by Jean Jouzel
Greenland: An Increasingly Negative Mass Balance
In 1912, the Swiss geophysicist Alfred de Quervain took the first glaciological measurements in Greenland when he traveled across it on skis. In 1932, two German scientists spent the winter on the ice at the center of Greenland at Eismitte Station at more than 3,000 meters in altitude: Alfred Wegener, father of the theory of continental drifting, who did not return from his journey, and Fritz Loewe, who lost a leg to gangrene. On the agenda: meteorology and glaciology in very difficult living conditions. Later the French polar expeditions of Paul-Émile Victor took the first measurements of the altitude and thickness of the inland ice sheet. From 1959 to 1974 the international glaciological expedition to Greenland led jointly by five European countries (including Denmark, the statutory godfather of the island) studied the mass balance of the ice cap. The Americans, within the framework of the International Geophysical Year but also motivated by strategic interests, were present on the field and developed, among other things, deep core drilling techniques in the ice;2 this was the starting point for the study of glacial archives. Geodesic chaining on the ground enabled scientists to establish profiles of the surface and to obtain surface ice velocities. During fieldwork, the thickness of the ice was measured using gravimetry or seismic prospection, but, as with the Alpine glaciers, one of the most important objectives was to establish the mass balance and thus to map accumulation and ablation conditions.
Accumulation depends on the quantity of snow that falls. The quantity decreases as one moves to the north, reaching a maximum of 2.5 meters of equivalent water per year in the extreme southeast and less than 0.15 meters toward the northeastern end. It also diminishes from the coast toward the interior, as the altitude increases and the temperatures become colder. Over all of Greenland, the accumulation is estimated at 520 billion tons—520 gigatons per year (gt/yr)—or 30 centimeters of water on average. Nevertheless, these figures are only 5–10% accurate.
Ablation occurs with the melting of the snow, the water refreezing in the deeper cold layers or flowing in the form of rivers in the marginal zones, and with the calving of icebergs in the coastal regions. The equilibrium line separates the zones of accumulation and ablation at an altitude that varies from 1,000 meters in the north to 1,800 meters in the south. Unlike glaciers in the central regions, which have flatter topography, some coastal glaciers have very high flow speeds, as is suggested by the existence of fields of crevasses and fjords filled with icebergs. These glaciers drain from vast zones of accumulation. All along their path, one can estimate speeds assuming that the inland ice sheet is in a state of equilibrium. At each point along the length of a flowline, the quantity of ice transported must be equivalent to the accumulation upstream, minus the ablation in the coastal zones. Thus we calculate an average speed taking into account the thickness of the ice. Less than one meter per year in the central domes, these speeds increase progressively and reach up to several hundred meters per year in the terminal tongues flowing into the sea. In the coastal zones, the contrast is great between these rivers of ice that flow next to other ice, which is seemingly immobile, attached to the bedrock. These high speeds reveal a sliding due to the presence of water at the base of the glacier. This is hardly surprising in the zones of ablation, but this phenomenon is also encountered in certain zones of accumulation; they are colder, but the geothermal flux and the thickness of the ice can lead to higher temperatures in the ice that is in contact with the rocky base. The ice velocities calculated for a state of equilibrium are close to reality except in coastal zones and in those where there is sliding. They suggest that the dynamic response of the great basins of accumulation of the inland ice is typically of several millennia. Using an oversimplified calculation, we can compare the average accumulation over the entire inland ice sheet—0.3 meters per year—to the average thickness—1,800 meters of ice equaling 1,620 meters of water—leading us to arrive at an “average time of residence” of 5,400 years.
We estimate, with accuracy on the order of 10%, that the loss of mass in the form of rivers (streaming) is around 300 gt/yr and that the output of the glaciers in the form of icebergs is on the order of 235 gt/yr, with an accuracy of ± 15%; thus, the Jakobshavn Glacier alone provides 23 gt/yr, a value comparable to the output of our great French rivers. The accumulation of snow thus seems more or less balanced by the flow of meltwater and the formation of icebergs, which then melt in the sea. However, the uncertainty in the various components of the calculations shows that this assessment of the mass balance remains approximate such that this method of comparison between accumulation and ablation does not allow scientists to determine whether the volume of Greenland ice is increasing, is in balance, or is diminishing.
Measuring the variations in altitude over the entire ice sheet as precisely as possible and thereby deducing the variation in its volume is a way of getting around that difficulty. That, which was only a dream some dozen years ago, is in the process of becoming reality, thanks to the satellite observations that even enable us to “weigh” the ice mass that covers Greenland.
Everything began accidentally. Not being able to see the surface, helicopters and light planes crashed on the ground in Antarctica quite simply because, in certain frequencies, the radar altimeters penetrate the ice and are reflected on the bottom rather than the surface. This in fact enabled us to obtain a detailed topography of the surface and the bedrock of the ice sheets. In the 1960s the Tiros satellite, put into orbit by NASA and equipped with optic receivers, enabled us to map the surfaces covered with ice—on continents as well as on the oceans. The European Remote Sensing (ERS) satellite, one of the first designed for polar regions launched by the European Space Agency (ESA), is equipped not only with an altimeter but with a radar imager that measures the surface velocities of the ice through interferometry or by following identifiable markers. The positioning of markers on the ground by the GPS system also allows us to measure ice speeds at the surface. Over time, the radar data from the satellite altimeter can determine the evolution of the volume of glacial masses, since we know the altitude to within a meter of every two-kilometer span. The data from laser altimetry obtained from airborne measurements are also used, such as those taken in 1993–94 and then in 1998–99; however, these covered only certain regions of Greenland. Since 2002, the satellite instrument GRACE (Gravity Recovery and Climate Experiment) has provided measurements of the field of gravity and its variations over time. After many corrections, such as incorporating data connected to tides, the data acquired above Greenland have enabled us to evaluate the geographic distribution of ice and its variations over time.
GRACE has already provided spectacular results. These satellite data show that Greenland likely began to shrink in volume in the 1990s, a phenomenon that has recently accelerated. Between 2000 and 2008 it lost 1.5 billion tons of ice, contributing on average each year to a sea-level increase of 0.46 mm.3 This contribution has increased during the past few years with an average level of 0.75 millimeters per year since 2006.
Antarctica: A Much More Recent Exploration
As a result of the intuition of thinkers, the maps of the Greeks pushed back far to the south the other summit of the world, that is, the counterpart of the Arctic. It was the Englishman James Cook who, on January 17, 1773, crossed the polar circle at 66° 33´ S and reported that there was no continent within reach of navigators. Seal and whale hunters had already ventured south of Cape Horn but kept their hunting grounds secret. It was Faddey Bellingshausen, on a mission for Tzar Alexander I, who earned the honor of discovering the continent in February 1820, while the sea ice was at its smallest. Twenty years later, three expeditions explored vast sections of the eastern coast. Among them, Jules Dumont d’Urville baptized Adélie Land with his wife’s first name. His mission was to locate the magnetic South Pole. This was done by James Ross, who discovered Victoria Land a bit farther east and reached 77°S, at the foot of the ice shelf that bears his name. The extreme south was not ment
ioned for another fifty years, until it was stated during the London congress in 1895 that “the exploration of Antarctic regions is the most important geographic work to be undertaken before the end of the century.”
The Belgian Adrien de Gerlache spent the winter of 1898 on his three-master caught in the sea ice, while the Norwegian Carstens Borchgrevink installed the first base on the continent. Other Europeans were there; the German ship Gauss, imprisoned in the ice, narrowly escaped the fate of the Antarctic, a Swedish ship crushed by the ice.
Jean-Baptiste Charcot spent the winter of 1904 onboard the Français and explored the coasts of the Antarctic Peninsula. He set off again in 1908, this time with the backing of the government, onboard the famous Pourquoi Pas. He spent the winter on a little island and explored the coasts, the sea ice, and the fauna during extensive fieldwork. He was one of the first to give priority to scientific objectives in an expedition. Charcot did not return to the south; he journeyed in the Arctic seas until his death with the disappearance of the Pourquoi Pas in 1936 during a storm off the coast of Iceland.
In December 1911 Amundsen was the first to reach the South Pole, just in front of the crew of the British explorer Robert Scott, whose five members never returned to the base camp. Shackleton got close to the pole in 1909 but backed off in time, saying in a message to his wife: “I thought that you would prefer a living ass to a dead lion.” Shackleton did not give up, however, and soon set off on a new adventure: crossing Antarctica from the Weddell Sea to the Ross Sea, passing by the South Pole. Having left in August 1914, he didn’t reach the continent because his ship, the Endurance, was caught in the ice in December. This was the beginning of an extraordinary voyage during which he was forced to abandon his boat, drifted on the sea ice, crossed wild oceans in lifeboats, and crossed a mountain chain in South Georgia. He returned to England at the end of 1916 without losing a single man. Shackleton died in 1922, once again en route to Antarctica. It is not surprising that he became a legend in polar adventure circles.
After the heroic period of discovery and of the first explorers came the era of observations and research, initiated by Charcot. Tractors, planes, airborne cartography, and telecommunications started to appear. In 1946 the American operation Highjump, directed by Admiral Richard Byrd, took thousands of airborne photographs. At the end of 1949, the French polar expeditions of Paul-Émile Victor established the Port Martin base in Adélie Land, an observatory that functioned for three years until it was destroyed by fire. During that period, the Australians established a base on the Wilkes Coast, while the Norwegians, Swedish, and British spent the winter on the other side of the continent, not far from the Argentines and Chileans who were present on the Antarctic Peninsula. All these bases were established in the coastal zones and, with its 14 million km2, Antarctica remained at this time a desert, especially since research was generally carried out in reaction to local events, such as the presence of a herd of emperor penguins or the blizzard on Adélie Land. Everything would change with the International Geophysical Year (1957–1958).
Twelve countries set up forty-eight stations, four of which, for the first time, were on the inland ice sheet: the Americans were at the South Pole, the Soviets at Vostok (the coldest place on Earth). The English on Queen Maud Land and the French on Adélie Land spent the winter in inland bases supplied by their coastal bases. The French, in the Dumont d’Urville and Charcot bases and during traverses, collected the first data on Adélie Land: they put markers in place and measured accumulations, thickness, and velocities of ice in the coastal zones. The glaciological fieldwork extended to the inland ice sheet: the English crossed the continent on tractors, from the Weddell Sea to the Ross Sea; the Australians, French, Japanese, and primarily the Soviets explored East Antarctica.
Antarctica: A Long Uncertain Mass Balance
Even more so than is the case for Greenland, and in spite of years of observation using classic measurement and satellite data, much remains to be done to understand the evolution of the Antarctic ice sheet that is the size of a continent. The trans-Antarctic chain borders and separates East Antarctica from West Antarctica. Starting from Victoria Land, it disappears under the ice and reappears intermittently in the Pensacola and Shackleton mountains and on the eastern edge of the Filchner Shelf. The ice moves depending on the slope of the surface, and the contour lines indicate that the ice of East Antarctica flows in part into the western zone, feeding the glaciers that flow into the Ross Sea or feed the Ross Shelf. The Antarctic Peninsula has a more temperate climate and attracts tourist boats with its accessibility and biological wealth. From a central plateau the ice flows in the eastern part toward the Weddell Sea, feeding, for example, the Larsen Shelf, while in the west, the glaciers can flow directly into the Bellingshausen Sea. Fed by the snow and the glaciers of the central regions, but also by local precipitations, the ice shelves have distinct dynamics because they float on the sea. In coastal regions the temperatures are lower than they are in Greenland, so that there is no significant surface melting and ablation is reduced to the output of the glaciers. In analyzing the change in mass of the ice sheet, it is customary to consider the eastern and western parts and the shelves separately.
From the elevated central regions, the contour lines allow us to demarcate the different basins of feeding that drain the ice toward the emissary glaciers. In East Antarctica, a dozen basins, whose sources go back a thousand kilometers upstream from the coasts, are identifiable; on these basins the accumulation of snow is on the order of 620 gt/yr, an amount that is a bit higher than the output of the glaciers that occupy 13% of the length of the coasts. The Lambert Glacier, the largest in the world, covers a surface area of close to twice that of France. The irregularities in the surface, revealing the tensions between the ice river and the much less dynamic neighboring zones, appear around 400 kilometers from the coast. The speed at the front of the glacier, which is 50 kilometers wide, can exceed one kilometer per year. It feeds the Amery Shelf, and its fluctuations in mass seem approximately in balance.
All of East Antarctica appears to be in a state of equilibrium, insofar as the few measurements of velocity carried out by satellite are in agreement with the calculated speeds of equilibrium. The variations in thickness measured from space have not been measured long enough and are not precise enough to challenge this conclusion: they have been available only since 1993, and the orbits of the satellites leave a great void above 82° S. These data nevertheless suggest a slight gain in mass of around 20 gt/yr, which could be connected to a slight increase in accumulation.
The situation is not as simple in West Antarctica, which is partially fed by the ice coming from East Antarctica. Furthermore, this region, whose uneven shores are largely open to different seas, is what is called a “marine” ice sheet because it rests on a bedrock located for the most part below sea level; thus glaciologists believe it is less stable than the East Antarctica ice sheet. All these characteristics cause some disequilibrium in the dynamic evolution of West Antarctica, which in the past few years has lost a great amount of mass. Consequently, Antarctica, on the whole, could begin to decrease in volume, as is suggested by the satellite data provided by GRACE. According to these data,4 Antarctica lost 104 gt/yr between 2002 and 2006, and more than double that (246 gt/yr) between 2006 and 2009. This loss of mass, and thus the contribution to the rise in sea level, would be close to that observed for Greenland. As for Greenland, these recent results indicate an acceleration that could, as we point out in part 3, be connected to climate warming.
Let’s continue on our historical journey by going back even further in time.
CHAPTER 3
Ice through the Ages
Today around 90% of the ice on land is found on the Antarctic continent around the South Pole. The second largest mass is the ice sheet of Greenland, near the North Pole. The rest of the land ice, as we have seen, is spread among the smaller ice caps of the Canadian or Siberian Arctic and in the form of mountain glaciers that remai
n only in high altitudes in tropical or equatorial regions. The polar regions thus constitute the preferred habitat of the planet’s ice.
The idea that the situation could have been different in the past and that in certain periods more ice was present on the continents is rather recent. First expressed in the first half of the nineteenth century, this theory aroused considerable disbelief in the field, and it took thirty years for its proponents, including Louis Agassiz, to convince the community of geologists of the existence of past glaciations.
The Time of the Pioneers
If you go to Chamonix today, there is a good chance that someone will tell you the story of the Mer de Glace, which reached the valley at the end of the nineteenth century. Since then, the glacier’s front has retreated by a kilometer and a half, leaving in its wake erratic piles of rocks that make up moraines. The discovery of the existence of glaciations was born out of the investigation of such moraines. Until the eighteenth century, geologists, called diluvionists, believed that these rocks had been transported great distances by the flood described in the Bible. It wasn’t until the following century that this explanation was questioned and the notion of great glaciations occurring in the past was proposed. John Imbrie, professor of oceanography at Brown University, combined his talents as a scientific storyteller with those of his daughter Katherine, a journalist, to recount this superb scientific saga.1 They showed how the idea of glaciations evolved at a time when the notion that colder periods had existed in the past was simply unimaginable.
The idea of past glaciations also germinated in the minds of several mountain-dwellers who, no doubt daily, observed the erratic blocks of stone, and it had already been suggested by several authors at the end of the eighteenth century and then at the beginning of the nineteenth. But Agassiz, in 1837, was the first scientist to defend the idea that these round rocky masses, found in places far from their point of origin, had not been transported by the flood but by glaciers that, at a time when the climate was much colder, had advanced far beyond their current boundaries. Many geologists remained skeptical. This is understandable because at the time no one knew of the existence of ice caps and ice sheets; it wasn’t until 1852 that people learned that Greenland was a huge mountain of ice; it was even later for Antarctica.