17.2: History of Melting

Ice sheet mass loss through recent times

Since the early 1990s, scientists have been using a new set of precise, powerful tools to measure the ice sheets—satellites, complemented by dedicated airborne surveys to measure the ice sheets at an unprecedented level of precision. Satellite instruments observe whole ice sheets and collect precise measurements of all their glaciers, from about 800 kilometers above ground, day in and day out, over days to years and decades, in a comprehensive and uniform fashion. These observations are possible as a result of massive technological advances and engineering achievements in remote sensing over the last 40 years.

There are three main ways to measure the ice sheets. In the mass budget technique, we compare the mass added to the continent by snowfall, reconstructed by regional atmospheric climate models, with the mass flux into the ocean along the periphery, obtained by combining ice thickness from airborne radar sounders and ice speed from satellite radar interferometers. The mass budget record goes back to the 1970s. The mass budget technique is difficult to use because it compares two large numbers with large uncertainties, but advanced satellite observations and regional atmospheric climate models constrained by a wealth of meteorological data have permitted us to obtain precise and detailed estimates of glacier changes with this method. The advantage of the mass budget method is that it documents changes in glacier dynamics separately from surface melt processes.

A second method to measure the ice sheets is altimetry in which scientists continuously measure the height of the snow and ice over time. If ice/snow accumulates, the height of the surface increases; if ice/snow melts, the height of the surface decreases. In the meantime, the elevation of the bedrock beneath the ice changes by only millimeters. This technique collects data from ice sheets and measures surface elevation with meter to decimeter precision (by radar and laser, respectively). A major difficulty of altimetry is in transforming the observed height changes into mass changes, since we do not know a priori whether the changes in height are due to changes in snow (density of 0.3) or ice (density of 0.9). On the other hand, the technique provides a critical view of where elevation changes are taking place, analogous to a warning signal.

Figure 17.2.1 Since 2002, scientists have used NASA’s Gravity Recovery and Climate Experiment (GRACE) mission to measure ice mass changes directly from space via changes in the gravity field of the Earth. GRACE has shown that the mass change in Greenland (left) has clear seasonal cycles and is negative; that is, the ice sheets are losing mass. The time series graph has a downward bend in it, showing that the mass loss is increasing with time. In Antarctica (center), the signal is noisier, with no clear seasonal cycles but large interannual to decadal cycles. The mass change is negative there too, and the acceleration is slightly larger than in Greenland. GRACE also measures the mass changes of the world’s glaciers and ice caps. The glaciers are melting fast, but their melting rates are accelerating less than those of the ice sheets. Adapted from Velicogna 2009.

Since 2002, scientists have used the time-variable gravity data from NASA’s Gravity Recovery and Climate Experiment (GRACE) mission to measure mass changes directly. GRACE has a large footprint, about 350 kilometers, which does not allow it to see small detail such as individual glaciers, but it detects changes in water mass with a precision of 1 centimeter of water on a monthly basis. The measured mass changes in Greenland are so precise that they capture seasonal cycles: gain in mass in winter, loss in mass in summer. In a graph of GRACE data, the seasonal changes in mass do not produce a straight line (Figure 17.2.1).

Instead the line is “bent.” This bend means that the mass loss is getting larger every year, or accelerating. The data show that over several years, the ice mass has decreased markedly. For every 361 Gt per year of extra mass melting into the ocean, sea level has risen 1 millimeter. This is more or less how fast Greenland ice is melting into the ocean today. The rate of acceleration shown by GRACE from 2002 to 2017 has been 430 Gt per year per decade.

For Antarctica, the mass change graph is noisier than the Greenland graph, and it shows no seasonal cycle but a large interannual to decadal variability. A few years of observations in Antarctica would not be sufficient to capture the long-term trend in ice mass. However, it can already be determined that overall, the ice sheet is losing mass—not as fast as Greenland and not over the entire periphery, but the time series graph has more curvature, indicating that the acceleration in mass loss is greater than in Greenland. The acceleration was 180 Gt per year per decade in 2002–2017.

With GRACE, scientists also quantify the mass loss of mountain glaciers, a set of about 150,000 glaciers and ice caps around the world. The glaciers and ice caps (GICs) turn out to melt as fast as the ice sheets. They experience a loss that increases by 110 Gt per year per decade. In total, melting ice from Greenland, Antarctica, and the GICs dominates sea level rise. From 2002 to 2017, the mass loss averaged 575 Gt per year with an acceleration of 430 Gt per year per decade. At this rate, we will exceed 1 meter of sea level rise by 2100 if we factor in a 20-centimeter sea level rise expected from the thermal expansion of the ocean. Glaciologists, however, fear that the contribution of land ice to sea level could become larger if major ice sheet instabilities take place.

Ice shelf collapse

How fast glaciers collapse into the ocean remains a central question in projecting the evolution of ice sheets in a warmer climate. In 1995 and 2002, large ice shelves in the Antarctic Peninsula collapsed following decades of slow decay from warm air and ocean temperatures. These ice shelves act like plugs on the glaciers upstream. Once gone, the glaciers are “free” to speed up. In 2002, following the collapse of the Larsen B ice shelf, the glaciers upstream of Larsen B sped up by a factor of 3 to 8. Fifteen years later, the glaciers are still flowing 5 times faster than when an ice shelf was present. The glacier response is therefore rapid, the impact on the mass loss is significant, and the effect is persistent for long periods of time.

As stated earlier, the collapse of Larsen B is an irreversible process on a human time scale. Studies have shown that the ice shelf had been stable during the entire Holocene; that is, it did not collapse in the prior 10,000 years. If the same process were to take place farther south, where larger ice shelves hold large sea level rise potential, the effect on global sea level would be measured in meters instead of millimeters. As an illustration, if all the glaciers around Antarctica were to speed up by a factor 6.5, sea level would rise by 4 meters per century.

Do we know where irreversible mass loss could take place in the ice sheets? In principle, the portions of the ice sheet most sensitive to climate change are marine based, that is, where the base of the ice is grounded below sea level. There the ice will remain in contact with the ocean waters during the retreat and be replaced by an ocean. Among marine ice sheets, the most sensitive sectors are those with a retrograde slope, as discussed earlier. Among the marine ice sheets capable of MISI, those closest to the sources of warm ocean water around Antarctica (and Greenland) are at risk because changing winds will bring more ocean heat to the glaciers.

Figure 17.2.2 In Greenland, there are three major marine-based basins with retrograde slopes: Petermann/Humbolt (P/H), the Northeast Greenland Ice Stream (NEGIS), and Jakobshavn Isbrae (JKS). All three are currently under attack by climate warming. In Antarctica, the mass loss is mostly from the Amundsen Sea Embayment (ASE), the Antarctic Peninsula, and sectors in East Antarctica (such as the Totten Glacier with a 3.9-meter sea level rise equivalent). Antarctica image from Fretwell et al. 2013. Greenland image from Morlighem et al. 2018.

In Greenland, we recognize three major marine-based basins: (1) the Jakobshavn Isbrae in central west Greenland, (2) the PetermannHumboldt drainage in central northwest Greenland, and (3) the 79 North–Zachariæ Isstrøm drainage in northeast Greenland (Figure 17.2.2). These basins hold sea level rise equivalents of 0.6 meters, 0.6 meters, and 1.1 meters, respectively. All three basins are currently under attack by climate warming. In 2002, the floating ice shelf that protected Jakobshavn Isbrae broke up in a few weeks, following years of melting from the bottom (due to warm ocean temperature) and above (warm air temperature), and the glacier sped up by a factor of 3. The glacier has been retreating along a retrograde slope at a rate of 0.6 kilometers per year. In the warm summer of 2012, the glacier was flowing at a record speed of 18 kilometers per year, or 54 meter per day, versus 3–4 kilometers per year in the 1990s. Based on the bed topography of the glacier, the retreat should continue for decades until the grounding line reaches a bed that is rising in the inland direction, more than 80 kilometers inland.

In the northeast, the floating section of Zachariæ Isstrøm collapsed in 2004 following years of slow decay of the permanent sea ice cover. Eight years after the collapse, we detected a glacier speedup of about 30%. The slower response reflects the geometry of the glacier: the grounding line was anchored on a ridge. As stated earlier, the glacier holds a 0.5-meter sea level rise equivalent. The glacier is now retreating along a retrograde slope for another 10–15 kilometers before the bed elevation rises again. Its neighbor, 79 North Glacier, is retreating more slowly because it is retreating along a prograde bed slope. Both glaciers are retreating for the same reason: warmer-than-usual waters have eaten away the floating section of the glaciers and removed the ice mélange that glues detached pieces of ice shelf together.

The third sector has been the most stable, but in 2010 a series of calving events removed one-third of the floating ice shelf of Petermann Glacier. The ice shelf moved to its most retreated position since the beginning of the twentieth century when first discovered by explorer Lauge Koch. Parts of the ice shelf have thinned by 100 meters in the last 8 years, suggesting that prolonged exposure to warmer conditions will eventually result in the collapse of the ice shelf. This glacier is connected to the interior of the ice sheet via a deep, marine-based channel.

In Antarctica, the northern part of the Antarctic Peninsula does not hold a lot of sea level rise potential, in the range of centimeters, but the southern part holds a lot of ice. At present, the northern part is melting away rapidly, and the southern part is changing slowly. In West Antarctica, the glaciers draining into Siple Coast have been slowly growing with time since the 1970s. This situation is an anomaly in Antarctica and is driven by internal dynamics rather than by climate. As the glaciers thicken, basal pressure rises until ice starts to melt under its own pressure, the bed becomes wet, and ice starts sliding. As ice slides faster and thins, it eventually loses its momentum, slows down, and refreezes to its bed, and the process starts again.

In the northern part of West Antarctica, Pine Island and Thwaites Glaciers hold a 1.2-meter sea level rise potential and stand in warm circumpolar deep water (CDW) at about +2°C. These glaciers drain from a basin below sea level with steep retrograde slopes in the interior. Since the mid-1990s, scientists have seen these glaciers slide to sea faster and thin. Grounding lines have retreated about 1 kilometer per year, or twice as fast as in Greenland, and the glaciers have lost vast quantities of ice to the ocean. Scientists have mapped the bed geometry of these glaciers in great detail since 2002, and we concluded in 2014 that we knew enough about the bed and the ocean conditions to conclude that the glaciers are in a trend of irreversible retreat. Warm water is fueling the retreat. We find no major bumps in the bed that will slow the retreat to a stop. If these glaciers retreat completely, losing all their water to the sea, they will entrain the collapse of the rest of West Antarctica and raise global sea level by 3 meters.

In a spectrum of slow, catastrophic changes, there is also good news. During a series of colder years (2009–2013) with a 60% drop in ocean heat, the glacier retreat in the Amundsen Sea Embayment slowed down by 1%. A warmer ocean therefore triggers the retreat but a colder ocean can slow it down. This is yet another illustration that MISI is complex rather than one-dimensional. Similarly, as the glaciers retreat in a nonuniform fashion and form new embayments with smaller ice shelves, warm water intrusion is more difficult, which slows the retreat. While the retreat may remain unstoppable, the rate of retreat depends on the rate at which ocean heat is delivered to the glaciers. As colder waters intruded Disko Bay in Greenland in 2017–2018, not only did the retreat of Jakobshavn Glacier stop, the glacier started to readvance.

The West Antarctic ice sheet is not the only source of instability in Antarctica. Other sectors at risk exist in East Antarctica.

East Antarctica has generally been viewed as stable and immune to change because it stands taller on the ground, most of the ground below the ice sheet is above sea level, the surface climate is colder, the bed slopes are not as steep as in West Antarctica, and there is scanty evidence for the presence of warm circumpolar deep water along the coast, due to a lack of observations. Altimeters on satellites, however, revealed that some parts of marine-based East Antarctica have been changing, with major glaciers thinning at rates of 0.4–0.7 meter per year. Other methods, such as the mass budget technique and use of GRACE mission data, suggest that these glaciers are slowly losing mass to the ocean. High-risk areas include the Totten Glacier, which holds a 3.5-meter sea level rise equivalent, that is, more than the marine part of West Antarctica; Denman Glacier, which holds a 1.5-meter sea level rise equivalent; and the sector drained by Cook ice shelf and Ninnis Glacier. Recent oceanographic data revealed that Totten Glacier stands in relatively diluted circumpolar warm water at +0°C. At present, the glacier is retreating on a nearly flat bed. In the inland direction, the bed rises for another 50–80 kilometers. This prograde slope offers a temporary protection from MISI on Totten. Beyond 50–80 kilometers, the basin drops down in the deep and broad Aurora Basin and its large reserve of ice. To the east, Denman Glacier is also at risk, grounded on a ridge at the edge of a deep trough with retrograde slopes, which reaches one of the lowest points in Antarctica, at 3,500 meters below sea level. These sectors at risk are closest to the sources of warm circumpolar deep water. Ongoing research will have to determine how and where this warm water reaches the East Antarctic coastline and what pathways exist to trigger a rapid retreat of the glaciers.