Über die Autorin bzw. den Autor

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

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"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

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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,...