given season for a given latitude could vary significantly from year to year because of changes in the earth's orbit. Second, he claimed that these seasonal variations were caused by two orbital phenomena known as precession and eccentricity. He was also aware that the tilt of the earth's axis changed, but he was unable to include obliquity into his calculations because the mathematical relationship for this aspect of the Earth's orbit had yet to be developed. Finally, Croll suggested that while the initial climatic effect of changes in the earth's orbit might be rather small, but that these changes were amplified significantly by climatic feedback mechanisms in the earth's climate system. To better understand Croll's ideas, it is helpful to learn more about earth's orbit and how it changes through time. Photo Credits:Courtesy of John Imbrie--Department of Geological Sciences, Brown University

8

Thomas G. Andrews, NOAA Paleoclimatology Program

Thomas G. Andrews, NOAA Paleoclimatology Program

8. Graphic of Earth's orbit adapted from Pisias and Imbrie [1986/1987].Viewed in the present, the tilted earth revolves around the sun on an elliptical path. The orientation of the axis remains fixed in space, producing changes in the distribution of solar radiation over the course of the year. These changes in the pattern of radiation reaching earth's surface cause the succession of the seasons. The warm weather of summer comes to the northern hemisphere, for instance, because during these months the northern hemisphere is tilted towards the sun (at the same time, the southern hemisphere experiences winter because it is tilted away from the sun).Croll was strongly influenced by the findings of astronomers like Adhamar and another Frenchman, U. Leverrier, who demonstrated that the earth's orbital geometry was not fixed over time. Croll argued that as the earth's orbit changed, the pattern of radiation received in various seasons changed too. These long-term variations in the earth's orbit, he claimed, did much to explain the waxing and waning of global climatein the last several million years. Photo Credits:Thomas G. Andrews, NOAA Paleoclimatology Program

9

Thomas G. Andrews, NOAA Paleoclimatology Program

9. Precession of Earth's orbit adapted from Pisias and Imbrie [1986/1987]Like a spinning top, the earth's orbit wobbles so that over the course of a precessionalcycle, the North Pole traces a circle in space. This wobble causes the precession of the equinoxes. Croll adapted part of his theory from Adhamar, who demonstrated that the cycle of precession takes about 22,000 years to complete. As shown in this figure, the position of the equinoxes and solstices shifts slowly around the earth's elliptical orbit. Precession changes the date at which the earth reaches its perihelion serving to amplify or dampen seasonal climatic variability. For example, the earth currently reaches its perihelion on January 3, close to the Northern Hemisphere's winter solstice. This timing of the perihelion and Northern Hemisphere's winter solstice reduces seasonal differences in insolation in the Northern Hemisphere because the hemisphere is closer to the sun in winter and hence relatively warmer. On the other hand, the earth is further away from the sun and relatively cooler during the Northern Hemisphere's summer, reaching its aphelion on July 5. However, 11,000 years ago, the reverse was true: the earth reached its perihelion during the northern summer, increasing the seasonal variability of earth's climate. Photo Credits:Thomas G. Andrews, NOAA Paleoclimatology Program

10

Thomas G. Andr ewsNOAA Paleoclimatology Program

10. Earth's eccentricity adapted from Pisias and Imbrie [1986/1987]Croll corrected some of the conceptual mistakes that plagued Adhamar's work, but he also added important innovations of his own. First, Croll incorporated recent findings by the French astronomer Leverrier demonstrating that the shape of earth's orbit changed slowly but consistently over time. As this graphic illustrates, the shape of the earth's orbit varies from nearly circular (eccentricity approaching 0.00) to more elliptical (eccentricity=0.06). These variations occur at a frequency of 100,000 years (which Leverrier demonstrated) and 400,000 years (which later scientists discovered). Croll understood that variations in orbital eccentricity had a small impact on the total amount of radiation received at the top of earth's atmosphere (on the order of 0.1%), but that the eccentricity cycle modulated the amplitude of the precession cycle. During periods of high eccentricity (a more elliptical orbit), the effect of precession on the seasonal cycle is strong. When eccentricity is low (more circular), the position along the orbit at which the equinoxes occur is irrelevant because all points on the orbit become, in effect, perihelia.As Croll's research indicated, variations in the precession and eccentricity cycles altered the amount of insolationreceived at different times of the year, creating variations in the distribution of solar energy received from season to season. But other scientists argued that these variations were too minor to explain the tremendous climatic oscillations that occurred as the earth alternated between glacial and interglacial periods. Croll's response to these scientists represented his third major innovation: the concept of climatic feedback mechanisms in the earth's climate system.Croll believed that the variation of winter insolation was the critical variable explaining climate change. When the

precessional cycle placed the earth at its aphelion during the Northern Hemisphere's winter, the northern winters would be significantly colder if this occurrence was coupled with a period of high eccentricity. Croll reasoned that snow would then begin to accumulate to a greater degree, eventually creating large snowfields and glaciers. As reflective snow and ice covered more of the Northern Hemisphere's land area, the earth would absorb less solar radiation. The climate would cool further as glaciers and ice sheets reflected a great deal of solar energy back into space. Croll also speculated about another feedback effect, this one involving the position of warm currents in the Atlantic Ocean. As the northern latitudes cooled, the strength of the trade winds would increase, drawing them southward towards the equator. This in turn reduced the strength of the Gulf Stream as warm currents turned south rather than north as they flowed towards the bulge of Brazil. Photo Credits:

Thomas G. Andrews, NOAA Paleoclimatology Program

11

Contributed by John Imbrie, Brown University

11. Portrait of Milutin Milankovitch by Paja Jovanovic, 1943, courtesy of Vasko MilankovitchCroll's arguments electrified the scientific community and provoked a great deal of debate and

research. Geologists scoured Europe and the Americas in search of evidence to support or refute Croll'sastronomical theory. Initially, the theory seemed to square well with the geologic evidence, but by the 1890s terrestrial field research raised serious questions about its accuracy. Croll's calculations predicted that the end of the last glacial period occurred some 80,000 years ago and yet reasonable estimates placed the age of two North American waterfalls that had formed after the great ice sheets retreated at 6,000-32,000 years (Niagara on the U.S.-Canada border) and 6,000-10,000 years (the Falls of St. Anthony in Minnesota). Clearly, something was wrong with Croll's theory. At the dawn of the twentieth century, few reasonable persons believed that variations in eccentricity and precession caused the ice ages.However, in the 1910s, the Serbian mathematician Milutin Milankovitch, began to embark on a series

of calculations that would eventually revive the orbital theory of climate change. Milankovitch's main contributions were threefold. First, he used new astronomical calculations by the German scientist Ludwig Pilgrim that took into account a third cyclical variation in the earth's orbit, that of obliquity or tilt. Secondly, he reasoned that summer rather than winter temperatures were the main contributors of ice sheet growth and decline. Lastly, he calculated summer radiation curves for the key latitudes of 55, 60, and 65 degrees N that seemed to correlate well with evidence then available from the geologic record. Photo Credits:Contributed by John Imbrie, Brown University

12

Thomas G. Andrews, NOAA Paleoclimatology Program

12. Earth's axial tilt, adapted from Pisias and Imbrie [1986/1987]Croll was aware that the obliquity of earth's axis varied through time, but Leverrier's astronomical calculations only allowed Croll to include eccentricity and precession in his theory. Milankovitch, on the other hand, benefited from innovations in astronomy that made it possible to incorporate changes in tilt into his calculations. Earth's axial tilt varies from 24.5 degrees to 22.1 degrees over the course of a 41,000-year cycle. Changes in axial tilt affect the distribution of solar radiation received at the earth's surface. When the angle of tilt is low, polar regions receive less insolation. When the tilt is greater, the polar regions receive more insolation during the course of a year. Like precession and eccentricity, changes in tilt thus influence the relative strength of the seasons, but the effects of the tilt cycle are particularly pronounced in the high latitudes where the great ice ages began.With Pilgrim's new calculations as his guide, Milankovitch embarked on an exhaustive series of calculations. Without a computer or even a calculator, the task was arduous indeed. While the calculations were complex, the reasoning behind them was quite simple. Croll had argued that winter insolation was the key factor in understanding the ice ages, but Milankovitch thought that summer insolation was more important. During periods of lower summer temperatures, he reasoned, less of the previous winter's snow would melt. Glaciation would soon begin after the snows of several winters piled up. Milankovitch set out to determine how variations in precession, eccentricity, and obliquity affected the amount of solar radiation received during the summer at particular latitudes. Photo Credits:Thomas G. Andrews, NOAA Paleoclimatology Program

13

Thomas G. Andrews, NOAA Paleoclimatology Program

13. Earth's insolation using data from Berger [1978]Most important to Milankovitch were the latitudes where the great ice sheets of the last ice ages formed. This graph shows how insolation has varied at 65 degrees N over the past 400,000 years. While subsequent refinements in astronomy are reflected in this curve, it is quite similar to the one calculated by Milankovitch. By the 1920s, Milankovitch's theory of orbital forcing was practically complete, though some of his calculations would take another two decades to finish. Once again, scientists used field studies to determine how well the theory of orbital forcing explained past climatic variations. Comparisons were initially positive. Sequences of glacial deposits in North America and Europe seemed to support Milankovitch's theory. But these deposits were very difficult to date accurately, and so any correspondence between the timing of glaciation and Milankovitch's insolation curves remained speculative. Nevertheless, most scientists supported the theory until the 1950s, when new developments in dating technology raised doubts about Milankovitch's ideas.One of the unintended results of the atomic age was the discovery of radiocarbon dating methods. Over a century after Louis Agassiz first proposed his glacial theory, geologists finally possessed a potent tool for determining the age of glacial deposits. They compared these dates with the insolation curves calculated by Milankovitch, and found that there were more glacial advances in the last 80,000 years than Milankovitch's theory could account for. By 1965, the theory of orbital forcing lay once again in disrepute. Photo Credits:Thomas G. Andrews, NOAA Paleoclimatology Program

14

Anne Jennings INSTAAR, University of Colorado, Boulder

14. A 10m-piston core being raised to the deck of the Hudson.Milankovitch's theory would be resurrected once again, however, this time from the depths of the world's oceans. Beginning in the 1960s and continuing to the present day, scientists have found overwhelming support for the Milankovitch theory in long cores such as this one extracted from the ocean floor. These oceanographic studies have two major advantages over the studies of terrestrial glacial deposits done in the 1950s that seemed to refute the Milankovitch theory. First of all, ocean cores provide a much more continuous record of glaciation than terrestrial deposits do. The reason for this being that subsequent waves of glaciation often erased or altered traces of earlier glacial and interglacial periods. Secondly, by the late 1960s, sediment cores pulled from the oceans could be dated with relative confidence as far back as 650,000 yrs. before present. In contrast, radiocarbon dating becomes much less accurate on materials over 40,000 yr. old. Thus, the initial land-based studies that claimed to provide chronologies for glaciation over the past 80,000 years relied heavily on some very tenuous dates. Photo Credits:Anne Jennings INSTAAR, University of Colorado, Boulder

15

David M. Anderson NOAA Paleoclimatology Program and INSTAAR, University of Colorado, Boulder

15. Marine sediment samples collected with a multicorer.Once a core is brought onboard, the sediment and the plastic core liner are extruded from the steel pipe, cut into sections, and split into two halves. An archive half is usually saved for future research, while the working section is sampled at periodic intervals. On board the Ocean Drilling Program drillship JOIDES Resolution, a core from the NW Arabian Sea has been heavily sampled by inserting white plastic plugs into the face of the split core. In this core, one can see faint alternations between dark, organic-rich sediment layers deposited during high productivity interglacial times, and light-colored layers deposited during low productivity glacial times in the NW Arabian Sea.Photo Credits:David M. Anderson NOAA Paleoclimatology Program andINSTAAR, University of Colorado, Boulder

16

D. B. S co tt , Cen tr e fo r M a ri ne G e ology , D a l hous i e Un i ver sit y, Ha li f a x, Nova S co ti a ,C a nada

16. Photo of Helenina anderseniSediment cores from the ocean floor contain types of information that scientists use to better understand the fluctuations of global climate. Perhaps the most important information is that gleaned from the microscopic shells of organisms such as this, called Foraminifera (forams for short). Forams provide two main types of information. First of all, different species of forams prefer different ocean temperature and nutrient conditions. Scientists can therefore learn much about the climatic conditions of a core site in the past by looking at which species once inhabited the area. Secondly, the shells of forams effectively lock in the oxygen and carbon isotopic composition of the waters in which they formed. Because past periods of glaciation changed the relative quantities of heavy oxygen (18O) and light oxygen (16O), scientists can use the isotopic composition of foram shells as a proxy signal for past changes in global ice volume.Other chemical measures are available as well by studying the composition of the shells. Data from ocean cores about past glaciation matches Milankovitch's theory remarkably well. Photo Credits:D. B. Scott, Centre for Marine Geology, Dalhousie University, Halifax, Nova Scotia, Canada

17

Nat RutterDepartment of Geology,University of Alberta,Edmonton, Canada

17. Baoji section of the Loess Plateau, ChinaNew evidence from terrestrial deposits also seems to support the theory of orbital forcing. This slide shows the upper part of a section of loess near Baoji in southern China's Central Loess Plateau. Note the alternation of four thin dark brown soils (S0 to S4) with relatively unaltered loess sections (dark to light tan). These soils represent interglacial periods when climates were wet enough to sustain vegetation development. During glacial periods, climates were colder, drier, and windier, leading to sparse vegetation cover and extensive mineral dust (loess) accumulation in many parts of the world. Photo Credits:Nat RutterDepartment of Geology, University of Alberta, Edmonton, Canada

18

Thomas G. Andr ewsNOAA Paleoclimatology

18. Climate forcing on glacial-interglacial timescalesAs the scientific quest to unlock the mysteries of the ice ages enters its fourth century, this slide summarizes our current understanding of the climate forcing (changes in insolation), and the climate response that we observe in the geologic record on glacial-interglacial timescales. The top panel is June insolation at 65oN, in watts/m2, calculated by Berger (1978). The three lower panels are all geologic records of glacial-interglacial change. δ18O is a measure of the ratio of the two stable isotopes of oxygen (18O and 16O). δ18O in foraminifer skeletons is affected by both temperature and the amount of 16O preferentially locked away in ice sheets, and up on this graph corresponds to negative d18O values, and indicates interglacial conditions.Down on this graph, more positive δ18O, corresponds to glacial conditions that are colder, and have more ice present. This SPECMAP record is a composite (average) of many δ18O records, intended to represent globally-averaged changes, while the record from ODP Site 677 (bottom graph), represents a single region in the North Atlantic. The final geologic record comes from a loess deposit. Photo Credits:Thomas G. Andrews, NOAA Paleoclimatology Program

19

Thomas G. Andrews, NOAA P aleoclimatology Program

19. Insolation and global ice volume fluctuations.When the data are filtered for solar insolation and global ice volume over the past 400,000 years, we see that insolation and global ice volume fluctuated at the same major frequencies: the precession cycle of 23,000 years and 19,000 years, the obliquity cycle of 41,000 years, and the eccentricity cycle of 100,000 years (n.b.: the data do not extend far enough back to test the 400,000 yr. eccentricity cycle). The curves are thus much more similar than they first appear. So what makes them look so different? The biggest difference between the insolation and global ice volume curves is the surprising amplitude of the 100,000-yr. eccentricity cycle in the ice-volume records. The Milankovitch theory predicts that changes in eccentricity have a smaller effect on climate than variations in precession and obliquity. But climatic records from across the globe suggest that the great ice sheets have advanced and retreated to a 100,000-yr. beat. Why this is so is one of the many questions that remain to be answered about ice ages and the forces that drive them? Photo Credits:Thomas G. Andrews, NOAA Paleoclimatology Program

20

Nancy Weiner, INSTAAR, University of Colorado, Boulder

20. From the time of Louis Agassiz to the present, scientists have sought to understand why glaciers once covered nearly one-third of earth's land surface. In the process, they have discovered much about earth's changing climate, but many questions still remain. Scientists are currently exploring some of the many climate feedback mechanisms in the search for the answers. Photo Credits:Nancy Weiner, INSTAAR, University of Colorado, Boulder