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Overview
The volume begins with the earliest recorded observations of sea ice, from 350 BC, but the majority of its information is drawn from the period after 1950, when detailed study of sea ice became widespread. Weeks delves into both micro-level characteristics—internal structure, component properties, and phase relations—and the macro-level nature of sea ice, such as salinity, growth, and decay. He also explains the mechanics of ice pack drift and the recently observed changes in ice extent and thickness.
An unparalleled account of a natural phenomenon that will be of increasing importance as the earth’s temperature rises, On Sea Ice will unquestionably be the standard for years to come.
Product Details
| ISBN-13: | 9781602230798 |
|---|---|
| Publisher: | University of Alaska Press |
| Publication date: | 06/15/2010 |
| Pages: | 664 |
| Product dimensions: | 7.00(w) x 10.00(h) x 1.50(d) |
About the Author
Read an Excerpt
On Sea Ice
By W.F. Weeks W.D. Hibler III.
University of Alaska Press
Copyright © 2010 University of Alaska PressAll right reserved.
ISBN: 978-1-60223-079-8
Chapter One
IntroductionHere was a crystalline world of azure and emerald, indigo and alabaster-dazzling to the eye, disturbing to the soul. P. Berton, describing the Ross expedition's first encounter with sea ice, Davis Strait, 1818
The reasons that individuals living in the Arctic need to understand the behavior of sea ice are both numerous and varied. Here, by the term sea ice, I refer to any type of ice that forms in or on the surface of the sea by the freezing of seawater. This definition excludes both icebergs, which are pieces of glacier ice adrift in the sea, and spray icing formed on either natural or man-made objects. Although at first glance this definition may seem to be unnecessarily restrictive, the reader will soon find that sea ice is surprisingly varied, reflecting the local environmental state during ice formation. However, as the great majority of readers of this book are probably not residents of the shores of the polar oceans and some will have never seen a piece of sea ice, they might well inquire why they should be interested in this seemingly obscure and physically remote material. Why should they not simply conclude that out of sight should equate with out of mind, and proceed on to other less esoteric matters?
To answer this question one needs to examine both the amount of snow and ice on the Earth and how it is distributed. Table 1.1 presents some of this type of information compiled from a variety of sources. When the maximum volume of sea ice that exists in the World Ocean during a given year (~75 x [10.sup.3][km.sup.3]) is compared with the present volume of glacier ice (~24 x [10.sup.6] [km.sup.3]), we find that the volume of sea ice is roughly 0.3% of that of glacier ice. A more realistic comparison would be to compare the volume of sea ice existing at any given time, recalling that maximum sea ice extent in one hemisphere roughly corresponds to a minimum extent in the other hemisphere. In this case the result would be even less impressive, with the volume of sea ice amounting to only ~0.2% of the volume of glacier ice. In making these approximate calculations I have arbitrarily assumed that all northern hemisphere sea ice has a thickness of 3 m and that all southern hemisphere sea ice is 1.5 m thick. Needless to say this is far from the case. However, the exact values are immaterial in that any reasonable estimate of average sea ice thicknesses would lead to the same general result: volumetrically, sea ice does not appear to be an important geophysical entity. When compared to the total volume of water in the World Ocean (~1370 x [10.sup.6][km.sup.3]), the volume of sea ice shrinks to total insignificance (0.004%). Even the total volume of all types of ice on the surface of the Earth comprises only ~1.7% of the total water volume on the Earth's surface. Approximate values for the volumes and surface areas of the Earth's sea and glacier ice can be found in Table 1.1.
There are two reasons why many scientists are currently interested in sea ice. The first reason is shared with ice in all its forms except for the small amount of ice suspended in the atmosphere. When ice occurs it forms the surface layer of the Earth. When sea ice exists it forms the surface of the sea, since sea ice forms from seawater by definition and ice floats in its own melt. Sea ice is also a good thermal insulator and serves as a platform for the deposition of snow, which is an even better insulator. In addition, the albedo of bare sea ice is ~0.8 and that of newly fallen snow is 0.85 to 0.9, meaning that during the polar summer only 10 to 20% of the incoming shortwave radiation is absorbed at the surface of snow- and ice-covered areas, with the rest being reflected back into either space or the atmosphere. Furthermore, during the winter the snow cover on the sea ice acts as a nearly perfect black body, radiating heat into space. In short, sea ice acts as an efficient lid on the surface of the polar oceans controlling the exchange of heat and water vapor between the comparatively warm ocean and the frigid atmosphere. As a result, sea ice is an important player in the puzzle of the world's climate system.
By now it is also probably obvious that the areal extent of the world's sea ice cover is vastly more impressive than its volume. At maximum extent sea ice covers ~35 x [10.sup.6][km.sup.2] during some time of the year. This amounts to ~7.3% of the Earth's surface or, more importantly, ~11.8% of the surface of the World Ocean. To put these numbers into some perspective, I note that the contiguous United States has an area of ~7.825 x [10.sup.6][km.sup.2] which means that in the northern hemisphere at maximum extent sea ice covers an area equal to slightly less than twice (192%) that of the contiguous United States. In the late summer the sea ice area shrinks to just slightly more (102%) than this value. A map showing the general distribution of sea ice in the northern hemisphere during both its maximum and minimum extent is given in Figure 1.1. As is obvious from this figure, at maximum extent sea ice does not advance parallel to lines of latitude. In both the Atlantic and Pacific Oceans the advance down the western sides of these ocean basins far exceeds the advance down the eastern sides. Also, the presence of shallower water greatly favors the occurrence of sea ice. In the summer the ice primarily retreats to locations within the Arctic Basin, plus locations between the islands of the northern part of the Canadian Arctic Archipelago and a southern extension off the east coast of Greenland referred to as the East Greenland Drift Stream.
In the southern hemisphere (Figure 1.2) at maximum extent sea ice covers an area equal to 2.6 times that of mainland Australia (7.633 x [10.sup.6][km.sup.2]) and shrinks to ~39% of the Australian area at the end of the austral summer. At maximum extent the ice advances farthest from the continent in the South Atlantic and South Pacific sectors, with the least advance occurring in the Indian Ocean sector. At minimum extent sea ice occurs in a fairly thin belt around the continent. However, there are several so-called ice massifs where consistently heavy sea ice can be found even at the end of summer. Examples include the extremely heavy ice in the western portion of the Weddell Sea, the Pine Island Bay area of the Amundsen Sea, and the Victoria Land-Balleny Islands region of the western Ross Sea. Considerably more will be said later in this book (Chapter 18) about recent variations in the amount and in the distribution of sea ice and in the significance of these changes.
The presence of sea ice is also associated with other grand-scale effects. For instance the freezing of seawater is a somewhat inefficient desalination process which concentrates water in the sea ice in the form of pure ice and rejects salt back into the underlying ocean in the form of cold, dense brine. Brine rejection is especially intense in regions such as the coastal margins of Antarctica, where the intense katabatic winds that flow off the continent continuously strip the newly formed sea ice away from the coast, forcing it to the north. As a result, open water and thin ice areas called polynyas are constantly being formed and refrozen in locales where thick ice covers would be expected considering the local climate. The resulting rapid growth of thin ice causes large amounts of cold, dense brine to form. This brine sinks to the bottom of the continental shelf and ultimately flows off the shelf contributing to the cold saline bottom water of the World Ocean.
In the Arctic, the other half of the sea ice desalination process also has an interesting oceanographic role to play. Every year some ~10% of the ice in the Arctic Basin exits the basin via the East Greenland Drift Stream, the name used to describe the cold, ice-laden current that flows toward the south off the east coast of Greenland. In that much of this ice has been desalinating for two or more years, the Drift Stream can be thought of as a river of freshwater flowing into the Greenland Sea. The scale of this freshwater flux is very surprising (~2366 [km.sup.3][yr.sup.-1]). It is over twice the total annual flow of North America's four largest rivers (the Mississippi, St. Lawrence, Columbia, and Mackenzie). When compared to the world's largest rivers, it proves to be exceeded only by that of the Amazon (Aagaard and Carmack 1989). The fresh, surface water layer resulting from the melting of this ice is transported with little dispersion at least as far south as the Denmark Strait and in all probability can be followed completely around the subpolar gyre of the North Atlantic. Even more interesting is the speculation that in the past this freshwater flux has been sufficient to alter or even stop the convective regimes of the Greenland, Iceland and Norwegian Seas and perhaps also of the Labrador Sea. This is a sea ice-driven, small-scale analog of the so-called halocline catastrophe that has been proposed for past deglaciations when it has been argued that large freshwater runoff from melting glaciers severely limited convective regimes in portions of the World Ocean. The difference is that, in the present instance, the increase in the freshwater flux that is required is not dramatic because at near-freezing temperatures the salinity of the seawater is appreciably more important than the water temperature in controlling its density. It has also been proposed that this process has contributed to the low near-surface salinities and heavy winter ice conditions observed north of Iceland between 1965 and 1971, the decrease in convection described for the Labrador Sea during 1968-1971, and perhaps to the so-called "great salinity anomaly" which has freshened much of the upper North Atlantic during the last 25 years. Is this all true? Perhaps. What is certain is that these speculations are currently actively being investigated.
For people who live in the polar regions, there are more obvious reasons for being interested in sea ice. It directly affects daily life by totally changing the nature of the sea. Like most things in nature, there are both good and bad aspects to these changes. For instance, sea ice makes a good hunting platform that is used by humans as well as other top-level predators. Its presence helps prevent coastal erosion by directly protecting the coast through the formation of a fast ice belt and by limiting the fetch available to generate waves during storms. It also provides a vertically stable platform from which to take geophysical observations in that the presence of sea ice damps out all but very long-period ocean waves.
Sea ice can also be used as a platform for a wide variety of operations. For instance, the United States Antarctic Program recently has operated winter resupply flights from fast sea ice runways using extremely large, heavy C-5 aircraft. On the downside, sea ice can be very unstable in a horizontal sense in that pack ice commonly drifts 1 to 3 km/day and can drift as much as 30 to 40 km/day during storms. During much of the year the presence of sea ice closes desirable shipping routes such as the Northwest Passage through the Canadian Arctic Archipelago and the Northeast Passage off the northern coast of Russia. Even in the summer, expensive icebreaker escorts are required along these routes although recently there have been summers when these routes were ice free for short periods of time. If offshore structures are to be built at locations on the continental shelves of the Arctic, the maximum forces, which they must be designed to withstand, are caused by ice moving against the structures. Furthermore, the seafloor of the polar continental shelves at water depths of less than ~60 m is regularly plowed by grounded sea ice masses that are pushed along by the surrounding moving pack. Gouges of up to 6-8 m are known, a fact that adds considerable uncertainty and expense to any engineering scheme that utilizes buried cables, pipelines, or other types of seafloor structures in the offshore Arctic. The presence of sea ice also changes the acoustic environment of the Arctic Ocean, a fact that has received considerable attention from the British, Russian, and United States navies, who have operated submarines in the Arctic since the cruise of the USS Nautilus across the Arctic Basin in 1957.
The properties and behavior of sea ice encompass a natural geophysical system that is fascinating in its own right. Sea ice is the one igneous rock that crystallizes from its melt at temperatures that humans can almost tolerate. Furthermore, the crystallization process commonly occurs at the surface where it can be observed. The ice is then overlain by snow, a sedimentary rock that is deposited in complex patterns. Added to this is the fact that both sea ice and snow undergo metamorphic transformations at rates that can be measured in terms of days. The whole system then races around undergoing high-speed plate tectonics and mountain building during deformation events whose time span can be measured in terms of hours. Yet none of this is ever mentioned in texts on geology. As my academic training was in geology, I have always found this to be somewhat amazing!
The study of sea ice is also able to profit from the extensive studies that have been made on crystal growth and solidification theory as applied to metals and ceramics. In fact, when the structure of sea ice is examined, it is found to bear a striking resemblance to structures formed during the solidification of impure melts of hexagonal metals such as zinc. Fortunately, sea ice studies also have something to contribute back to metals and ceramics, as ice is transparent, allowing one to optically examine its internal characteristics at temperatures within a few degrees of the initial freezing temperature.
Finally, in using this book the reader should keep the following in mind. The scientific study of sea ice has essentially all been carried out during the last 100 years, with most of the effort concentrated on the period after 1950. During most of this period and perhaps for a period of several thousand years, the polar sea ice covers appear to have remained remarkably stable in extent and presumably in thickness. However, this does not mean that every year was the same; clearly there were changes in extent from year to year and presumably there were associated changes in thickness. What this does mean is that there were no obvious strong trends indicating systematic changes in ice conditions. Much of the general description of ice types and ice conditions distributed throughout this book are based on observations taken during this apparently static period of historically "normal" ice conditions. Today it will be a rare reader who does not realize that since the late 1980s static would be a very poor term to use in describing the current state of sea ice. Currently the world's sea ice covers are undergoing rapid changes in extent and thickness, changes that are likely to have far-reaching repercussions. These recent trends are described in some detail in Chapter 18.
I hope that the above discussion will encourage the reader to delve further into this book as well as provide a general idea of a few of the topics that will be discussed.
(Continues...)
Excerpted from On Sea Ice by W.F. Weeks W.D. Hibler III. Copyright © 2010 by University of Alaska Press. Excerpted by permission.
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Table of Contents
PrefaceAbout This Book
Acknowledgments
Figure Sources
1. Introduction
2. Historical Background
3. The Ocean Setting
4. An Introduction to Sea Ice Growth
5. Components
6. The Phase Diagram
7. Sea Ice Structure
8. Sea Ice Salinity
9. Sea Ice Growth: The Details
10. Properties
11. Polynyas and Leads
12. Deformation
13. Sea Ice–Seafloor Interactions
14. Marginal Ice Zone
15. Snow
16. Ice Dynamics by W. D. Hibler III (with W. F. Weeks)
17. Underwater Ice
18. Trends
19. Conclusions
Appendix A. Symbols
Appendix B. Acronyms and Abbreviations
Appendix C. Terminology and Glossary
Appendix D. Sampling
Appendix E. Thin Sections
Appendix F. Remote Sensing
Bibliography
Index