Vanishing Ice: Glaciers, Ice Sheets, and Rising Seas

Antarctic ice shelves are highly impressive. You have to sail up close to them to appreciate their size and apparent solidity…. Every so often, a massive slab on the seaward edge of one of these monsters will flex a little too much; perhaps the surface crevasses will start to make their way deeper; tides work the ice up and down, making more crevasses, more cracks until, finally, the slab breaks off and sails away, forming one of those tabular icebergs, flat-topped and square-shouldered, the size of a floating city, or even a county…. And it is these shelves, on the [Antarctic] Peninsula, that have started to alarm scientists by falling apart.

Lately, many ice shelves have been in serious trouble, and are even endangered in some places. Within a matter of weeks in 1995, around 2,000 square kilometers (770 square miles) of the Larsen A Ice Shelf, near the northern tip of the Antarctic Peninsula, shattered into a slivered jumble of icebergs. While ice shelves routinely calve icebergs, the disintegration was then considered unprecedented. In early 2002, an enormous chunk of ice, roughly the size of Rhode Island in area, split off the Larsen B Ice Shelf to the south of Larsen A, and rapidly disintegrated within a month.³ The size and rapidity of this particular event astounded scientists. Overall, Larsen B lost a total of 9,166 square kilometers (3,540 square miles) between the 1960s and 2008–2009 (table 2.1). Following their disintegration, satellites observed that large glaciers feeding into Larsen A and B had lurched forward, thinned, lowered in elevation, and disgorged myriad small icebergs into the ocean.⁴

Glaciers feeding the remnant Larsen B shelf continue to accelerate and thin. Rifts slice across the ice. These features imply that “the final phase of the demise of Larsen B Ice Shelf is most likely in progress” and may now be “nearing its final act” within the decade,^5,6 Ala Khazendar of the NASA Jet Propulsion Laboratory and University of California and his team have been closely monitoring the incipient demise of the shelf with laser altimeters, depth-probing radar, and radar interferometry for measuring glacier velocities. Khazendar says that although “it’s fascinating scientifically to have a front-row seat to watch the ice shelf becoming unstable and breaking up, it’s bad news for our planet.”⁶

The Larsen B Ice Shelf had remained stable for the 11,000-year period since the end of the last ice age—based on information derived from marine sediment cores and seismic reflections.⁷ Its sudden collapse was therefore more likely related to recent warming than to surging tributary glaciers or sudden ungrounding (fig. 2.1).

Larsen C, to the south of Larsen A and B and now the largest surviving ice shelf on the Antarctic Peninsula, had remained fairly stable since the 1980s, although the shelf showed signs of thinning and lowered elevation.⁸ On July 12, 2017, a giant iceberg the size of Delaware (nearly 5,800 square kilometers [~2,240 square miles]) split off from Larsen C.⁹ A fissure in the ice had been slowly growing for decades, but recently began to spread northward until it finally unleashed the berg. What next for Larsen C? While calving is a normal part of an ice shelf’s life cycle, icebergs this large are rare. The stability of Larsen C will depend on whether future icebergs break free of at least two “pinning points,” or underwater ridges that help anchor the ice shelf. Should the ice shelf lose that support, it would no longer effectively restrain the glaciers feeding into the shelf from speeding up and discharging more ice.

These shelves are not unique. Starting in the 1950s, at least 10 ice shelves on the Antarctic Peninsula shrank by roughly 34,634 square kilometers (13,372 square miles) and fell apart one by one (table 2.1). Nor is ice shelf thinning confined to the Antarctic Peninsula. Since 2003, Antarctic ice shelves have lost 310 cubic kilometers per year (74.4 cubic miles).¹² While Antarctic Peninsula and West Antarctic ice shelves account for over two-thirds of this loss, the colder, more stable East Antarctic shelves also contribute a growing fraction. Unusually warm air temperatures and warm ocean incursions are the likely culprits. If warming persists, disintegration of ice shelves will likely propagate southward to the Antarctic mainland, potentially destabilizing much larger ice shelves. In the words of Theodore Scambos, a glaciologist at the National Snow and Ice Data Center in Boulder, Colorado: “It’s the trailer for the movie that’s going to unfold over the rest of Antarctica for the next 50 to 100 years.”¹³

Analogous Arctic phenomena echo the breakup of Antarctic ice shelves. Ellesmere Island, Canada, lost over 90 percent of a once-continuous band of ice shelves during the twentieth century, with further breakups continuing since 2000. In July 2017, another massive iceberg in northwest Greenland suddenly detached from Petermann Glacier, following two large calving events on the same glacier in 2010 and 2012 (fig. 1.5).

Jakobshavn Isbrae, a large glacier in western Greenland, drains 6.5 percent of the Greenland Ice Sheet.14 This glacier may have calved the iceberg that sank the Titanic.¹⁵ In their 2012 award-winning documentary film Chasing Ice, nature photographer James Balog and his Extreme Ice Survey team vividly captured a major calving event in 2008 on this glacier. One team member excitedly recalls that it was like “watching Manhattan break apart in front of your eyes.” Before 1997, this large glacier flowed into a 15-kilometer (9.3-mile) floating ice tongue in a deep-ocean fjord. In the early 2000s, its ice tongue began to disintegrate and the glacier sped up—as much as four times faster during the summer of 2012 than in the 1990s.¹⁶ In August of 2015, the glacier shed another huge chunk—some 12.5 square kilometers (nearly 5 square miles), claimed by some to be the biggest calving event on record.¹⁷ Regardless of whether or not this set any record, Eric Rignot, a scientist at NASA’s Jet Propulsion Laboratory, was “struck by the sheer size of this calving event, which shows that the glacier continues to retreat at ‘galloping speed.’” These dramatic events underscore the recent rapid Arctic warming¹⁸ that triggered the disintegration of ice shelves and tongues encircling many high Arctic islands and tidewater glaciers. The next section probes why and where ice shelves are crumbling.

Some of the fastest-warming places on Earth are located, surprisingly enough, in Antarctica. Over the past 50 years, Byrd Station on the West Antarctic Ice Sheet (WAIS) has warmed by 0.42°C (0.76°F) per decade.¹⁹ Springs have warmed the most. The −9°C (16°F) annual temperature line—the apparent temperature limit for ice shelf stability—has been slowly creeping southward on the Antarctic Peninsula, reducing ice shelf area by nearly 35,000 square kilometers (13,400 square miles) since the 1960s²⁰ (table 2.1). Referring to the small Sjögren Glacier’s fjord on the northern Antarctic Peninsula, which according to Douglas Fox²¹ once “held ice 600 meters thick as recently as 1995…but now it holds seawater instead.” Regarding the demise of the Larsen A and B Ice Shelves, he wrote: “That more ice shelves will collapse is a foregone conclusion. An average summer temperature of zero degrees Celsius seems to represent the highest temperature at which an ice shelf can exist. And the invisible line where summer averages zero degrees Celsius (32°F) is creeping south along the Antarctic Peninsula tip toward the mainland, along with higher mean annual temperatures. Every ice shelf that the line crosses has collapsed within a decade or so. Next up, south of Larsen B and Scar Inlet, is the Larsen C ice shelf,” the largest remaining shelf on the peninsula, which as if on cue unleashed a huge iceberg in July 2017.

Ice shelves may be even more vulnerable to basal melting, or thawing from beneath, as the ocean warms. Changes in wind circulation over the past several decades have brought warmer air to the Antarctic Peninsula, increasing surface melting that has helped to destabilize the ice shelves. Changed wind patterns have also facilitated the upwelling of warmer deep water. Comparatively mild, dense, salty Circumpolar Deep Water (CDW) now penetrates into numerous submarine troughs that cut across the continental shelf off West Antarctica beneath the ice shelves.²⁴ (Submarine troughs—basically drowned fjords—were gouged out by glaciers during past ice ages at times of lower sea level.) While surface water is near the freezing point, CDW can be several degrees warmer. It therefore melts the ice shelf from below. The resulting meltwater cools, freshens, and lightens the CDW. Seaward-outflowing CDW is then replaced by sinking colder, saltier, and denser overlying shelf water. CDW incursions have thinned ice shelves in the Amundsen Sea Embayment sector of West Antarctica within the past two decades. Ice shelves there melted at a rate of around 19 meters (62 feet) per decade between 1994 and 2012.²⁵ Basal melting is fast outpacing calving as a cause of Antarctic ice loss.²⁶

On the Antarctic Peninsula, more so than in West Antarctica, increases in regional air temperature have contributed to the remarkable string of ice shelf breakups (e.g., the Larsen A and B and Wilkins Ice Shelves; fig. 2.1; table 2.1). Less sea ice around the Antarctic Peninsula (although it is expanding elsewhere in Antarctica—see ”Why Is Antarctic Sea Ice Expanding?” later in this chapter) may also have facilitated ice shelf collapse. Large calving events tend to occur in summer, when sea ice cover is minimal. (Sea ice inhibits wave action and helps prop up an ice shelf.)

Floating ice shelves or ice tongues act like buttresses supporting an old, decaying building. The “backstress” exerted by ice shelves or tongues slows the advance of many Antarctic ice streams, as well as Alaskan and Greenlandic tidewater glaciers. The removal of this support, by processes described in the previous section, is like releasing the floodgates on a dam. Once the ice shelf weakens or collapses, the glacier can surge forward rapidly (fig. 2.2). Following the breakup of the Larsen B Ice Shelf in 2002, its tributary glaciers began to accelerate forward.²⁷ Around 28 percent of all Antarctic ice losses between 1992 and 2011 came from the Antarctic Peninsula.²⁸ However, the fairly small glaciers on the Antarctic Peninsula would add at most only half a meter (20 inches) to sea level if they all melted. More importantly, however, surging glaciers following the demise of their fringing shelves may foreshadow future behavior of the much larger West Antarctic Ice Sheet (WAIS).

Although not yet disintegrating, many ice shelves fringing the WAIS are thinning and weakening as a result of the slight increase in nearby ocean temperatures. The Amundsen Sea Embayment region, including the Pine Island, Thwaites, and Smith Glaciers, once described as the “weak underbelly of West Antarctica,” is warming at a rate comparable to that of the Antarctic Peninsula. The modest ocean warming may have sufficed to initiate the recent acceleration of the Pine Island and neighboring Thwaites Glaciers by eroding their ice shelf bottoms. Pine Island Glacier alone now discharges 20 percent of the total ice lost from the WAIS.²⁹

The WAIS is potentially unstable because large portions of the ice sheet rest on a substrate like a cereal bowl, with edges that slope down steeply landward. A landward-retreating grounding line allows more seawater to penetrate under the ice, setting it afloat (fig. 2.2; see also fig. 6.6). As more ice at the grounding line is eroded and undermined, landward retreat continues, the ice sheet surges forward, and calving increases. This runaway process, originally proposed by J. H. Mercer in 1978, has been called the marine ice sheet instability (MISI).³⁰ While most experts view a catastrophic collapse of the WAIS as an extremely improbable event this century, chapters 6 through 8 will examine recent evidence for greater potential instability than previously thought, raising the prospects of increased sea level rise.

In Greenland, smaller ice tongues at the mouths of marine glaciers play a role analogous to that of Antarctic ice shelves. Oceanfront changes in Greenland have sped up many of its tidewater glaciers.³¹ The Jakobshavn Isbrae glacier accelerated following loss of its ice tongue, as changes in atmospheric circulation beginning in the late 1990s drove warmer ocean water shoreward.³² Its rapid surging may be related to a MISI already under way, which has led to a 12.5-kilometer (7.8-mile) retreat of the grounding line over a 20-year period.³³ Recent surveys of the glacier bed depth indicate a reverse slope within a deep section of the confining fjord, which persists for at least the next 50 kilometers (31 miles) farther inland (a situation similar to the WAIS MISI, described in the previous paragraph). Fast retreat may persist for many decades until the grounding line reaches a shallow section of the fjord or climbs above sea level.

Towering icebergs loom over a barren, brooding Arctic seascape in Frederic Edwin Church’s masterpiece painting The Icebergs (1861) (fig. 2.3). A broken ship’s mast lies trapped in ice in the foreground—a grim reminder of the dangers of the Arctic and the puny scale of human achievements in the face of nature. In 1859, Church (1826–1900), an American artist of the Hudson River School, sailed to Newfoundland and Labrador, where he sketched scenes for his Arctic paintings. The broken mast commemorates Sir John Franklin’s ill-fated 1845 voyage to the northern seas in search of the Northwest Passage. Trapped in sea ice, Franklin and his men were forced to abandon their ships, Erebus and Terror, and set out in lifeboats. One by one, the men succumbed to illness, hypothermia, and lead contamination from food cans or the ships’ pipes. After numerous unsuccessful search parties in the following years and retrieval of several relics from the expedition, the remaining mystery was finally solved with the location of the two ships on the ocean floor in 2014 and 2016, respectively (see chap. 1).

Icebergs were “once plentiful,” some even “grand and imposing,” near Battle Harbor, Labrador, a booming nineteenth-century fishing and sealing town.34 Louis Legrand Noble (1813–1882), a writer who accompanied Frederic Church on his journey, described the “wandering alp of the waves,” stating that “icebergs, to the imaginative soul, have a kind of individuality and life. They startle, frighten, awe; they astonish, excite, amuse, delight and fascinate; clouds, mountains and structures, angels, demons, animals and men spring to the view of the beholder.” Individual icebergs received fanciful names: the “Alpine Berg,” the “Great Castle Berg,” and the “Rip van Winkle Berg.” Other icy giants were compared to England’s Windsor Castle, or to Niagara Falls.

Church sketched a mountainous iceberg floating near Twillingate, Newfoundland, on July 4, 1859. In the summer of 2008, another “4th of July Iceberg,” reduced to a few exposed masses peeking above water, sat in a sheltered cove, “perspiring” and “melting into rivulets that cascaded down the slope of ice.”³⁵ As ice cracked apart, the sound reverberated “like a cannon.” Twillingate is now a prime tourist destination for viewing the passing parade of icebergs.

Icebergs form when chunks of ice break off tidewater glaciers and ice streams, or split off ice shelves. Most Arctic icebergs calve off major tidewater glaciers of Greenland and northern Ellesmere Island, Canada, with lesser inputs from the Svalbard archipelago north of Norway and islands of the Russian Arctic.36 Greenland glaciers produce ~10,000 to 30,000 icebergs per year, or as many as 40,000 by some estimates. Many pass through the Fram Strait into the East Greenland Current and around southern Greenland. Swept north counterclockwise around Baffin Bay, they head south again along the coast of Baffin Island and Davis Strait toward the Labrador Sea. The Labrador Current carries the bergs farther south to “Iceberg Alley” along the Labrador and Newfoundland coasts. On average, only 1–2 percent make it as far south as St. Johns, Newfoundland (48°N latitude).³⁷ The peak season for icebergs drifting to 48°N extends from March to June, with two-thirds observed by April. However, actual numbers fluctuate greatly from year to year, reaching a high of ~2,200 in 1984 and none in 1966 and 2006. Many survive as far south as 39°–41°N, until they encounter the warmer waters of the Gulf Stream where they rapidly melt. (The southernmost iceberg ever spotted in the Atlantic reached around 30°N in 1926, some 240 kilometers [150 miles] southeast of Bermuda.)

Unlike the recent decline in Arctic summer sea ice (see “Losing Arctic Sea Ice” later in this chapter), iceberg numbers show no consistent trend, in spite of an apparent decline since the late 1990s. Nevertheless, more icebergs reached 48°N latitude in 2014 than in any year since 1998, underscoring the high degree of annual variability in iceberg production. Yearly iceberg counts may be related to winter sea ice coverage in the Davis Strait and Labrador Sea.³⁸

Around Antarctica, icebergs split off ice shelves, such as the Ross and Ronne-Filchner shelves. They drift with the prevailing coastal currents; some run aground in embayments, while others are eventually entrained clockwise by the Antarctic Circumpolar Current and drift as far north as 55°–60°S, with a few occasionally spotted near southern New Zealand.

Although Inuit hunters, the Vikings, and whalers were quite familiar with icebergs, the tragic sinking of the Titanic in April 1912 was probably the most publicized historic encounter with a hulking mass of floating ice. Long a menace to shipping, icebergs form by breaking off the edge of a glacier that ends in the sea, or off an ice shelf still attached to the shore. Moved by winds and currents, they slowly drift toward warmer waters where they eventually melt. The last tiny bits to melt fizz like seltzer due to the numerous air bubbles trapped in ice, originally snow compressed into firn and ultimately glacial ice.

The International Ice Patrol (IIP), founded in 1913 in response to the sinking of the Titanic, monitors the presence and movement of icebergs in the North Atlantic and Arctic Oceans, along major shipping lanes.39 Operated by the United States Coast Guard, the International Ice Patrol is funded by a consortium of 13 nations from North America, Europe, and Japan involved in transatlantic shipping. The IIP tracks icebergs from aircraft (and increasingly, satellites) between February and August; the balance of the year is covered by the Canadian Ice Service, which cooperates closely with the IIP. Passing ships also provide information on ice conditions.

The United States National Ice Center (NIC), operated jointly by the National Oceanic and Atmospheric Administration, U.S. Coast Guard, and U.S. Navy, provides accurate and timely information on ice and snow conditions. For example, the NIC generates daily and weekly maps of sea ice extent and ice edge around polar regions, maps the sea ice edges around Alaska and the Great Lakes (the latter when ice is present), provides daily maps of Northern Hemisphere snow cover, and provides weekly locations of major Antarctic icebergs. Synthetic aperture radar (SAR) on satellites, such as the Canadian RADARSAT-2 and European CryoSat-2, now provide high-resolution images independent of weather and time of day or night. Other instruments that track icebergs from space include MODIS, Landsat 7 and 8, Thematic Mappers, and ASTER, which acquire multispectral images in the visible, near- tomiddle-infrared regions (appendix B).

Icebergs look bright white because of myriad tiny trapped air bubbles that scatter all wavelengths of light. Bubble-free ice, on the other hand, appears a soft aqua to deep neon blue, because ice absorbs more of the red wavelengths, allowing more blue and blue-green light to be transmitted or reflected.

Icebergs come in a wide variety of sizes and shapes, ranging from the smallest, colorfully named growlers and bergy bits a few meters across to massive giants over hundreds of meters across (table 2.2). Iceberg B-15, which calved off the Ross Ice Shelf, Antarctica, in March of 2000, holds the current world record at 11,000 square kilometers (4,250 square miles).⁴⁰ It subsequently broke into several pieces, the largest of which, B-15A, was over half the size of the original. Shapes vary from very large, flat-topped, tabular masses such as B-15 to cube-shaped, steep-sided blocks to alp-like pinnacles or spires to domes, wedges, or quite irregular shapes. Church’s paintings vividly capture some of these fantastic forms (fig. 2.3). Spired, domed, and steep, pinnacle-shaped icebergs occur more frequently in the Arctic, whereas flat, tabular masses are more characteristic of Antarctica. (The closest Arctic analogues to Antarctica’s tabular icebergs are the flat-topped ice islands calved off the ice shelves of northern Ellesmere Island.) A chaotic mix of ice and icebergs less than 2 meters (6.5 feet) is referred to as brash ice.

The life expectancy of an iceberg varies greatly, depending on size and drift patterns. Large icebergs that travel from Greenland around Baffin Bay may last 3 years or more before exiting Davis Strait into the Labrador Current.⁴² During the summer and fall in the Labrador Sea, a small, irregular iceberg may disintegrate within days, while larger ones can survive several weeks.

Tides, storm waves, underwater currents, and collisions with nearby icebergs exert stresses on the floating extension of a marine glacier or ice shelf. Friction along the base and edges of a moving glacier stretches the ice and opens cracks, or crevasses. These crevasses may then become incorporated into the iceberg. Variations in ice velocity set up strains that determine the location and depth to which crevasses will penetrate. Other crevasses develop at the boundary between the floating and grounded portions of the ice (i.e., where ice rests on solid bedrock), or near the shelf’s seaward edge (fig. 2.2). Repeated bending and flexing by ocean waves helps break up large, tabular bergs and ice shelves.⁴³ Major calving events in Antarctica, such as that of B-15, occurred after huge rifts, up to several hundred miles long and a few miles wide, formed in the ice shelf and penetrated the full thickness of the shelf. Large pieces of ice split off more readily once crevasses reach the base.

Icebergs deteriorate further by breaking apart and by melting, especially in warmer water. Seawater, generally several degrees warmer than ice, melts it at or below the waterline.44 This underwater erosion may unbalance the iceberg and topple it over, posing a danger to passing ships. In addition, wave action can erode a notch into the ice, eventually undermining the iceberg enough to cause pieces to split off.⁴⁵

Aside from their threat to shipping, exemplified by the tragic sinking of the Titanic, icebergs indirectly influence global sea level and ocean circulation. Since icebergs displace their weight in water and float, they do not raise sea level when they melt. However, the number of icebergs shed by glaciers does affect sea level indirectly. All other things staying equal, an increasing quantity of calved icebergs is one measure of diminishing ice mass on glaciers or ice sheets. Unlike ice already afloat, the newly added ice will raise sea level.

Melting of icebergs dilutes the salinity of seawater, which in turn may weaken the sinking of dense, cold North Atlantic Deep Water and slow down the Atlantic Meridional Overturning Circulation (AMOC), as described in chapter 1. Such changes in ocean circulation could boost regional sea level along the east coast of North America. But a different form of floating ice may more strongly influence regional and global climate.

Unlike ice shelves and icebergs, sea ice has no prior connections to land; it freezes directly from seawater. In the winter season, sea ice completely fills the Arctic Ocean and encircles Antarctica. What is sea ice, and in what ways does it differ from an ice shelf or iceberg? How do losses of sea ice affect the vanishing cryosphere? The next section will seek answers to these questions and more.

Until fairly recently, sea ice has presented a nearly impenetrable obstacle to ships navigating Arctic waters. Most of the historic expeditions to the Arctic in search of a Northwest or Northeast Passage ended in failure or disaster, victims of the harsh climate and treacherous sea ice. This is rapidly changing with the sharp decline in Arctic Ocean sea ice cover since the 1980s—one of the most conspicuous signs of an endangered cryosphere. The September 2012 Arctic sea ice extent set a new record low (see fig. 1.4), while 2016 tied with 2007 as second lowest.⁴⁶ A nearly ice-free summer Arctic Ocean could become a reality within decades (see “Losing Sea Ice,” later in this chapter).

Sea ice strongly influences the climate system. The high albedo of sea ice cools the surroundings, while a thick ice cover insulates the ocean from heat losses and inhibits the exchange of gases, such as water vapor and carbon dioxide, between ocean and atmosphere (see chap. 1). Freezing sea ice expels salt, which modifies the density of seawater, and thereby may alter ocean circulation. Changes in regional temperatures also modulate sea ice extent and thickness, initiating a string of feedbacks that in turn may affect the climate over broader regions. Finally, sea ice constitutes a major habitat for the marine mammal, fish, and plant ecosystems upon which many Arctic indigenous populations depend (chap. 9).

Sea ice, unlike icebergs or ice shelves, forms directly from ocean water. The average ocean water salinity of around 35 parts per thousand (ppt), or 3.5 percent, depresses the freezing point from 0°C (32°F) to around −1.8° to −1.9°C (28.8°–28.6°F). In calm water, sea ice forms tiny, platy ice crystals under 2–3 millimeters (0.08–0.12 inch) in diameter, floating on the surface (box 2.1).

BOX 2.1

The crystal lattice of ordinary hexagonal ice illustrated in figure 1.2a is characterized by a c-axis with sixfold symmetry (in the vertical direction) and three equal a axes that lie 120° apart in a plane perpendicular to c. Typical ice crystals are flattened hexagonal plates perpendicular to c (like the snowflake in fig. 1.2b) and hexagonal prisms, where c is the long axis. Sea ice initially forms flat, platy hexagonal crystals, oriented with c-axes in the vertical direction, i.e., perpendicular to the ocean surface. Further growth occurs sideways, often as dendritic (branching) arms, like those of snowflakes. These fragile crystals break apart easily, but quickly reassemble and merge into a thin layer of translucent ice. By the time crystal grains are more or less touching, further horizontal expansion is inhibited, and therefore growth continues downward. In a type of “crystalline Darwinism,” those plates favorably oriented with their c-axes horizontal (i.e., now parallel to the ocean surface) grow preferentially downward into larger, elongated columnar crystals, characteristic of congelation ice.*

* The preferred growth direction points downward (in the plane of the a-axes); Wadhams (2003).

The platy crystals soon spread into a thin layer of frazil, or grease ice (named after its appearance), that coalesces into a thin, translucent sheet of nilas ice (fig. 2.4a). Nilas ice first turns gray, then white, eventually thickening into a more stable, smooth-bottomed sheet called congelation ice, characterized by downward-pointing, elongated crystals (box 2.1). Although the growing ice crystal squeezes out most of the salt in a process called brine rejection, some seawater is trapped in fluid inclusions. When this ice refreezes after the summer melt season, the salinity is reduced even further.

In rougher water, the frazil crystals collide and are compressed into slushy cakes that develop into larger slabs of pancake ice (named after their shape), often with raised rims due to multiple collisions (fig. 2.4b). These coalesce into still larger floes that often override or raft onto one another, or form ridges, and ultimately expand into a continuous sheet of consolidated pancake ice. Ice ridges that form by collision between thicker floes can rise 2 meters (6.6 feet) or more above water, with a correspondingly thicker submerged portion. In the Arctic, first-year ice that lasts into spring ranges from at least 30 centimeters (1 foot) to 1–2 meters (3.3–6.6 feet) deep.⁴⁷ Multiyear ice that has survived two or more summers may grow as thick as 4–6 meters (13–19.71 feet) deep.

By summer, the overlying winter’s snow and ice begin to melt, forming surface pools. Meltwater thaws its way or trickles downward through small pores, crevasses, and channels, flushing out much of the remaining brine. Thus, sea ice that survives the summer melt season is purer than first-year ice. The thin, young sea ice admits more of the sun’s energy, which enhances bottom melting.

November of 1978 ushered in a new era of Arctic sea ice observations with the launch of the Nimbus-7 Scanning Multichannel Microwave Radiometer (1978–1987). It was followed by the Special Sensor Microwave Imagers (SSM/I) aboard U.S. Defense Meteorological Satellite Program (DMSP) satellites beginning in 1987. These and other satellites reveal a steady decline in sea ice covering the Arctic Ocean since 1979, with greater losses since the late 1990s.48 Recent satellite measurements demonstrate a clear correlation among vanishing sea ice, a darker Arctic Ocean, and greater polar warming.⁴⁹ The albedo declined from 0.52 to 0.48 between 1979 and 2011. Arctic air temperatures have been rising faster than those in the rest of the Northern Hemisphere, especially toward the latter part of the twentieth century—a phenomenon known as “polar amplification” or “Arctic amplification.” In recent decades, Arctic temperatures have climbed almost twice as fast as those in the rest of the world. The years 2011 to 2015 were warmer than at any time since 1900.⁵⁰

The September (summer) minimum sea ice extent plunged by 13.2 percent per decade between 1979 and 2017 as compared to the average for 1981–2010⁵¹ (fig. 2.5). Across the Arctic, the melt season lengthened by five days per decade between 1979 and 2016.⁵² Although more ice melts toward the end of summer than in spring, the earlier melt onset is more significant because it occurs at a time of year with nearly 24-hour daylight and solar energy input to sea ice reaches a peak.

Declining Arctic sea ice: average September Arctic sea ice extent, 1979–2017. (National Snow and Ice Data Center, “Arctic Sea Ice 2017: Tapping the Brakes in September,” October 5, 2017, http://nsidc.org/arcticseaicenews/2017/10/arctic-sea-ice-2017-tapping-the-brakes-in-september/; figure courtesy of the National Snow and Ice Data Center)

Winter maximum (February/March) sea ice extent has followed a persistent downward trend for the last 39 years and reached record low levels 4 years in a row, from 2015 to 2018. Arctic sea ice hit its lowest winter maximum extent to date on March 7, 2017.⁵³ A year later, March of 2018 ranked second lowest.⁵⁴ Most troubling is the plummeting older, thicker multiyear ice, down from 20 percent of the total Arctic ice extent in the 1980s to less than 5 percent by 2014.⁵⁵ Ice older than 4 years constituted only 1.2 percent of the total by 2016; most sea ice is now first-year ice.⁵⁶ The marked decline in old sea ice and its replacement by younger, thinner ice may be one of the most important changes affecting the Arctic cryosphere.⁵⁷

Unlike other Arctic regions, the North Pole still possesses a nearly intact sea ice cover at the end of the melt season, ranging from 1.25 to 2.6 meters (4.1–8.5 feet) in thickness.58 Nevertheless, several recent years have witnessed record summer ice minima, especially 2007, 2012, and 2016. Twice as much ice melted from below between 2008 and 2013 than between 2000 and 2005. Even in this region, sea ice may not remain intact for long.

Global warming affects vertical atmospheric temperatures differently near the poles than at the tropics.60 This too contributes to sea ice loss. As tropical surface temperatures increase, more moist air masses rise aloft and develop into thick storm clouds. Heat released by condensation of water vapor in the clouds then warms the overlying atmosphere more than near the surface. On the other hand, cold, dense Arctic air, even if warmed by a few degrees, stays near the surface and does not mix with air aloft. Instead, any added heat penetrates into the upper ocean, where it melts more sea ice and reinforces Arctic amplification. Abundant late summer–early autumn low clouds at a time of minimal sea ice cover also increase evaporation and relative humidity. The open water and fall-winter release of stored ocean heat help strengthen Arctic warming.⁶¹ The shrinking Arctic sea ice cover therefore results from natural atmospheric variability, together with thinner ice cover and increased summer melting.⁶²

Is Arctic sea ice therefore trapped in an irreversible downward spiral? Is summer sea ice approaching a point of no return—a so-called “tipping point”? How soon will we see an ice-free Arctic Ocean in summer? The record-shattering (to date) September of 2012 surprised experts and generated intensive speculation over the fate of Arctic sea ice. Such a speedy retreat took climate modelers by surprise. This led some researchers to speculate that summer Arctic sea ice could disappear within a few decades. But this opinion may be premature because of the large year-to-year variability (see, e.g., fig. 2.5) and capacity for ice recovery under favorable conditions. Negative feedbacks could partially counteract the ice-albedo and cloud feedbacks, at least temporarily. For example, thinner ice grows back faster than thick ice. As thin ice floes converge, they can raft and form thicker and stronger ridges. Therefore, a few cold years could temporarily slow down or reverse the recent negative trend. Such random fluctuations in natural processes could in fact account for as much as 40 percent of the recent sea ice losses.⁶³

If the jet stream continues to exhibit slower-moving, higher-amplitude waves, these harsh weather conditions will grow even more intense and stay in place longer, multiplying their potential for death and destruction. If the theories…are correct, there is no going back to our old climate unless we find a way of growing more Arctic sea ice.

The chain of cascading climate changes triggered by diminishing Arctic sea ice may reverberate far to the south. Be prepared for an increasingly “weird” jet stream, according to Jeff Masters, director of meteorology at Weather Underground. “Our recent jet stream behavior could well mark a crossing of a threshold into a new, more threatening, higher-energy climate,” he says.⁶⁵ Arctic amplification has reduced the temperature contrast between midlatitudes and the North Pole. Some scientists therefore speculate that a weakened north-south temperature gradient makes the jet stream more sluggish and wavier, much like looping meanders in the flat floodplain near a river’s mouth. The slowly propagating, sinuous air circulation pushes warmer temperatures north and Arctic cold south and sets the stage for “blocking patterns,” or persistent extreme weather in some regions: heat waves or droughts in summer or colder, snowier winters.⁶⁶ Low summer/autumn sea ice may also stack the deck for more frequent outbreaks of a “warm Arctic/cold continent” pattern, analogous to the negative phase of the AO, in which a weaker, wavier jet stream brings outbreaks of frigid Arctic temperature and snowstorms to the continental United States, Europe, or eastern Asia, while conversely, balmier conditions prevail in parts of Alaska or Siberia.

While Arctic sea ice extent has been sliding downhill since the 1980s, its Antarctic counterpart has held steady or grown slightly. Antarctic winter sea ice extent reached record maxima in 2012, 2013, and 2014. Why the contrarian Antarctic behavior? The answer, in part, lies in the marked geographic differences between the two polar regions. The Arctic Ocean, unlike the Southern Ocean, is almost entirely surrounded by land, so that most sea ice just stays there. Warmer ocean water can penetrate into the Arctic from the south via the Barents Sea and the Bering Strait, facilitating sea ice melting. Sea ice surrounding Antarctica, on the other hand, unconstrained by landmasses, drifts northward with prevailing winds and currents into warmer waters where it eventually melts. Thus, nearly all Antarctic sea ice that forms in winter melts the following summer and is therefore much thinner than in the Arctic (typically only 1–2 meters [3–6 feet] thick versus 2–3 meters [6–9 feet] thick, respectively). Furthermore, strong westerly winds and currents (e.g., the Antarctic Circumpolar Current) prevent warmer air and water from reaching Antarctica, keeping it cold. The Southern Ocean also delivers more moisture to Antarctica, covering sea ice there with a thicker, better-insulating layer of snow than is found in the drier Arctic.

Paradoxically, global warming may have indirectly led to the expansion of Antarctic sea ice. This apparent contradiction may be understood by closely examining the thread connecting recent thinning of Antarctic ice shelves, ice sheet losses, and sea ice growth.69 The Southern Ocean surrounding Antarctica has warmed significantly at depths between 100 and 1,000 meters (330–3,300 feet) in recent decades, as have most oceans elsewhere, whereas melting directly beneath ice shelves has cooled and freshened the upper 100 meters.⁷⁰ Recall that Antarctic ice shelves can be up to hundreds of meters thick, whereas sea ice is only several meters thick. Antarctic sea ice freezes rapidly in winter and expands into the colder, fresher surface water. On the other hand, the base of the ice shelves is bathed by warmer Circumpolar Deep Water, which thins and weakens them, occasionally to the point of spectacular disintegration, as in the case of the Larsen B and Wilkins Ice Shelves. Glaciers and ice streams abutting thinned ice shelves have often advanced and shed icebergs that cool the upper ocean layer and stimulate sea ice growth.

Changing regional wind patterns may also play an important role.⁷¹ Shifting winds may have physically pushed sea ice to cover wider areas or locally warmed or cooled the ocean surface. Changes in both winds and ocean temperatures may have led to the contrarian growth of Antarctic sea ice, and its paradoxical recent reversal, but further research is needed to resolve this issue.

In spite of any apparent Antarctic gains, sea ice continues its global downward slide.⁷² Furthermore, the once-expanding Antarctic sea ice cover has shown signs of contraction ever since September 2016, a period that saw record or near-record lows. The icy continent experienced its second-lowest summer sea ice minimum in late February of 2018, following the record low set on March 3, 2017. Whether this is a fluke or the beginning of a new trend, only time will tell.

While the greatest volume of ice resides on land and sea, a hidden world of ice also lies buried in soil near the poles and within high mountain valleys. The next chapter explores this hidden world and how it too is rapidly changing, with potentially far-reaching consequences that extend beyond its borders.

ICE SHELF	PERIOD	CUMULATIVE LOSS (km²)
Jones	1950s–2008/9	29
Larsen Inlet	1986–1989	407
Larsen A	1950s–2008/9	3,624
Larsen B	1960s–2008/9	9,166
Larsen C	1960s–2008/9	5,295
Larsen C	July 2017	~5,800
Müller	1956–2008/9	38
Northern George VI	1950s–2008/9	1,939
Prince Gustav	1950s–2008/9	1,621
Wilkins	1950s–2008/9	5,434
Wordie	1960s–2008/9	1,281
Total		34,634

	HEIGHT		LENGTH
CATEGORY	METERS	FEET	METERS	FEET
Growler	<1	<3	<5	<16
Bergy bit	1 to 4	3 to 13	5 to 14	15 to 46
Small	5 to 15	14 to 50	15 to 60	47 to 200
Medium	16 to 45	51 to 150	61 to 120	201 to 400
Large	46 to 75	151 to 240	121 to 200	401 to 670
Very large	>75	>240	>200	>670