CHAPTER TWO

Fundamental Questions of Global Change

The world is experiencing many changes. Global temperatures, on land and in the oceans, are increasing. Sea levels are rising, global weather patterns are shifting, and the chemistry and biology of the world’s lands and oceans are changing. It is a unique time in history in that we now have great capacity to observe many of these changes and understand many of the reasons behind them.

Antarctica and the Southern Ocean are intimately involved in global processes that provide the key to understanding those changes. Formation of the deepest water in the global ocean circulation occurs in the Southern Ocean, as does upwelling to the sea surface of all the deep waters from other oceans. The Southern Ocean is an extremely important region of the globe for air-sea exchange of carbon dioxide, second only to the northern North Atlantic. The strong westerly winds that circle the Antarctic continent influence global atmospheric circulations. The Antarctic continental plate played a central role in the history of the formation of the continents and the resulting oceanic and atmospheric circulation patterns observed today. Understanding processes in Antarctica and the Southern Ocean is critically important to understanding processes in the global system.

Antarctica and the Southern Ocean comprise an unparalleled natural laboratory in which to study a multitude of constantly changing conditions. Short-term changes happen within lunar and annual cycles and within the context of longer-term oscillations of years to decades. In recent decades, changes to the global climate from human activities have been superimposed upon these natural variations, and the poles reflect these changes. Indeed, the Arctic has experienced large temperature changes already. The Southern Ocean has also experienced significant warming, with oceanic fronts being pushed 60 miles closer to the continent, but the situation in Antarctica is complicated by the influence of the Antarctic ozone hole, another human-induced change that has uniquely affected this region. These complex environmental forces need to be studied in order to understand how they affect global processes, and also to measure their impact on life, from bacteria to worms, microarthropods, fish, birds, and marine mammals. Antarctica and the Southern Ocean are critically important locations for observing physical, chemical, and biological changes that are happening on a global scale (National Research Council, 2010b).



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CHAPTER TWO Fundamental Questions of Global Change T he world is experiencing many changes. Global temperatures, on land and in the oceans, are increasing. Sea levels are rising, global weather patterns are shifting, and the chemistry and biology of the world’s lands and oceans are changing. It is a unique time in history in that we now have great capacity to observe many of these changes and understand many of the reasons behind them. Antarctica and the Southern Ocean are intimately involved in global processes that provide the key to understanding those changes. Formation of the deepest water in the global ocean circulation occurs in the Southern Ocean, as does upwelling to the sea surface of all the deep waters from other oceans. The Southern Ocean is an ex- tremely important region of the globe for air-sea exchange of carbon dioxide, second only to the northern North Atlantic. The strong westerly winds that circle the Antarctic continent influence global atmospheric circulations. The Antarctic continental plate played a central role in the history of the formation of the continents and the resulting oceanic and atmospheric circulation patterns observed today. Understanding pro- cesses in Antarctica and the Southern Ocean is critically important to understanding processes in the global system. Antarctica and the Southern Ocean comprise an unparalleled natural laboratory in which to study a multitude of constantly changing conditions. Short-term changes happen within lunar and annual cycles and within the context of longer-term oscil- lations of years to decades. In recent decades, changes to the global climate from hu- man activities have been superimposed upon these natural variations, and the poles reflect these changes. Indeed, the Arctic has experienced large temperature changes already. The Southern Ocean has also experienced significant warming, with oceanic fronts being pushed 60 miles closer to the continent, but the situation in Antarctica is complicated by the influence of the Antarctic ozone hole, another human-induced change that has uniquely affected this region. These complex environmental forces need to be studied in order to understand how they affect global processes, and also to measure their impact on life, from bacteria to worms, microarthropods, fish, birds, and marine mammals. Antarctica and the Southern Ocean are critically important locations for observing physical, chemical, and biological changes that are happening on a global scale (National Research Council, 2010b). 23

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F U T U R E S C I E N C E O P P O R T U N I T I E S I N A N TA R C T I C A A N D T H E S O U T H E R N O C E A N This chapter explores important questions related to environmental change that will drive research in Antarctica and the Southern Ocean over the next 20 years. The ques- tions here are not an exhaustive list, but rather highlight important research areas: • H ow will Antarctica contribute to changes in global sea level? • W hat is the role of Antarctica and the Southern Ocean in the global climate system? • W hat is the response of Antarctic biota and ecosystems to change? • W hat role has Antarctica played in changing Earth in the past? The following sections generally include the following subsections for each of the is- sues discussed: • D escription of the global context for the issue; • C urrent trends or understanding of the issue; • Q uestions to better understand the issue in the future; and • R equired tools and actions to better understand the issue. 2.1 HOW WILL ANTARCTICA CONTRIBUTE TO CHANGES IN GLOBAL SEA LEVEL? Global Context Antarctica’s ice sheets are maintained through a dynamic balance: snow and ice accu- mulate over the continent, flow to the margins, and are lost to the sea. Temperatures are rarely above freezing, even during summer (except in the Peninsula), and ice is primarily lost by calving or melting when it comes into contact with relatively warm ocean waters. Antarctica holds enough ice to raise global sea levels by more than 60 m (Huybrechts et al., 2000) (see Box 2.1). A big question persists: As the world warms, how much will ice loss accelerate, ice sheets shrink, and sea levels rise? What Is Currently Known About Antarctica’s Contribution to Sea level Rise? Earth’s geologic history provides some insight into Antarctica’s relationship with global sea levels. During the Last Glacial Maximum, roughly 20,000 years ago, atmo- spheric carbon dioxide concentrations were 180 parts per million by volume, one-third lower than preindustrial values (Sigman and Boyle, 2000); Earth was colder on aver- age by about 5°C; and larger ice sheets caused global sea level to be more than 130 m lower than today (Fairbanks, 1989). Through a combination of rising atmospheric carbon dioxide levels, changes in Earth’s orientation and orbit around the Sun, and instabilities inherent in large ice sheets, a massive deglaciation occurred that caused 24

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Fundamental Questions of Global Change BOX 2.1 THE CONNECTION BETWEEN ICE AND SEA LEVEL RISE Where land and ocean meet, the sea surface height changes regularly on short timescales as a result of tides and weather. On longer timescales sea level changes because of thermally controlled expansion or contraction of water in the ocean and because of changes in the amount of water stored on land in the form of groundwater and land ice. Also observed are changes in relative sea level due to the subsidence or lifting of the coast, but even larger sea level changes come from changes in the amount of water stored on land in the form of ice. Paleoclimate records show how much variation in sea level has been experienced by Earth before. During the ice ages sea level varied by more than 130 m (400 ft) (Fairbanks, 1989); these variations were driven by variations in the amount of ice stored on land. At the Glacial Maximum the sea level was low enough to walk from Siberia to Alaska, while at other times sea level was 5-6 m (15-20 ft) higher than today. Evidence suggests that most of this sea level rise during the Glacial Minimum was from the melting of the West Antarctic Ice Sheet (WAIS). Such sea levels would put much of Washington, DC, and lower Manhattan under water, not to mention many low-lying coastal areas around the world. The WAIS may be unstable (Bamber et al., 2009; Katz and Worster, 2010) and could potentially cause a significant sea level rise. Robust models for predicting the behavior of the WAIS under various climate conditions are needed now (Joughin and Alley, 2011). sea level to rise at an average rate of 10 mm per year for more than 10,000 years (Figure 2.1). Coral records indicate that the sea level increased at a rate in excess of 40 mm (about 1.6 in) per year during one interval around 15,000 years ago (Fairbanks, 1989). Antarctica and its ice sheets contributed about 20 m to the overall 130 m rise in sea level and they appear to have been at least partially responsible for the rapid rise noted 15,000 years ago (Clark et al., 2002). Following the transition from the last glacial period, sea level was relatively stable for a period of approximately 7,000 years (Figure 2.1). However, increasing atmospheric carbon dioxide (CO2) levels and warming since the advent of the Industrial Revolu- tion raise concerns of significant sea level rise in the future. Presently, sea level is rising at approximately 3.5 mm per year as a combined result of thermal expansion of the oceans and melting of glaciers and polar ice sheets (note that sea ice disappearance does not contribute to sea level rise as it is already part of the ocean volume) (Beckley et al., 2007; National Research Council, 2010b). Sea level rise has been measured by a combination of tidal gauges and satellites, including altimetric data from the Jason satellites. 1 Since 2001, ice mass loss has also been measured from gravity field mea- surements from the GRACE2 (Gravity Recovery and Climate Experiment) satellites 1 See http://sealevel.jpl.nasa.gov/missions/. 2 See http://earthobservatory.nasa.gov/Features/GRACE/. 25

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F U T U R E S C I E N C E O P P O R T U N I T I E S I N A N TA R C T I C A A N D T H E S O U T H E R N O C E A N FIGURE 2.1 Changes in sea level since the last glacial period, showing a 130-m rise along with the rela- tively fast rates of rise beginning about 15,000 years ago. SOURCE: Image created by Robert A. Rohde/ Global Warming Art. Based on data from Fleming, 2000; Fleming et al., 1998; and Milne et al., 2005. ( Ward, 2004). Starting from being nearly in balance during the early 1990s, Antarctica has been losing ice at an increasing rate and now contributes more than 0.5 mm to sea level rise each year (Rignot et al., 2011). Antarctica’s accelerating ice loss is, at least in part, attributable to disintegration of floating ice shelves. Although the loss of floating ice shelves does not contribute to sea level rise directly, the ice shelves provide a back pressure against the flow of ice, essentially buttressing the interior ice locked up on land and preventing it from flowing quickly. Once ice shelves are lost, continental ice flows more rapidly into the sea. As predicted more than 30 years ago (Mercer, 1978), ice shelves along the Antarctic Peninsula of Antarctica have been the first to significantly deteriorate (Morris and Vaughan, 2003), owing to the overall warmer conditions in this region. This ice shelf loss has been followed by an acceleration of ice flow into the sea (Scambos et al., 2004), similar to events that have been observed in Greenland (Thomas, 2004). The Antarctic Peninsula does not contain much ice because it is located in warmer latitudes and is narrow, so the immediate consequences for sea level are not large. However, the question remains whether the loss of floating ice shelves and conse- quent acceleration of continental ice observed in the Antarctic Peninsula is a harbin- ger of what is to come in West Antarctica or other parts of East Antarctica. On the continental interior, summer temperatures atop Antarctica’s ice shelves gener- ally remain several degrees below freezing. A major question is whether future warm- ing will lead to summer melting and jeopardize the stability of the ice shelves. Most 26

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Fundamental Questions of Global Change of the Antarctic continent has not warmed as much as the global average in recent decades, but paleoclimate records from the last interglacial period and climate model predictions for the end of this century indicate, respectively, that Antarctic tempera- tures have changed and will change more than the global average over longer time- scales (Clark and Huybers, 2009). In addition to surface warming from the atmosphere, ocean warming may also lead to thinning and possible destabilization of ice shelves. Indeed, the grounding line of the Pine Island Glacier has been observed to be migrat- ing inward toward the continent, apparently because of increased subsurface melting of that ice shelf caused by warming ocean water (Thomas et al., 2004). The geometry of Antarctica’s ice also raises the concern that ice loss could substan- tially accelerate. Parts of the East Antarctic Ice Sheet and most of the West Antarctic Ice Sheet rest upon ground that is below sea level. The ice that extends above sea level literally weighs down upon the ice underneath, pressing it onto submerged ground. As the thickness of the ice sheet tapers toward its margins, it can lose contact with the ground to form floating ice shelves. In these regions, the ice sheet melts more rapidly because of the relatively warm ocean waters in which it bathes. When an ice sheet that is grounded below sea level loses ice, more of it will tend to float, which can lead to more rapid flow, more melting of ice, and even more rapid ice loss. Thus, loss of ice leads to more loss of ice, constituting a positive feedback that has the potential to ac- celerate sea level rise (Nicholls et al., 2007; Thomas and Bentley, 1978). For this reason, the West Antarctic Ice Sheet is sometimes referred to as the “weak underbelly” of Antarctica (Hughes, 1981). Importance to the United States The estimated range of sea level rise expected to occur by 2100 is 0.4 to 2 m (National Research Council, 2011e; Pfeffer et al., 2008), but these are back-of-the-envelope cal- culations based on extrapolations from current trends. Indeed, the 2007 Intergovern- mental Panel on Climate Change (IPCC) report (IPCC, 2007) almost entirely neglected to account for changes in the rate at which Antarctic ice is discharged into the ocean on the basis that not enough is known about how to model these processes. Antarc- tic contributions to sea level are therefore largely considered “a known unknown,” wherein ignorance of likely outcomes hinders society’s ability to understand what will happen and what consequences might follow. Globally, rising sea level is expected to threaten the homes and livelihoods of hun- dreds of millions of people by the second half of this century (see Box 2.2). In an assessment of exposure to coastal flooding by 2070, Miami and New York City ranked 27

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F U T U R E S C I E N C E O P P O R T U N I T I E S I N A N TA R C T I C A A N D T H E S O U T H E R N O C E A N BOX 2.2 THE RISKS OF SEA LEVEL RISE High rates of economic and demographic growth during the past century have multiplied populations and the infrastructure placed along coastlines worldwide. This leads to not only local communities and commercial centers being placed at great risk from rising sea levels, but also to nations being faced with extremely high economic, societal, and security challenges. Examples of problems already being faced in the United States from rising seas include shoreline retreat along most U.S. exposed shores and intrusion of seawater into freshwater aquifers in coastal areas, which threatens freshwater supplies (National Research Council, 2010a). More than one- third of U.S. residents live near a coast, and more than $1 trillion is contributed annually to the nation’s economy from activities that occur on or along a coast (USGCRP, 2009). Future sea level rise poses risks to U.S. communities, coastlines, and infrastructure along much of the eastern and southern United States, the West Coast, and Alaska (see figure). Potential mid-Atlantic wetland survival. Areas where wetlands would be marginal or lost under three sea level rise scenarios (in mm per year). SOURCE: CCSP, 2009. 28

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Fundamental Questions of Global Change 6th and 17th, respectively, in threatened impacts to the world’s major cities (Nicholls et al., 2007). In particular, rising sea level threatens to cause more frequent flooding by increasing the height of storm surges and the peak level of tidal cycles. Overtopping coastal levees on even a single occasion can have dire consequences, as evidenced by the results of Hurricane Katrina in New Orleans in 2005. Higher sea level also threatens wetland habitats, as the U.S. Climate Change Science Program reported (Titus and Anderson, 2009), namely that most of the mid-Atlantic coastal wetlands will be lost in the next century if local sea level rises by as much as 1 m. The U.S. Navy has taken steps to examine the potential impacts of climate change, including those from sea level rise, on future naval operations and capabilities (National Research Council, 2011e). Global average sea level is, of course, less relevant than how much sea level will rise in specific locations—primarily where the sea meets where people live and work—and here lies a poignant wrinkle. Loss of ice weakens the local gravitational attraction that the ice sheet exerts on the ocean, leading to a reduction in sea level at the margin of the ice sheet. Further afield from where the ice loss occurs, sea level rises by more than its global average, with the specific locations of maximal rise depending upon the ro- tation of Earth and the geometry of the ocean basins. Local variations in sea level also depend upon changes in ocean circulation and storm activity. As it happens, loss of ice from West Antarctica would cause about a 15 percent greater sea level rise along the eastern and western United States than the global average, with the largest increase centered approximately at Washington, DC, highlighting how the United States is uniquely exposed to the fate of West Antarctica and the Antarctic ice sheet (Mitrovica et al., 2009) (Figure 2.2). Questions for the Future Two critical questions arise: How much will Antarctica contribute to a rising future sea level and how quickly? Antarctica’s ice sheets are strongly intercoupled with the fluid and solid portions of Earth, and developing an ability to predict their future behavior depends upon designing a comprehensive modeling and observing strategy. To give a sense of the system intercoupling, consider that determination of how much ice Antarctica has been losing in the past decade, based on satellite measurements of gravitational anomalies, requires knowledge of the rate at which the underlying bed- rock is lifting. Determining bedrock uplift requires understanding the structural prop- erties of the rock, as well as how much ice Antarctica has lost since the Last Glacial Maximum, some 20,000 years ago. As another example of linked system complexity, whether ice loss will accelerate depends, in part, upon the stability of the ice shelves 29

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F U T U R E S C I E N C E O P P O R T U N I T I E S I N A N TA R C T I C A A N D T H E S O U T H E R N O C E A N FIGURE 2.2 Sea level changes in response to a collapse of the West Antarctic Ice Sheet represented as an additional change relative to the global average of 5 m; this highlights the significant local deviations. Sea level rise is 15 percent higher than the global average along the U.S. coastline. Changes over land can be ignored. SOURCE: Mitrovica et al., 2009, reprinted with permission from the American Association for the Advancement of Science. bordering Antarctica, which in turn depends on their temperature, and therefore the circulation and temperature of the oceans and atmosphere. Required Tools and Actions The committee recommends five actions that are needed to advance prediction of Antarctica’s contribution to sea level in the future: • Develop greater predictive capacity for the flow of ice into the ocean. Relative to the ocean and atmosphere, the dynamics of the cryosphere are poorly under- stood. This is partly because of difficulties inherent in observing and model- ing ice flow: it is difficult to make physical measurements deep within and beneath ice sheets and ice shelves; many timescales of ice motion are longer than those afforded by instrumental records; and ice is a non-Newtonian fluid, whose motion depends sensitively upon its interactions with sediment or rock at its bed. As stated, the 2007 IPCC report neglected the possibility of change in the rate at which Antarctic ice is discharged into the ocean because not enough was known (IPCC, 2007), underscoring the need for further theoretical and observational work on ice sheets. Requisite work can be broken down ac- 30

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Fundamental Questions of Global Change cording to ice interactions with the ocean, atmosphere, and solid Earth and are described in separate bullets below. Improved theoretical understanding and technical capacity is also needed, as detailed next. • Increase scientific and technical capacity to observe and model ice sheets. The cadre of theoreticians and those making observations related to the Antarctic ice sheet is small relative to the scope of the problem. Teams of collaborators would need to include glaciologists, geologists, oceanographers, atmospheric scientists, and so on, and expansion of existing efforts across federal agen- cies and academia. Those components of ice sheets that can change relatively rapidly, especially those associated with ice streams and ice shelves, require particular attention. • Determine how the ocean transports heat to ice shelves and how this may change in the future. Antarctica loses the vast majority of its ice via interactions with the ocean. The amount of melting beneath ice shelves depends upon trans- port of heat by the oceans, which is driven by a complex mix of wind stress and changes in water density brought about by heating, cooling, and fluxes of salt- or freshwater. Recent modeling studies (Pollard and DeConto, 2009) high- light how an increase in ocean heat flux could lead to rapid inward migrations of ice shelf grounding lines and loss of ice volume. Developing instrumenta- tion and an observational program with which to monitor the conditions be- neath ice shelves is a high priority (see Appendix C for enabling technologies). In conjunction with increasing observations, improved models capable of accurately representing the transfer of heat from the ocean to the cryosphere need to be developed and tested (also see Section 2.2). • Improve monitoring of surface temperature and ice accumulation. It is not en- tirely certain whether the temperature of Antarctica is or is not increasing. A general warming trend was reported for surface atmospheric temperatures, based on surface and satellite observation (Steig et al., 2009). But a recent report, using similar data but different statistical methods, found little evi- dence of warming (O’Donnell et al., 2011). At the heart of this discrepancy is the sparsity of the international Antarctic observational network, which places heavy demand on statistical methods for estimating temperature variations in regions where direct observations are not being made. Nonetheless, there are both model analyses and paleoclimate observations that strongly suggest that Antarctica will eventually warm significantly more than the lower latitudes (Clark and Huybers, 2009). A warming of several degrees Celsius could lead to significant summer melting atop the ice shelves and cause their disintegra- tion, as recently observed for the Larsen ice shelf (see Figure 2.3) (MacAyeal et al., 2003; Mercer, 1978). Similar to the limited and widely scattered Antarctic 31

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F U T U R E S C I E N C E O P P O R T U N I T I E S I N A N TA R C T I C A A N D T H E S O U T H E R N O C E A N temperature observations (often obtained at international bases around the continent), there are large gaps in monitoring snow accumulation over Antarctica, as well as a significant partial evaporation of snowfall. Because satellite observations of ice temperature and snow accumulation are not suf- ficiently reliable, a comprehensive surface observing network is needed to define these basic surface conditions. • Improve mapping of conditions and structures beneath the ice sheet and measur- ing uplift of underlying bedrock. Subglacial topography and the composition of the underlying rock are important determinants of glacial flow. Determining which regions are below sea level is important for evaluating instabilities in the ice. However, the subglacial topography and geology of Antarctica is less well known than the topography of Mars (Gwinner et al., 2010). Comprehen- sive radar mapping of Antarctica is required. Determining the rate of uplift of the bedrock beneath Antarctica, which is still adjusting to the unloading asso- ciated with the last major deglaciation (between 18,000 and 7,000 years ago), is also critical for monitoring and assessing the changes of the mass of the ice sheet. In particular, correct interpretation of gravitational anomalies monitored FIGURE 2.3 In 2002, the Larsen B ice shelf collapsed and delivered 3,250 km2 of ice into the ocean. These images are derived from satellite data from the MODIS (Moderate Resolution Imaging Spectroradiom- eter) instrument. SOURCE: Cavalieri et al., 2008, National Snow and Ice Data Center, University of Colorado, Boulder. 32

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Fundamental Questions of Global Change from space requires measuring changes in the elevation of both the underly- ing bedrock and the overlying ice sheets. Bedrock uplift rates can be assessed both through Global Positioning System measurements as well as through models that incorporate the geologic history of changes in the size of Antarc- tic ice sheets. Lack of knowledge of the amount of bedrock uplift provides the largest source of uncertainty in determining the rate that Antarctica is losing its ice (Chen et al., 2009) (also see Section 2.4). It is only through observations made in Antarctica that scientists were alerted to such phenomena as the ozone hole, rapid disintegration of the Larsen B ice shelf, accelera- tion of glaciers once the ice shelves were lost, and draining and filling of subglacial lakes. Given how limited direct observations of the Antarctic continent have been and how human actions are now prodding the climate system, many surprises seem possible in the future. In order to expect or learn from any surprises, there will need to be careful monitoring of Antarctica, including its ice, overlying atmosphere, and peripheral oceans. Observations made in Antarctica can be likened to an early warning network that, when adequately interpreted, analyzed, and placed into the context of a developed theoretical understanding, will alert society to acceleration of Antarctica’s ongoing contribution to changing sea level or, possibly, uncover new mechanisms by which Antarctica can change sea level. 2.2 WHAT IS THE ROLE OF ANTARCTICA AND THE SOUTHERN OCEAN IN THE GLOBAL CLIMATE SYSTEM? Although Antarctica and the Southern Ocean are physically distant from the Northern Hemisphere, they are directly connected to the entire global climate system. Some of the connectivity with lower latitudes is rapid, through the atmosphere, with adjust- ments on short timescales of the order of days to months. Some of the processes are more remotely connected and have longer timescales; these include the Southern Ocean’s role in the global ocean overturning circulation and rate of carbon dioxide up- take. In stark contrast to the rapid warming of the Arctic, Antarctica and the Southern Ocean present a mixed picture of both climate change and climate variability. Significant progress in understanding changes in the southern high-latitude coupled climate system over the next 20 years will require construction and operation of an observing system for the atmosphere, ocean, sea ice, and glacial ice. In parallel, suc- cessful predictive modeling will require greatly improved coupled modeling of all of the elements of the climate system and continuing improvement of the data- 33

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F U T U R E S C I E N C E O P P O R T U N I T I E S I N A N TA R C T I C A A N D T H E S O U T H E R N O C E A N snow and ice-free areas. These areas range from biologically more complex terrestrial ecosystems on the Antarctic Peninsula and “oases” near the East Antarctic coast to the less complex ecosystems in the McMurdo Dry Valleys (Fox et al., 1994). On both land and sea, warming and ice melt will increase the area of exposed surfaces, provide new habitats for colonization by organisms, and cause changes in ecosystem functioning. As sea ice disappears, new areas of ocean surface will be exposed to increased solar radiance, and biological productivity may increase. Natural colonization rates will in- crease and species ranges will expand. Rapid expansion of the biogeographical ranges of native plant species has been noted in maritime Antarctica, as have increases in biological production in continental lakes. These changes in range expansion and growth rates of native species due to warming could lead to fundamentally different organization of Antarctica’s ecosystems, includ- ing more complex ecosystem structures, and an increase in the biotic factors (preda- tion, competition, pathogens) that control rates of biogeochemical processes rather than the current dominance by physical factors (National Research Council, 2010b). Understanding current distributions of native species is central to detecting and pre- dicting the effects of climate change. Fortunately, there has been significant progress in identifying some species and the factors that determine their ranges, which can be used to predict future range expansions, new community assembly, and altered ecosystem function. Questions for the Future The mechanisms of ecosystem response to global change remain controversial ( Trivelpiece et al., 2011), but there is a growing consensus that climate change generally affects ecosystems by destroying existing habitats or enabling new ones (see above) and by disrupting the trophic and other phenological connections among prey and predator populations. Evidence for the effects of climate change on the structure and function of marine, freshwater, and terrestrial systems is still based on a few observa- tional ecological studies and even fewer laboratory and field manipulation experiments (National Research Council, 2011d). Advances in knowledge of the structure and func- tion of Antarctic ecosystems have been substantial, yet researchers are still unsure of the spatial and temporal variability of ecosystem responses to climate change and other global changes. Major questions related to environmental change include the following: • How vulnerable or resilient are marine, freshwater, and terrestrial food webs to changes such as warming, enhanced water availability, habitat disturbance, ocean acidification, pollutant accumulation, and loss of sea ice? 64

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Fundamental Questions of Global Change • What are the functions of Antarctica’s diverse ecosystems in biogeochemical cycling and how will they change? • Are the marine and terrestrial ecosystems of Antarctica organized differently than ecosystems elsewhere on the globe? And does this temper their re- sponses to change? • Could Antarctic ecosystems switch to an unknown, alternative state with dif- ferent structure and functioning? • Is the Peninsula a harbinger of larger-scale changes to come? Will ecosystems of the continental interior follow the lead of the Peninsula? Required Tools and Actions Scientists do not yet know if environmental changes proceed from north to south or from the continental margin to its interior. Lack of geographically extensive, long- term observation records and the paucity of observations south of the Peninsula and McMurdo regions impede rigorous testing of these questions. The fortuitous location of a planetary climate change hotspot in a region with advanced scientific facilities at the many research stations along the Peninsula provides an unparalleled opportunity to understand and predict the future course and consequences of climate change. Meteorological and biotic data needed for model projections also come from long- term observations in lakes, streams, soils, permafrost, and glaciers of the McMurdo Dry Valleys. But there is a dearth of observations at other terrestrial, coastal, and interior sites to indicate the future effects of climate change. To place these local changes in a continent-wide context, and predict the future course of change across Antarctica, a comprehensive coordinated observing and prediction system encompassing all the major elements of the Antarctic environment is needed, including the terrestrial ecosystems, permafrost, surrounding ocean, sea ice, ice shelves, ice sheets, and sub- glacial habitats. The variables to be captured by a comprehensive Antarctic Observing Network are described later in this report, including geophysical climate observations and coordinated measurements of diagnostic ecosystem structure and biogeochemi- cal function (see Section 4.4). Antarctica is an international laboratory for studies of global change and ecosystem responses to environmental variability. In the coming decades Antarctica will continue to undergo significant changes due to human activities. Climate change has already altered the ecosystems of the peninsula, and its impacts may well reach across the continent in the coming century. Increased tourism, overfishing, and other human ac- tivities will impose new burdens on the management of Antarctica. Moving forward in the coming two decades, it will be increasingly necessary to come to terms with these 65

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F U T U R E S C I E N C E O P P O R T U N I T I E S I N A N TA R C T I C A A N D T H E S O U T H E R N O C E A N possible realities. Yet because of the unity of purpose imposed by the Antarctic Treaty, Antarctica provides the world with the only example of an entire continent reserved primarily for scientific research. A continental-scale, interdisciplinary, observation- prediction-management system will be needed to provide timely data to support decision making, adaptive management, and governance of the continent as the press of human intervention on its climate, natural resources, ecosystems, and biogeochemi- cal cycles becomes ever more intense. The committee’s vision for science in Antarctica in the next two decades is an inte- grated observing, information, and modeling effort enhanced by powerful new ge- nomic tools, geochemical tracers, and increased modeling efforts to build a predictive understanding of ecosystem response to rapid climate change. 2.4 WHAT ROLE HAS ANTARCTICA PLAYED IN CHANGING THE PLANET IN THE PAST? Global Context The interaction between solid Earth tectonics and the changing planet is complex, multifaceted, and tightly connected. On a very long timescale, the movements of tectonic plates (where plates may consist of entire continents and ocean basins), their fragmentation, or the collision of several plates have dramatic consequences. Conse- quences range from earthquakes and volcanoes to the construction of new mountain ranges, the opening of gateways between vast oceans, and the triggering of global climate shifts. New mountain ranges provide the high topography where glaciers first grow—glaciers that can become the nucleation point for major ice sheets. In Antarctica these tectonic processes have driven the uplifting of vast reaches of Earth’s surface, producing spectacular mountain ranges such as the Transantarctic Mountains that cut across the continent and the Gamburtsev Mountains that are hid- den completely beneath the thick cover of the East Antarctic Ice Sheet (Figures 2.12 and 2.13). The tectonic and glacial histories of Antarctica are tightly linked. Without its high topography, the history of the Antarctic ice sheet would have been quite differ- ent. Without the continental glaciation the mountains of Antarctica would have been quite different. Understanding the mechanisms and timing of the formation of these mountains is linked to the understanding of the changing climate of Antarctica and of the planet in the past. 66

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Fundamental Questions of Global Change FIGURE 2.12 The tectonic processes that lead to the formation of mountain ranges seen here are linked to the glacial histories of Antarctica. SOURCE: Bedrock elevations relative to sea level from Lythe et al., 2001. What Is Currently Known About Antarctica’s Geologic History? The tectonic opening of key oceanic passageways has controlled global climate and shifted global circulation patterns within the atmosphere and the deep oceans. Over 200 million years ago, Antarctica was the centerpiece of the Gondwana, a massive 67

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F U T U R E S C I E N C E O P P O R T U N I T I E S I N A N TA R C T I C A A N D T H E S O U T H E R N O C E A N FIGURE 2.13 Cross-sectional profile of the Antarctic ice sheet based on BEDMAP bed topography (Lythe et al., 2001) and surface topography (Liu et al., 1999). The inset indicates the location of profile end points. SOURCE: G. Clarke; NRC, 2007a. supercontinent consisting of what later became Antarctica, India, Australia, South America, and Africa. Around 180 million years ago, this supercontinent began to break apart, and Antarctica commenced moving into its present polar position. The climate of the planet was significantly different when Antarctica arrived at the South Pole (roughly 100 million years ago). Because of thick ice, there is no knowledge of the geology of most of East Antarctica, but outcrops at the Transantarctic Mountains and along the Antarctic Peninsula, for example, show that at that time lush forests grew there and were inhabited by dinosaurs and mammals (Francis et al., 2008). With the final separation of the supercontinent and the 10-fold drop in global atmospheric carbon dioxide (CO2) levels from 3,000 parts per million (ppm) in the Cretaceous, to around 500 ppm approximately 34 million years ago, both Antarctica and the globe cooled (Arthur et al., 1988; Jenkyns et al., 1994; Kuhnt et al., 1986). As a seaway formed between South America and Antarctica between 34 and 24 million years ago, the isolation of the southern continent began (see Figure 2.14). The Antarctic Circumpolar Current (see Box 2.4) began circulating and likely reduced the amount of heat that the ocean previously brought from the midlatitudes to the edges of Antarctica. Thus, tec- tonic fragmentation and falling CO2 levels shifted Antarctica from a green continent to a white continent encased in ice. Understanding the opening of the Southern Ocean 68

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Fundamental Questions of Global Change FIGURE 2.14 This figure illustrates (left) the Gondwana supercontinent, (middle) the transition and open- ing of the oceanic passageway around Antarctica, and (right) present-day geography and bathymetry. SOURCE: ODSN-Geomar. as Gondwana fragmented is critical to understanding how Antarctica became glaci- ated and why the global climate became much colder. Researchers have learned much about the processes of past climate change from Antarctic sediment and ice cores. The Antarctic ice cores provide key insights into past changes in the global atmosphere while sediment cores reveal how the ice sheets have waxed and waned in the past (see Section 3.1). This information is crucial to constraining global climate models of the past and of the future. While ice sheets mantle the entire Antarctic continent and the most dramatic envi- ronmental change is being observed along the edges, the crucial location for under- standing the mechanisms of how these thick, slow-moving, enormous pieces of ice will change is the interface between the ice and the underlying rock. The presence of water or water-saturated sediments acts as a lubricant to the ice sheet, enabling ice to slide from the center of the continent to the ocean. Subglacial lakes (National Research Council, 2007a) usually formed in rifted basins and are linked to the onset of fast ice flow (National Research Council, 2007a). Tectonically driven heat flow varia- tions are key controllers of basal melting rates and the distribution of subglacial water. Knowledge of the basal conditions—what is happening beneath the ice where ice meets water or solid rock and sediment—is key to understanding ice sheet dynamics and how the ice sheets will move. 69

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F U T U R E S C I E N C E O P P O R T U N I T I E S I N A N TA R C T I C A A N D T H E S O U T H E R N O C E A N Questions for the Future Projections of future Antarctic ice sheet dynamics depend heavily on acquiring knowl- edge of what lies below that ice, including the physical properties of the ice column and the properties of ice-rock and ice-sediment interfaces. Constructing useful models of ice sheet and shelf movements to estimate their potential for destabilization will depend on obtaining major expansions of core sampling, annual ice budget measure- ments, and ice sheet velocity mapping. Without carefully defining basal conditions, especially with respect to how slippery the beds are and how much melt is present at the base of each ice sheet, ice sheet models will not be able to produce reliable estimates of how the ice sheets will change in the future. Contemporary estimates of continental ice movements are shown in Figure 2.15. Required Tools and Actions Understanding the role Antarctica has played in global systems over time will require a holistic study of both the thick ice sheets and the underlying Earth crust. Major gaps remain in the fundamental knowledge of the structure of the Antarctic continent. Determining how the continent was formed is one key to understanding the role Ant- arctica has and will play in the global system. Studies of the base of the ice sheet will require sampling through the thick ice and should include accurate measurements of heat flow. Projections of future changes suggest that warm ocean waters will eventu- ally reach the margins of East Antarctica, an ice sheet that presently appears relatively stable. Key approaches include systematic surface and airborne geophysical obser- vations in both East and West Antarctica along with sampling of the rock beneath the thick mantle of ice. Both studies of the fundamental architecture of the Antarctic continent and essential ice sheet dynamic studies will require airborne radar and laser observations from long-range aircraft complimented by coincident gravity and magnetic field measurements. Sampling the base of the ice sheet and the underlying bedrock requires development of a new generation of rapid drilling systems for access to the bed with minimal contamination of the environment. While much emphasis has been placed on the apparent instability of portions of the West Antarctic Ice Sheet, much of East Antarctica remains absolutely unknown yet is critical to the understand- ing of the continent and the ice sheets. Regions to be sampled include the enigmatic Gamburtsev Mountains, subglacial lakes, and other major subglacial provinces. Marine drilling targets range from the Weddell Sea coast to new sites in the Ross Sea. Dynamic drilling programs including the Integrated Ocean Drilling Program and Antarctic Geo- logical Drilling have been very effective at determining the climate history of Antarc- 70

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Fundamental Questions of Global Change FIGURE 2.15 Composite surface speed of ice from RADARSAT-1. Speed is represented by a color log 10 scale (0 in deep blue to 1,000 m/yr in red). This type of information is crucial to develop reliable ice sheet models. SOURCE: Jezek, 2008. tica. Ongoing marine geophysical studies of the surrounding oceans including high- resolution marine bathymetry, and marine seismic measurements will be important to examine the mechanisms and timing of prior major tectonic events. Initiatives to expand knowledge of the geology and glaciology of Antarctica through collaborative international efforts have already begun (Bell, 2008). However, the large scale of the necessary observations and the observing network remains daunting con- sidering the difficult task of diagnosing and monitoring the motions and melting of an ice and land mass that is approximately 1.4 times the area of the United States (British 71

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F U T U R E S C I E N C E O P P O R T U N I T I E S I N A N TA R C T I C A A N D T H E S O U T H E R N O C E A N Antarctic Survey, 2005). In parallel with major advances and expansions in sampling networks, advanced mathematical modeling with understanding of the basic fluid me- chanics of the continental ice sheet is needed. Validation activities based upon actual sampling of the ice sheet’s properties would improve these models. In 20 years, the committee envisions that there will be an improved understanding of the tectonic evolution of Antarctica, including the formation of the major mountain ranges, the distribution of key geologic terrains beneath the ice sheets, and the open- ing of major ocean basins surrounding the continent. Understanding tectonic evolu- tion will inform the understanding of the basal geologic framework and the condi- tions necessary for developing accurate ice sheet models. 72

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Red lights help maintenance workers doing routine repairs on the South Pole Telescope. SOURCE: Daniel Luong-Van/NSF.