by Jean Jouzel
Figure 1.1. The Arctic regions, indicating the deep ice core drilling sites in Greenland.
One need only fly over Siberia in the winter to grasp the importance that snow has in the everyday lives of Russians. It’s not surprising that their language is particularly rich in terms describing snow in all its forms, nor is it surprising that they have bequeathed many expressions to us, including the word zastrougi to describe the very typical relief of the great expanses of snow as seen on the high plateaus of the ice sheets swept by winds. Zastroug is in fact an old Russian word that means “hand plane” (referring to the manual planing of wood), and sastrugi are the shapes on the surface of the snow that form through erosion and in the direction of the wind. Such shapes are similar to those that are seen on a plank that has been manually planed.
Greenland, Antarctica, and Ice Shelves
The largest and most voluminous glaciers are indisputably the inlandsis (“ice in the middle of land”) as they are called in Danish. They are on a completely different scale: one is in the north (Greenland) (Figure 1.1) and the other in the south (Antarctica) (Figure 1.2), home of the South Pole. These two enormous ice masses, which cover around 11% of the total surface of all continents, weigh heavily upon the Earth’s crust. They cover that crust and their weight pushes down the rock base by around 30%. Whereas Antarctica is more or less centered on the South Pole, Greenland is an island around 2,500 kilometers in length and located over latitudes of 60° and 80° N. Both have true rivers of ice that flow toward the sea in the form of emissary glaciers, or extend into the sea in the form of floating ice shelves, mainly in Antarctica. The profiles of the ice sheets are noticeably parabolic. On the relatively flat central parts, the wind sculpts little dunes that resemble those in dry deserts, the zastrougis. In the coastal zones, where the slope is greater and the ice currents marked, crevasses form that make it difficult to gain access to the inland ice sheets.
In Greenland, ice covers a surface of 1.76 million km2, or 80% of the total surface of the island, which with 40,000 kilometers of coastland is the largest in the world. The ice sheet makes up the largest part (1.7 million km2), to which are added glaciers and glacial ice caps located on the periphery and on coastal islands. The surface of the ice sheet is characterized by the existence of two domes, one in the south reaching 2,873 meters and one in the north at 3,236 meters. The dividing lines of the ice flowing to the coast depend on this asymmetry in altitude. There, toward the coast, myriad glaciers create paths through a discontinuous and low mountain chain. The widest is the Humboldt Glacier located in the northwest. The front is 120 meters high and 80 kilometers wide. These glaciers isolate the nunataks (“land in the middle of glaciers”). The largest and the most active is the Jakobshavn Glacier on the west coast, around 70° N. It drains the ice from the ice sheet over a length of more than 300 kilometers. The icebergs it releases invade the fjord that the glacier has dug, which, even in the summer, assures the presence of sea ice. The icebergs that come from the largest ice rivers are of modest size and relatively regular shape. The fjords, which have been sculpted by the glaciers, are filled by icebergs over dozens of kilometers. The ice attains a thickness of more than 3,400 meters in the central regions, where the floor is a concave basin that seems to have been sunken under the weight of the ice. The ice becomes much thinner, or even nonexistent, when rocky outcrops appear in the coastal regions; it is then on average close to 1,800 meters thick. The volume of ice is around 2.9 million km3.
Figure 1.2. Antarctica, indicating deep ice core drilling sites.
While certain zones of the North Atlantic are relatively tempered by the Gulf Stream, this is not the case in Greenland, which has a continental climate. The average annual temperatures can be close to 0°C in the coastal regions, which causes the melting of ice in the summer. On the ice sheet, temperatures can become as low as –30°C in the central zones. One can also encounter surfaces where the snow contains greater or lesser amounts of water infiltrations that refreeze during the winter. The zone where the snow is “dry” is found in the central regions above 70° N. If, in general, temperatures decrease from the coast to the interior, snowfall is more abundant in the south where it can reach several meters. There is much less snowfall over the rest of the ice sheet where the annual layer decreases to about a dozen centimeters.
On the other side of the Earth, the surface of the Antarctic continent, 12.3 million km2, or more than twenty times the area of France, is almost completely covered with ice. The exposed rocks on the Antarctic Peninsula, in the chain of Transantarctic Mountains and on some coastal ranges, represent only 1% of the continent. Geographically, we can describe the ice sheet as being made up of three parts. At 10 million km2, East Antarctica is the largest; its central and most elevated part is relatively flat but rises to more than 4,000 meters. This ice rests on an ancient floor, a part of which is currently located above sea level but at below 1,000 meters in the Marie Byrd Land and reaching –2,540 meters in the Bentley Subglacial Trench. Given this very irregular floor, it is difficult to determine the thickness of the ice from the topography. The layer is much thinner in the coastal regions, but it reaches 4,800 meters at 40 kilometers away from the coast of Adélie Land, whereas the surface altitude is 2,400 meters. It is in East Antarctica that we find the true center of gravity of the ice sheet at around 81.5° S and 73° E; this point is the farthest away from the coasts and the conditions there are such that it has been dubbed “the pole of inaccessibility.” The ice flows toward the sea and many glaciers find their path through the coastal mountain chains. The largest, the Lambert Glacier, is 120 kilometers wide and approximately 40 kilometers in length; it feeds the Amery Ice Shelf. But this ice also feeds West Antarctica through the trans-Antarctic chain, which culminates at Mount Kirkpatrick at an altitude above 4,500 meters. The topographies of the bedrock of these two areas are very different.
The area of West Antarctica (1.8 million km2) is more complex since it includes several domes as high as 2,300 meters. The Antarctic Peninsula (around 0.5 million km2, 300,000 of which are covered by ice) abuts West Antarctica and stretches away, thinning, toward South America. The surface elevation reaches up to 2,300 meters and the average thickness of the ice is on the order of 1,000 meters. In all, the continental ice in Antarctica represents a volume of 24.7 million km3.
Antarctica is also characterized by the presence of many huge ice shelves fed by the ice flowing from the ice sheets. When the ice reaches the sea, it begins to float. The shelves on the perimeter of the Arctic Ocean are few in number and not very large. Favored by the topography of the subglacial floor that often descends below sea level at the edges of the glaciers, by the presence of wide bays fed by large glaciers, and by the existence of islands that serve as anchors, they occur in great numbers in Antarctica, where they occupy close to half the length of the coasts.
In West Antarctica two of them have surface areas comparable to that of France. The Ross Ice Shelf covers close to 500,000 km2, with an average thickness of 430 meters. On the Antarctic Peninsula, the Ronne-Filchner Ice Shelf is a bit smaller (450,000 km2), but the ice is somewhat thicker (660 meters). Ice shelves are also fed by the snowfall and, in certain zones, by the ice that is formed at the bottom by freezing seawater. The very flat surfaces are broken only near the anchoring points where crevasses mark the line where continental ice begins to float. The fronts of ice shelves are like cliffs from which icebergs sometimes break off. These bergs are called tabular icebergs, and their name indeed describes their shape. In all, the surface of the ice shelves covers a bit more than 1.5 million km2.
The two large Ross and Ronne-Filchner ice shelves have an important hidden characteristic: they float. They also stabilize millions of cubic kilometers of ice in West Antarctica, most of which rests on the Earth’s crust, well below sea level. This innate characteristic will perhaps not be so hidden in the future. We are indebted to a British geologist from Ohio State University in Columbus, John Mercer, for having been the
first to point out the potential danger of collapse to the entire ice cap of West Antarctica by the rupture of the two floating shelves in the event of the warming of the planet following an increase in the greenhouse effect caused by human activities.1 The consequences of this would be an accelerated melting of the ice sheet of West Antarctica and potentially a five-meter rise in sea level. Mercer’s theory was met with skepticism in 1978, but this possibility has since been suggested by many other scientists.
Ice: An Agent and Indicator of Climate Change
The variations in the extent of snow- or ice-covered zones are amplifying the fluctuations in temperature on a global scale. A cooling leads to an extension of ice, whose strong albedo contributes to the maintaining of low temperatures; by contrast, a warming of the climate reduces the area of ice-covered surfaces, which increases zones with a weak albedo and thus contributes to an increase in temperatures. Ice cover, through its extent and thickness, is both an agent in the climatic system and an indicator of any change: first, on a seasonal scale when the expanses of snow cover or sea ice follow the changes of the thermometer; in the longer term, on the scale of decades, when the change involves the advance or the retreat of mountain glaciers and the expansion or reduction of their volume; and finally, on the level of centuries and millennia, with respect to the ice shelves, ice caps, and ice sheets of the polar regions.
There are many interactions between the cryosphere and the other components of the climatic system on a global scale. The cryosphere plays an important role in atmospheric and oceanic circulation. Simply stated, the cold air is denser and the low temperature zones are also those of high atmospheric pressure; similarly, deep oceanic circulation originates from the high-latitude oceans. On a more regional scale, the snow cover or the presence of sea ice reduces the atmospheric exchanges with the continents and the oceans. Above the cold zones, the atmosphere contains much less water vapor, and it allows the infrared rays emitted by the Earth to pass through, back to space. The result, then, is all the more negative, as it leads to even lower temperatures.
Furthermore, the phase transition between water vapor and ice requires a large amount of energy. It takes close to 80 calories to melt a gram of ice that is already at 0°C. In the spring, as soon as the snow disappears, temperatures climb very quickly in calm weather; the calories previously used for melting snow then heat the atmosphere. Regarding the energy expended in the climatic system, a few figures speak for themselves: to heat the ocean by 1°C, it takes a thousand times more heat than to increase the temperature of the atmosphere by the same amount. It would take close to twice as much more to cause 28 million km3 of ice, which currently constitutes our white planet, to melt.
The White Planet and Sea Levels
An agent within the climatic system, an indicator of its changes, the cryosphere also plays an important role in the variations in sea levels, one of the key variables for humanity in the evolution of the climate. The changes in the volume of continental ice directly influence sea levels. We will see that the warming that followed the end of the last ice age, some 20,000 years ago, caused an increase in sea levels of around 120 meters. More subtly, the melting of our mountain glaciers added to the thermal expansion of the oceans has contributed to an increase of 10 to 20 centimeters observed in the last century. In this realm, the behavior of the ice comprising the ice sheets represents one of the major uncertainties involving the future of our planet.
Sea ice and ice shelves that float on the seas do not affect the level of the water as they melt, just as melting ice cubes in a beverage do not raise the level of liquid in a glass. On the other hand, glaciers, ice caps, and ice sheets are formed from precipitation connected to the evaporation of ocean water. They represent a volume of ice close to 28 million km3 (Table 1.1), or an equivalent amount of water (around 25 million km3), ice having a density of close to 0.92. Spread over the ocean’s surface, this volume represents a layer of around 70 meters. This doesn’t mean that the sea would rise by as much, since a part, around 10%, of the ice of the ice sheets is currently below sea level and the new distribution of the masses would lead to a readjustment of the Earth’s crust. Taking these readjustments into account, one can then estimate an increase of around 64 meters. Let us very clearly specify that the melting of all the existing ice is not predicted by any scenario of the evolution of our future climate. But before exploring further what the ice reveals about our climatic future, let’s look at a bit of history.
CHAPTER 2
From Exploration to Scientific Observation
In the eighteenth century, the way in which an educated man perceived our white planet was quite different from that which has just been presented. Of course, people knew about the existence of mountain glaciers and eternal snows that covered the highest peaks, but no geographer imagined that the amount of snow could fluctuate over time. The Arctic—at least its peripheral regions—was not a completely virgin land since native peoples lived there, but no chronicle mentions anyone reaching the North Pole or traveling across all of Greenland. As for Antarctica, that continent was terra incognita. On January 17, 1773, James Cook became the first person to cross the polar circle, declaring upon his return from what was then his second expedition: “I went around the austral hemisphere, following a high latitude, and ran along it in order to irrefutably prove that no continent exists, unless it [the continent] is close to the pole and out of reach of the sailors.”
Certain zones in the center of Antarctica remain largely unexplored to this day, but our knowledge of these polar regions has increased enormously in recent times. Their geography and topography keep very few secrets in this era of satellites, but progress has also reached more intimate aspects of the evolution of various components of the white planet: the flow and mass balance of mountain glaciers, polar ice caps, and ice sheets; the conditions prevailing at their base; the thickness and processes of formation of the ice shelf and permafrost; and so forth. We cannot resist the pleasure of including a few anecdotes about (and mentioning some names among those who became known through) the discovery then the exploration of these extreme regions. However, it is above all the scientific aspects of the explorations, the methods of observation used by researchers, and a few notable results that should be emphasized. We will leave aside for the moment one of the questions that quite naturally comes to mind: Does the recent evolution of the cryosphere independently suggest a climatic warming, as has been observed during the last few decades?
The Flow of Mountain Glaciers
In August 1820, three guides on their way to Mont Blanc were thrown into a crevasse by an avalanche. Their remains were found on the face of the Bossons Glacier, 3,500 meters lower, in August 1861. From this tragedy one can estimate an average speed of flow of the glacier of 180 meters per year. More recently, on September 19, 1991, a couple hiking in the Tyrol near the border of Italy and Austria found a human body at an altitude of 3,200 meters. The man was small (1.6 meters tall), his body was 5,000 years old, and the objects scattered around him, including an ax, were characteristic of the Bronze Age. Many questions arose: How was he able to be mummified quickly, which enabled his preservation? Why wasn’t he crushed by the weight of the ice and dislocated by its currents? Even the slowest glaciers are renewed in a few centuries. In fact, the very great age of the man demonstrates that there are niches in the cavities of the rocky relief where the ice can stagnate for a long time.
Looking at mountain glaciers, we see huge white expanses near the peaks and lower down, a winding surface design, with crevasses and seracs, rocky moraines: this is what suggests a flowing, like calm, lazy rivers that are transformed into agitated currents. In the first half of the nineteenth century, the Swiss Louis Agassiz and then other observers intrigued by the glaciers measured their speed of displacement. They noted that the speeds are higher in the center than on the edges—and faster on the surface than on the bottom—because of the friction of the edges on the glacial valleys and as a result of con
tact with the rocky ground. Ice is not rigid matter; its plasticity causes it to change shape, through the effect of gravity, under the pressure of its own weight (1 m3 of ice weighs more than 900 kilograms) and by following the slope of the glacial valleys. The speed of flow varies along the glacier; it is higher when the slope is greater, in which case the glacier is thinner. Another important factor in the ice flow is the decrease in viscosity of the deep ice with the warming associated with the geothermal flux. With the melting that occurs at lower altitudes, water penetrates into the glacier, which, owing to the presence of a liquid layer at its base, can slide and sometimes cause catastrophic advances. The dynamics of a glacier are thus very dependent on the presence or the absence of melting water. This is what distinguishes temperate glaciers from cold glaciers. In practice, the latter exist only at very high altitudes and in high-latitude regions. In the Alps, a cold glacier at the summit becomes temperate as it flows toward its front.
Let’s look at the Mer de Glace Glacier closely. It is the largest French glacier, which stretches up to 12 kilometers in length, between 3,900 and 1,400 meters in altitude, and covering an area of 40 km2. The feeding basin extends into a cold zone, and the height of the snow can be considerable there, reaching eight meters at the end of winter in the upper part of the Vallée Blanche. The Glacier du Géant, by way of the Vallée Blanche, the Glacier de Leschaux, the Glacier de Talèfre, the Glacier des Périades issued from Mont Blanc du Tacul and from the high peak of the Géant—they all converge to form the Mer de Glace, some 15 kilometers wide. Overall, the zone of accumulation represents two-thirds of the surface. Lower, one enters into the zone of ablation marked by the disappearance of the névé and the presence of seracs due to changes in slope. With warmer temperatures, the ice layer can lose as much as a dozen meters in thickness per year, the terminal tongue of the glacier marking a front, the evolution of which can be followed visibly. The thickness of the glacier varies along its entire length from one to a few hundred meters (400 meters at the foot of Mont Blanc du Tacul) but much less in the terminal tongue. In this pathway, speeds vary with the slope and thickness of the glacier, which then assures continuity in the flow of the ice. Speeds measured over the last kilometers of the Mer de Glace indicate a rapid decrease from more than 200 to 100 meters per year upstream, where the slope is greatest, to 50 meters per year at the terminal tongue where the glacier becomes increasingly thin.