Über die Autorin bzw. den Autor

Shawn J. Marshall is the Canada Research Chair in Climate Change at the University of Calgary.

Von der hinteren Coverseite

"Distinguished glaciologist Shawn Marshall leads a lucid tour through the essentials of ice in the environment, with more than enough information to satisfy the casual student, and additional pointers to steer the serious scientist."--Richard Alley, Pennsylvania State University

"Marshall presents a comprehensive, accessible, and authoritative treatment of the cryosphere, and makes excellent use of field and model examples to illustrate key points. New metaphors and descriptions are offered for familiar topics that bring with them new insight and utility. I enjoyed reading this book."--Robert Bindschadler, emeritus scientist, NASA Goddard Space Flight Center

This is one of the best, most up-to-date books about the state of the cryosphere and an excellent read."--Eric Rignot, University of California, Irvine

Auszug. © Genehmigter Nachdruck. Alle Rechte vorbehalten.

THE CRYOSPHERE

PRINCETON UNIVERSITY PRESS

Contents

Preface...................................................vii1 introduction to the cryosphere..........................12 Material Properties of Snow and ice.....................113 Snow and ice Thermodynamics.............................364 Seasonal Snow and Freshwater ice........................655 Sea ice.................................................1046 Glaciers and ice Sheets.................................1277 Permafrost..............................................1658 cryosphere–climate Processes......................1809 The cryosphere and climate change.......................201Glossary..................................................241Notes.....................................................249Annotated Bibliography....................................251Index.....................................................285

Chapter One

In this place, nostalgia roams, patient as slow hands on skin, transparent as melt-water. Nights are light and long. Shadows settle on the shoulders of air. Time steps out of line here, stops to thaw the frozen hearts of icebergs. Sleep isn't always easy in this place where the sun stays up all night and silence has a voice. —Claire Beynon, "At Home in Antarctica"

Earth surface temperatures are close to the triple point of water, 273.16 K, the temperature at which water vapor, liquid water, and ice coexist in thermodynamic equilibrium. Indeed, water is the only substance on earth that is found naturally in all three of its phases. Approximately 35% of the world experiences temperatures below the triple point at some time in the year, including about half of earth's land mass, promoting frozen water at earth's surface. The global cryosphere encompasses all aspects of this frozen realm, including glaciers and ice sheets, sea ice, lake and river ice, permafrost, seasonal snow, and ice crystals in the atmosphere.

Because temperatures oscillate about the freezing point over much of the earth, the cryosphere is particularly sensitive to changes in global mean temperature. In a tight coupling that represents one of the strongest feedback systems on the planet, global climate is also directly affected by the state of the cryosphere. Earth temperatures are primarily governed by the net radiation that is available from the Sun. Because solar variability is modest on annual to million-year timescales (less than 1% of the solar constant), the single most dynamic control of net radiation is the global albedo—the planetary reflectivity—which is heavily influenced by the areal extent of snow and ice covering the earth. The simple but illuminating global climate models of Mikhail Budyko and William Sellers explored this feedback in the late 1960s, demonstrating the delicate balance between earth's climate and cryosphere.

GEOGRAPHY OF EARTH'S SNOW AND ICE

Perennial ice covers 10.8% of earth's land surface (table 1.1 and figure 1.1), with most of this ice stored in the great polar ice sheets in Greenland and Antarctica. Smaller glaciers and icefields are numerous—the global population is estimated at more than 200,000—but these ice masses cover a relatively small area of the landscape. An additional 15.4% of earth's land surface is covered by permafrost: frozen ground that ranges from a few meters to hundreds of meters deep.

In contrast to this permanent ice, seasonal snow and ice fluctuate dramatically. Snow cover is the most variable element of the cryosphere. From 1966 to 2011, the northern Hemisphere winter snow cover reached an average maximum extent of 46.7 × 10⁶ km²: almost half of the northern Hemisphere land mass (figure 1.3). There is almost complete loss of this snow each summer, with permanent snow cover limited to the interior of Greenland and the accumulation areas of other high-altitude and polar ice caps.

Because the Southern Hemisphere continents are situated at lower latitudes (excepting Antarctica), southern snow cover is less extensive. It is also less studied, with satellite composite images of total snow-covered area available only since 2000. The South American Andes, high elevations of southeastern Australia, much of new Zealand, and the islands off of Antarctica all experience seasonal snow, as do the high peaks in tropical east Africa. Based on the July 0°c isotherm, the total area of this maximum snow cover is estimated to be 1.2 × 10⁶ km², with most of this snow residing in the Patagonian icefields of South America. Combined with the permanent blanket of snow over Antarctica, this gives a peak Southern Hemisphere terrestrial snow cover of 15.1 × 10⁶ km², approximately one-third that of the northern Hemisphere.

There is less of a seasonal cycle for the Southern Hemisphere snowpack, as most of Antarctica is too cold to experience summer melting. Snows are perennial across the frozen continent, with melting confined to the coastal periphery. As a tangential but delightful consequence of this, earthshine is exceptionally bright in December and January, when the Sun is sojourning in the Southern Hemisphere and reflected sunlight from Antarctica adds to the solar illumination of the Moon. in a sense, everyone in the world can see the Antarctic snows in the lunar orb.

The white blanket that spreads over the land surface each winter has a direct parallel in the high-latitude oceans, where sea ice forms a thin veneer that effectively transforms water to land for much of the year. Figure 1.2 illustrates a "field" of snow-covered ice floes aligned by the wind during sea-ice breakup in early summer (June 2005). Sea ice is made up of a combination of first-year and multiyear ice. First-year ice forms anew from in situ freezing of seawater each year. Multiyear ice has survived at least one summer melt season, persevering through two main mechanisms: (i) some ice remains at high latitudes as a result of being landfast, stuck within a channel or bay, or cycled within ocean gyres that trap rather than export the ice; (ii) ice floes ridge or pile up in areas of convergence, producing thick, resilient ice. These mechanisms often operate in concert and are more prevalent in the Arctic than the Antarctic, resulting in a thicker ice cover and more multiyear ice in the north.

Relative to the continents, seasonal cycles of ice in the oceans are more hemispherically symmetric (table 1.1), although there are interesting north–south contrasts. Passive microwave remote sensing for the period 1979–2011 indicates an average minimum northern Hemisphere ice area of 4.8 × 10⁶ km², typically reached in September. Maximum ice cover is usually attained in late winter, with an average March ice-covered area of 13.6 × 10⁶ km². Sea Figure 1.2. Snow-covered sea ice floes and melt ponds during spring breakup, Button Bay (Hudson Bay), Manitoba, Canada. The ice floes are aligned by the wind. Scientific instrumentation (a ground-based microwave scatterometer) is visible in the center of the picture. (Photograph by John yackel.) ice in the Southern ocean has a larger seasonal cycle, with relatively little multiyear ice. Annual mean sea-ice cover in the south is 8.7 × 10⁶ km², varying from 1.9 × 10⁶ km² (February) to 14.5 × 10⁶ km² (September).

Combining the hemispheres, global sea-ice area is relatively constant, varying from 15.4 × 10⁶ to 20.8 × 10⁶ km², with a minimum in February and a peak in November. Global ice extent—the area of the oceans containing sea ice, as demarcated by the ice edge—varies from 18.4 × 10⁶ to 27.3 × 10⁶ km². Mean annual global ice area and extent are 18.5 × 10⁶ and 23.9 × 10⁶ km².

Combining the snow and sea-ice cover, the seasonal cryosphere blankets 59 × 10⁶ and 30 × 10⁶ km² in the northern Hemisphere and Southern Hemisphere, respectively. Figure 1.3 illustrates the geographic distribution. Additional elements of the cryosphere include seasonally frozen ground and freshwater (river and lake) ice.

This snow and ice cover influences the surface albedo and energy budget of the planet fluxes of heat and moisture between the atmosphere and surface and the patterns of circulation in the ocean and atmosphere. Each element of the global cryosphere interacts with and affects weather, climate, and society, and each is highly sensitive to global climate change.

This book explores the physics and characteristics of the global cryosphere, with an emphasis on cryosphere–climate interactions. Chapter 2 presents an overview of the structure of snow and ice in its various manifestations on earth, including the material properties that make it such a peculiar and intriguing substance. The thermo dynamics of snow and ice are examined in chapter 3. Chapters 4–7 provide an overview of Earth's different cryospheric realms: seasonal snow, freshwater ice, sea ice, glaciers, ice sheets, and permafrost. I discuss cryosphere–climate processes and the role of the cryosphere in the global climate system in chapter 8. Chapter 9 concludes with a perspective of cryospheric changes throughout Earth history, including a brief overview of cryospheric vulnerability to recent and future climate warming.

This offers an entry to some of the important aspects of snow and ice in the global climate system. It is not possible to provide a comprehensive overview within the pages of this volume, so I confine the focus to a basic physical introduction to the cryosphere and cryosphere–climate interactions. Many fascinating aspects of the cryosphere, such as avalanche science, snow and ice micro physics, ice cores, and cold regions geomorphology, are overlooked. Interested readers will find many excellent texts that delve deeper into snow and ice science, and suggestions for further reading for each chapter are provided in the Annotated Bibliography.

Chapter Two

Even as our cloudy fancies take Suddenly shape in some divine expression ... This is the poem of the air, Slowly in silent syllables recorded —Henry Wadsworth Longfellow, "Snowflakes"

Much has been written of the ephemeral beauty and singularity of snowflakes. There is little in nature that can rival a snowflake's intricate, delicate architecture. Snow crystals are simultaneously unique yet ubiquitous in blanketing the landscape of much of the world during the winter months. Snow and ice have unusual material properties and a very specific crystalline structure, imparted by the molecular character of water and the nature of intermolecular bonds in the ice crystal lattice. This chapter provides an overview of some of the macroscopic properties of snow and ice that shape the cryosphere and its influence on earth's climate.

CRYSTAL STRUCTURE

Water—familiar, household H₂O—has a simple molecular arrangement, but this simplicity and familiarity disguise the fact that water is a rather peculiar substance. Hydrogen atoms within a water molecule are held to the oxygen atom by strong covalent bonds. The hydrogen atoms are grouped together on one side of the oxygen atom with a bond angle of 104.5°. two electron pairs sit on the other side of the oxygen. This structure gives water molecules a strong polarity, with a positive charge on the side with the hydrogen atoms and a negative charge opposite to this, associated with the electron pairs. This dipolar nature creates strong intermolecular bonds between water molecules, as hydrogen atoms are attracted to the electron pairs of adjacent molecules. The resulting intermolecular hydrogen bonds are even stronger as a result of water's small molecular size, which allows close packing.

Water molecules group in a tetrahedral form, which should produce bond angles of 109.5°. The strong repulsion between the electron pairs distorts this, producing a lower bond angle of 104.5° in the liquid or vapor phase and giving water molecules a "bent" shape. In the solid phase, ice crystals form from individual water molecules bonded in symmetric, hexagonal plate structures (figure 2.1).

A number of different ice-crystal structures have been experimentally identified, but hexagonal ice (I_h) is the only structure that forms at the range of temperatures and pressures relevant to earth's climate. Cubic ice (I_c) is found in ice crystals in the extreme cold temperatures of the upper atmosphere, where it can form at temperatures below 150 K. This structure is also expected at the low temperatures felt elsewhere in the solar system. More exotic, high-density ice structures have been experimentally produced at high pressures, but these are not found naturally on earth.

The hexagonal symmetry of ice crystals results from the tetrahedral bonds of H₂O as water molecules freeze into a crystal lattice (figure 2.1a). In the solid phase, each hydrogen atom is still shared with an adjacent oxygen atom via hydrogen bonds, but the crystal lattice structure opens up to the tetrahedral bond angle of 109.5°. This results in an open, hexagonal planar structure.

Snowflakes in the atmosphere are not usually initiated through spontaneous (homogeneous) nucleation. Supercooled water droplets exist to temperatures as low as -40°c, as the low vapor density and continual movement of the air makes it difficult for ice crystals to nucleate (compared with, e.g., lake ice that forms in water). At subfreezing temperatures, water in clouds consists of a mixture of vapor, supercooled droplets, and ice crystals, something known as mixed clouds. A surface for nucleation, such as pollen, dust, or another ice crystal, helps to seed ice-crystal growth. Where present, such cloud condensation nuclei provide a surface for condensation as well as deposition and greatly facilitate cloud development.

Once nucleated, ice crystals grow in clouds through vapor deposition, as well as through collision and coagulation with other ice crystals. Because of curvature effects, the saturation vapor pressure over a plate-like ice surface is lower than that at the surface of a spherical liquid water droplet. This creates a favorable vapor pressure gradient that allows nascent ice crystals to out-compete water droplets with respect to vapor diffusion, and snowflakes grow at the expense of supercooled water droplets in a cloud. Once a snowflake become massive enough, it drifts to the ground, commonly experiencing modification en route through collisions or partial melting. These processes often cause snowflakes that reach the ground to be rounded or fragmented, rather than the textbook dendritic, stellar crystals of figure 2.1b.

DENSITY OF SNOW AND ICE

The geometric arrangement of this lattice structure is spacious, giving water the most unusual property of having a solid phase that is less dense than its liquid phase. At 0°c, water has a density of 1000 kg m^-3, whereas pure ice (I_h) has a density of 917 kg m^-3. Ice floats in its own melt, one of few substances to do so. Diamonds, germanium, gallium, and bismuth, all structurally similar to ice, also float in their own liquid. Imagine the sight of sunlight sparkling off of a diamond-berg in a sea of liquid diamond! But this is not to be found at earth's surface temperatures and pressures.

Figure 2.2 plots the density of pure ice and water as a function of temperature. This plot also illustrates the unusual density inversion for freshwater. Pure water has its maximum density at 4°c, and it becomes less dense as it cools below this. The reason for this is not fully understood, but it is related to the angle of the hydrogen bonds in the rigid, low-density crystal lattice that characterizes water in its solid phase. To quote James trefil, "water never quite forgets that it was once ice."

Water density continues to decrease below 0°c in supercooled water droplets (figure 2.2a). This density inversion is specific to freshwater. Salt content in seawater makes it more dense: 1028 kg m^-3 for surface water with a temperature of 0°c and a salinity of 35 ppt. Dissolved ions in salt water also interfere with the molecular packing of water molecules, causing it to behave more like a regular liquid. Where salinity exceeds 24.7 ppt, density increases continuously as temperature drops to the freezing point. This is the case in most of the world's oceans. Seawater with a salinity of 35 ppt freezes at -1.9°c.

(Continues...)

Excerpted from THE CRYOSPHEREby Shawn J. Marshall Copyright © 2012 by Princeton University Press. Excerpted by permission of PRINCETON UNIVERSITY PRESS. All rights reserved. No part of this excerpt may be reproduced or reprinted without permission in writing from the publisher.
Excerpts are provided by Dial-A-Book Inc. solely for the personal use of visitors to this web site.