The Cryosphere

THE CRYOSPHERE

Princeton Primers in Climate

David Archer, The Global Carbon Cycle

Geoffrey K. Vallis, Climate and the Oceans

Shawn J. Marshall, The Cryosphere

THE CRYOSPHERE

Shawn J. Marshall

Copyright © 2012 by Princeton University Press

Published by Princeton University Press

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ISBN 978-0-691-14525-9

ISBN (pbk.) 978-0-691-14526-6

Library of Congress Cataloging-in-Publication Data

Marshall, Shawn.

The cryosphere / Shawn J. Marshall.

p. cm. — (Princeton primers in climate)

Includes bibliographical references and index.

ISBN 978-0-691-14525-9 (hardcover) — ISBN 978-0-691-14526-6 (pbk.)

1. Cryosphere. 2. Climatic changes. I. Title.

QC880.4.C79M73 2011

551.31—dc23

2011017995

British Library Cataloging-in-Publication Data is available

This book has been composed in Avenir and Minion Pro

Printed on acid-free paper. ∞

Printed in the United States of America

10  9  8  7  6  5  4  3  2  1

Contents

Preface

1 Introduction to the Cryosphere

2 Material Properties of Snow and Ice

3 Snow and Ice Thermodynamics

4 Seasonal Snow and Freshwater Ice

5 Sea Ice

6 Glaciers and Ice Sheets

7 Permafrost

8 Cryosphere–Climate Processes

9 The Cryosphere and Climate Change

Glossary

Notes

Annotated Bibliography

Index

Preface

ALL CANADIANS COME INTO A WORLD THAT IS SHAPED by snow and ice, and I am exceptionally privileged in this regard. I was born in the heart of winter in Matheson, Ontario, a small mining town in the northern part of the province where one can count on snow cover from Halloween to Mother’s Day. For half a year the snow is part of your fabric, and for the other half of the year one can bike, run, and paddle the landforms and lakes formed by the great ice sheets that carved the landscape. I took up Nordic ski racing, where I became a student of the subtle influences of snow temperature, moisture, and texture on grip and glide. I have enjoyed the company of too many friends to name (and I include my graduate students in this) on ski trips and in glacier field work. Faron Anslow, Joe Shea, Tara Moran, Kate Sinclair, Gwenn Flowers, Dave Hildes, and Phil Hammer stand out within this group. Thanks also to several pagophilic friends and colleagues that helped with this text, including Jacqueline Dumas, Eric White, Cecilia Bitz, Camille Li, and Tom Lambert.

Professionally, I am indebted to many colleagues for exposing me to the scientific wonders of snow and ice. As an undergraduate student at the University of Toronto, I had a fortunate confluence with Dick Peltier, who loaned me a copy of Imbrie and Imbrie’s Ice Ages: Solving the Mystery. This exposed me to the incredible rhythms and unresolved mysteries of Ice Age cycles: Earth’s definitive testimony to the importance of the cryosphere in climate dynamics. I went on to study glaciology at the University of British Columbia, where I enjoyed graduate courses from J. Ross McKay, Dave McClung, and Garry Clarke in permafrost, avalanche processes, and glacier dynamics. Garry, my Ph.D. mentor, brought me to Trapridge Glacier in the Yukon: a magical setting, one of the best-studied glaciers in the world, and the incubator for many of the ideas, instruments, and research methods that underlie current understanding of glacier dynamics. Helgi Björnsson entrained me in studies of ice caps in Iceland, where fire and ice clash with spectacular results. Martin Sharp invited me to field studies on Ellesmere Island in the Canadian high Arctic, where the spring landscape is a brilliant white as far as the eye can see.

This text is intended as a brief introduction to the topic, but I hope that these pages capture the essential physics and character of the cryosphere and inspire others to further exploration of the cryosphere’s role in Earth’s climate. Although I have seen snow and ice in many guises, I am still a student of cryosphere dynamics. I have striven to provide a balanced perspective on all aspects of the global cryosphere, but I suspect that the depth of my understanding of certain issues, and lack thereof, shines through in places. The suggestions for further reading provide greater detail on all topics and will help to fill gaps in my coverage. Sincere thanks to Bob Bindschadler, Koni Steffen, and two anonymous reviewers for their suggestions, which have improved this text. Alison Kalett at Princeton University Press has been a delight to work with, and I thank Alison and the production staff at the Press for their enthusiasm, support, and flexibility.

Lawren S. Harris: Ellesmere Island (1930). (The McMichael Canadian Art Collection, Kleinberg, Ontario, Canada. Gift of Mrs. Chester Harris.)

THE CRYOSPHERE

1 INTRODUCTION TO

THE CRYOSPHERE

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.

Table 1.1

Area and Volume of the Global Cryosphere

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.

Figure 1.1. Global (a) Arctic and (b) Antarctic sea ice cover, February 1, 2011. Also shown are the Greenland and Antarctic Ice Sheets. Data provided by the National Center for Environmental Prediction/NOAA and the U.S. National Snow and Ice Data Center. Images from The Cryosphere Today, University of Illinois at Urbana-Champaign (http://arctic.atmos.uiuc.edu/cryosphere/).

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.

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.)

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 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 thermodynamics 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.

Figure 1.3. Areal coverage of global glaciers, permafrost, and winter snow and sea ice cover. (Image adapted from the UNEP/GRID-Arendal Global Outlook for Ice and Snow (2007). Original map design and cartography by Hugo Ahlenius; http://maps.grida.no/go/graphic/the-cryosphere-world-map.)

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 microphysics, 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.

2 MATERIAL PROPERTIES

OF SNOW AND ICE

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 (Ih) is the only structure that forms at the range of temperatures and pressures relevant to Earth’s climate. Cubic ice (Ic) 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.

Figure 2.1. (a) Hexagonal symmetry and tetrahedral structures of the ice crystal lattice. Each oxygen atom (white sphere) is joined with four other oxygen atoms through covalent bonds with hydrogen atoms (black spheres). (b) Stellar dendrite snowflake, 0.25 mm, as imaged by a low-temperature scanning electron microscope. (Microphotograph courtesy of William Wergin and Eric Erbe, Beltsville Agricultural Research Center, Beltsville, Maryland.)

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