Changes to the Terrestrial cryosphere over the next 100 years
- This page is part of the topic Antarctic climate and environment change over the next 100 years
Recent observations of ice sheet behaviour have forced experts to radically revise their view of ice sheet sensitivity to climate. Existing models, based primarily on earlier views of the essential governing dynamics of ice sheet flow, fail to reproduce observed behaviour, removing a primary tool for anticipating future changes. Predictions of the future state of ice sheets must, therefore, be based upon a combination of inference of past behaviour, extension of current behaviour, and whatever proxy data and analogues exist. The broad range of time scale response of the ice sheet guarantees that future behaviour will be composed of a superposition of the continuing gradual response to past climate change and the more rapid responses to present and future changes.
Currently there exists a patchwork of changes across the Antarctic ice sheet - growth in some areas, loss in others, some related directly to climate, some related to changes in oceans, driven by climate change. No single external driver, nor single aspect of climate change (atmospheric temperature, snowfall rate, or ocean conditions) will dominate in all areas. Rather the dominant effect in each area will depend on climate in that area, and the particular sensitivities of the ice sheet in that region.
Most certain is the expectation that in a warmer world there will be less ice and higher sea level. Five million years of paleoclimate data support this expectation (Naish et al., 2009; Pollard and DeConto, 2009). Both data and modelling indicate the loss of the West Antarctic Ice Sheet during periods prior to one million years ago in which global temperature was ~3º warmer than pre-industrial levels. The data also demonstrate that sea-level change, and therefore the rate of ice loss, will be neither uniform nor monotonic. The implication is that the loss of ice and, thus, future ice-sheet behaviour, will be episodic. Translated into a picture of future ice sheet behaviour, this suggests there will be a combination of coherent activity and disjoint activity from different regions of the continent, among individual glacier basins and groups of glacier basins.
The most likely regions of near-future change are those that are changing most today (Antarctic Peninsula, and Pine Island Glacier). Most models agree that future warming in the Antarctic will be strong, and so it is likely that – depending on location - both snowfall and melt will increase later this century. Even small amounts of additional accumulation would cause significant volumetric growth due to the vast area of the ice sheet; average accumulation across all of Antarctica is only 15 cm/yr (water equivalent) and much lower in the cold, high interior of East Antarctica. But warming (especially around the margins) also leads to increased melting that not only would remove ice mass by runoff, but could cause the margin of parts of Antarctica to adopt some of the dynamic character of the present-day margin of Greenland, where surface meltwater penetrates to the ice sheet bed causing accelerated flow (Zwally et al., 2002b; Joughin et al., 2008). It must be borne in mind that models disagree over many other details of atmospheric climate change in Antarctica, and generally share little agreement in precise projections of future ocean warming, which might have the most significant influence of all on the ice sheets.
East Antarctic ice sheet
Present changes in the East Antarctic ice sheet (EAIS) are a patchwork of interior thickening at modest rates and a mixture of modest thickening and strong thinning among the fringing ice shelves (see Figure 4.34). As discussed in The East Antarctic cryosphere in the instrumental period, the cause of the current slight interior thickening is probably a long-term dynamic response to a distant change in climate (e.g. a past cooling event), and not a response to recent increased snowfall. This effect is likely to continue and change only slowly. Eventual atmospheric warming over the East Antarctic interior is projected in most GCMs as ozone depletion is reversed, but there may not be a straightforward connection between the expected atmospheric warming and an increase in snowfall over the Antarctic continent. Many GCM projections show a similar degree of sensitivity; that a 1ºC increase in mean annual temperature would cause around 5% increase in mean net surface accumulation (equivalent to 0.3 mm annual decrease of global sea level) (Meehl et al., 2007). In the most recent IPCC assessments, this contribution to sea level due to increasing snowfall was included, and was highly significant. Without its effect, sea level rise projections would generally be 5 cm higher.
Coastal changes are more difficult to anticipate. Most of the additional snowfall may be limited to the coastal areas, compensating for present processes responsible for the observed thinning of ice shelves, however the compensation will likely only be partial. Equally likely is an amplification of the present rapid thinning of the Cook Ice Shelf and the mouth of the Totten Glacier, spreading of ice shelf thinning to other coastal areas of the EAIS, and perhaps isolated initiations of summer acceleration of grounded coastal ice by lubrication of meltwater penetrating to the ice sheet base where summertime temperatures exceed 0°C.
There are marine basins beneath the East Antarctic ice sheet, especially in Wilkes Land (between 100°E and 160°E), and the potential for these to harbour even more dramatic and rapid ice loss remains unquantified. The general discussion of this potential is presented in the next section because it is already happening in West Antarctica.
West Antarctic ice sheet
The current loss of mass from the Amundsen Sea embayment of the West Antarctic ice sheet is equivalent to that from the entire Greenland ice sheet (50 Gt per year) (Lemke et al., 2007). This sector of the West Antarctic ice sheet is by far the most active, well ahead of the Ross Sea sector where the stagnation of Kamb Ice Stream dominates the positive mass balance contribution from that sector. Little net change is measured from the remaining Weddell Sea sector.
The future of West Antarctica is always cast against the backdrop of the “marine-based ice sheet instability”, a concept first posed by Weertman (1974). Accelerating and irreversible retreat of marine-based glaciers resting on back-sloping beds has been confirmed by a more detailed, full-stress tensor, numerical analysis (Schoof, 2007). The observed doubling of the Pine Island Glacier’s speed in less than 30 years demonstrates that marine based outlet glaciers are capable of dramatic accelerations in the relatively short period of a few decades (Joughin et al., 2003). Figure 5.18 illustrates one model’s prediction of changes in the Amundsen Sea sector over the next millennium.
The process believed responsible for this dramatic behaviour is a gradual thinning of the fringing ice shelves seaward of the Amundsen Sea outlet glaciers - in effect, a slow motion version of the glacier acceleration that was observed to follow ice shelf disintegrations in the Antarctic Peninsula. The chain of events leading to this thinning begins with increased circumpolar circulation in the atmosphere above the Southern Ocean, driven by the increased pressure gradient between the ozone hole cooled Antarctic and the warmer Southern Hemisphere mid-latitudes. The surface waters drift northward due to the Coriolis effect, encouraging a greater upwelling of warm Circumpolar Deep Water. Now raised, these warmer waters are able to get onto the continental shelf, where their flow is directed toward the outlet glaciers by following the troughs carved by these glaciers in past glacial periods. Once they reach the floating ice shelves, they are responsible for extremely high melting rates of many tens of metres per year.
GCM predictions are for a continuation of the positive phase of the Southern Annular Mode, which will continue the stronger circumpolar circulations, thus continuing to upwell warmer waters onto the continental shelf in the Amundsen Sea. A doubled outflux in the glaciers in this sector would contribute to an extra 5 cm of sea-level rise per century. Ultimately, this sector could contribute 1.5 metres to global sea level, so a contribution from this sector alone of some tens of centimetres by this century’s end cannot be discounted.
The relationship between sub-ice melt rates and ocean temperatures is just beginning to be explored. A recent modelling study by Pollard and DeConto (2009) suggests that the West Antarctic ice sheet will experience a complete collapse when nearby ocean temperatures warm by roughly 5ºC. A corollary to this is that if ocean circulation patterns change and warmer portions of the ocean access the ice shelves, an equivalent collapse would unfold. Global climate and regional ocean modelling is needed to predict when and if future ocean temperatures and melt rates under the Antarctic ice shelves will increase to that extent, and improved ocean-ice models are required to explore whether the details of this interaction increase the thermal sensitivity of this system. The AR4 climate models suggest that there will not be a full collapse in this century.
Other sectors of the West Antarctic ice sheet are doing little to offset this potential contribution to sea-level rise. There is modest ice sheet growth within the Ross Sea sector as the basin of the near-stagnant Kamb Ice Stream continues to thicken through snowfall and the adjacent Whillans Ice Stream slowly decelerates. Continued deceleration of Whillans Ice Stream is likely—probably the result of freezing and stiffening of the subglacial till. Contributions to ice sheet growth are limited to the rate of snowfall, so even a second stagnant ice stream will not be able to offset the ice lost from the Amundsen Sea sector, and there is the intriguing probability that the Kamb Ice Stream will reactivate - an event that is not predictable, but one that has scientists’ attention and that is being monitored by a host of sensors.
Measurements made in the Weddell Sea sector of West Antarctica do not raise alarms now or for the future. The open ocean is held well away from where ice streams first enter the Filchner-Ronne Ice Shelf. Like ice streams from the Ross Sea sector that feed the Ross Ice Shelf, the vast size of these ice shelves is probably the greatest insurance that sudden changes in the deep Southern Ocean will not greatly impact the flow rates of these major ice flows this century. Substantial thinning of even portions of these large ice shelves would require a re-evaluation of the potential impact of the considerable reservoir of ice held upstream.
The empirical observation that glaciers in the northern Peninsula flowed faster following the collapse of an ice shelf provides a possible analogue for the changes occurring in the Amundsen Sea sector of West Antarctica. There, the collapse of a substantial ice shelf has not been observed, but the surrounding ice shelves are thinning at rates in excess of 5 metres per year in places (Shepherd et al., 2004; Bindschadler, 2002). This thinning has perhaps caused ice to detach from the bed, or reduced the side-drag of ice shelves, decreasing resistance to the motion at the front of the Pine Island and Thwaites glaciers, in the same way that resistance from the Larsen ice shelf was removed by its disintegration. Models of ice flow suggest that this effect could provide an explanation for the acceleration of these glaciers (Payne et al., 2004; Thomas et al., 2004b).
The potential for counterintuitive behaviour of the Antarctic ice sheet does exist and must not be dismissed lightly given our limited understanding of recent ice-sheet behaviour. As one example, the supply of warm water that drives sub-ice-shelf melting beneath the Filchner-Ronne Ice Shelf is driven by sea-ice production in the Weddell Sea. Warmer or less windy conditions in the Weddell Sea would reduce sea-ice production, and could therefore decrease ocean overturning and reduce the delivery of warm water to the ice shelf, leading to less sub-ice-shelf melting. Another example is that in places along its ice front, the gap between the Ronne Ice Shelf and the seabed is less than 50 meters. Any substantial increase in thickness, for example caused by an increase in discharge upstream, could quickly cause new areas of ice shelf/seabed contact, decelerating ice-shelf flow, and resulting in thickening upstream and a local advance of the ice-sheet.
Within the Antarctic Peninsula, recent climate changes have had a dominant impact on the behaviour of glaciers and ice shelves. That being the case, predictions of future climate can be used to help predict the future of this ice. Unfortunately, there is a demonstrable lack of skill in current GCMs in simulating recent changes on the Antarctic Peninsula, because they do not work well at the regional scale, and thus poor confidence in future projections of ice behaviour there at that scale.
Nevertheless, some mechanisms for recent warming are understood. Warming rates vary seasonally. Winter warming, related to a loss of sea ice to the west, exceeds summer warming, caused by an increase in the Southern Annular Mode. Both these trends cannot continue indefinitely (winter will never become warmer than summer). More precipitation occurs in winter than summer, so warmer winters will probably bring more snowfall. Yet the number of days of melt (temperatures above 0ºC) is an important parameter tied to glacier growth or shrinkage and these days are projected to increase, offsetting some part of the increased snowfall.
Applying the concept of a thermal limit to ice shelf viability (Vaughan and Doake, 1996), increased warming will lead to a southerly progression of ice shelf disintegrations along both coasts of the Antarctic Peninsula. As in the past, these may well be evidenced and preceded by an increase in surface meltwater lakes. Prediction of the timing of ice shelf disintegration is not yet possible, but field programmes on some of the ice shelves thought to be most vulnerable to collapse now join a host of satellite sensors that monitor ice shelf health and watch for the expected signs of disintegration. These data should improve understanding of the causal mechanisms and lead toward a predictive capability. With a total volume of 95,200 km3 (equivalent to 242 mm of sea level; Pritchard and Vaughan, 2007), roughly half that of all glaciers and ice caps outside of either Greenland or Antarctica, the mechanism of fast acceleration of these glaciers as an immediate consequence of ice shelf disintegration means this ice could impact global sea level faster than the more gradual melting of all other glaciers and ice caps.
Glaciers on the Antarctic Peninsula have been generally overlooked in global projections of sea-level rise (e.g. Meehl et al., 2007). In assessing their likely future contribution to sea-level rise it is important to identify glacier volume changes directly, and when negative mass balance began. Other parts of the world have long-running glacier monitoring programmes. For the Antarctic Peninsula such datasets do not exist, although it is hoped that using photogrammetric measurements from historic photographs it may be possible to determine changes in glacier surface height, and hence volume and mass balance over past decades (e.g. Fox and Cziferszky, In Press).
Efforts to measure the mass balance of ice sheets have borne fruit in recent years with multiple methodologies that have provided a picture of increasing ice loss in the past one or two decades. The rapid rate of change makes it difficult to extrapolate these observations into the future. These same observations have revealed unsettling weaknesses in most time-dependent ice sheet models, reducing confidence in their predictions of the future of the Antarctic ice sheet.
Nonetheless, a conservative expectation of changes no greater than those already observed results in an increasing loss of ice in the Antarctic Peninsula and in the Amundsen Sea sector of West Antarctica, more than offsetting the slow growth in East Antarctica and the stagnant Kamb Ice Stream in West Antarctica. More disturbing is the possibility, albeit impossible to quantify at this time, of a number of climate influences that could amplify loss of Antarctic ice and accelerate future sea- level rise.
Most past predictive efforts on sea-level rise, such as those by the IPCC, have aimed at the predictable components, shying away from the less predictable elements, especially the response of continental ice sheets to climate change. Efforts are now being made to develop more integrated models that incorporate some of the ice-climate interactions that are now inferred as central to recent changes. In some cases, more field work is required before a sufficiently deep understanding of the process is possible. However, simplified schemes can be introduced to numerical models now for use in future assessments by the IPCC and others. For the moment there are no comprehensive, objective projections that can be cited, and the future evolution of the Antarctic ice sheet is better described through a more subjective, discursive approach.
Summary and needs for future research
The instrumental period has been a period of accelerating data collection culminating in the recognition of astonishingly rapid physical changes. These achievements have been fueled primarily by the array of extraordinarily capable satellite sensors. The availability of these data for research has continued to increase through a variety of data sharing agreements, further increasing their value. The research community has become increasingly reliant on their existence and availability. However the continuation of these exceptional resources cannot be taken for granted and many of the most valuable sensors, such as those on ICESat, MODIS and Landsat, are already beyond their design lifetime, with replacement missions either not planned or planned for launches so far in the future that prolonged gaps in coverage are probable. These gaps must be minimized and, if possible, eliminated. The prospect of slowly going blind to ice sheets at precisely the moment when their behaviour has suddenly become very dramatic carries with it the undesirable consequences that not only will we not be able to follow the continuing evolution of areas already changing dramatically, but we will not be able to detect new areas of change at an early stage, limiting our ability to understand the causes of these changes. Recommendations for satellite observations of the cryosphere are given in the Cryosphere Theme document produced for the Integrated Global Observing Strategy (IGOS) Partnership (http://www.eohandbook.com/igosp/cryosphere.htm).
As the IPCC’s AR4 noted, climate models lack the ability to predict the future of ice sheets primarily because our understanding of ice flow dynamics cannot predict “future rapid dynamical changes in ice flow”. Satellite data have identified the signatures of change in many regions and helped scientists suggest possible causes, however airborne and field studies are the only means to determine the root causes and quantify the sensitivity of the ice sheet to them. The major processes currently under investigation are: the disintegration of ice shelves and the consequent acceleration of feeding glaciers; ocean-ice interaction beneath the fringing ice shelves in both relatively warm and cold water environments; and the influence of a newly appreciated active subglacial hydrological system on ice flow. In all these areas, the emphasis is on directly accessing the active region, usually with attempts to maintain instruments that can survive to produce temporally extensive records of key parameters. These field studies are essential and must be pursued. Their success will be a quantitative understanding of these processes at a level that allows models to incorporate the key processes of ice flow, thus leading to improved predictions of future ice flow and ice sheet shape.
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