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Landscapes on the Edge: New Horizons for Research on Earth’s Surface CHAPTER TWO Grand Challenges in Earth Surface Processes A number of overarching intellectual questions regarding natural and anthropogenic processes are fundamental to understanding the form, composition, and evolution of our planet’s surface. From among these questions, the committee has identified nine grand research challenges, each of which is introduced in the form of a primary research question with associated opportunities for further investigation. The grand challenges are exemplary, but they cannot provide a full assessment of all current research in Earth surface processes. Focus was given, instead, to integrating a range of topics that are representative of the breadth and depth of research in the field. The nine challenges are supported by information gathered from the community of Earth surface scientists1 during this study (Appendix B). 2.1 WHAT DOES OUR PLANET’S PAST TELL US ABOUT ITS FUTURE? One of the most remarkable aspects of the Earth surface system is the extent to which it records its own evolution. The visible landscape incorporates a wealth of information about its past, and the archive of sediments and sedimentary rock extends the record to very early in our planet’s history (Figure 2.1). The Earth’s record keeping has given us, among other things, a detailed picture of the evolution of the atmosphere, oceans, tectonics, life, and the surface system itself. Both scientific curiosity and societal demand drive us to read this fragmentary and often puzzling archive in an attempt to understand how the surface environment in which we exist has evolved through time and therefore how it may change 1 As used in this report, the term “Earth surface scientists” refers to scientists from disciplines concerned with the form, composition, properties, function, and evolution of Earth’s surface, including biogeochemistry, ecology, environmental science, geochemistry, geography, geology, geomorphology, glaciology, hydrology, oceanography, sedimentology, soil science, and stratigraphy. Increasingly, scientists from disciplines such as atmospheric science, engineering, geophysics, and social science are participating in research on Earth surface processes as well.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface FIGURE 2.1 The stratigraphic section in the Grand Canyon records a time span from about 1.8 billion to 250 million years ago—a substantial fraction of Earth’s history. SOURCE: National Park Service. in the future. Our growing ability to quantify processes specific to the near-surface environment also lets us reconstruct Earth’s past in increasing detail including aspects of its geochemistry, biotic processes, topography, and particle and solute fluxes. The information stored in landscapes and sediments provides a kind of “time telescope” that allows us to view what is in effect a temporal sequence of alternative Earths—recognizably our planet, yet often surprising and unfamiliar. In addition, the history of landscapes strongly influences their present state and future evolution. In the Earth’s northerly regions, the current landscape pattern was largely created by glacial processes that peaked around 18,000 years ago, and present-day debates about agricultural best practices and practical water quality standards require a sophisticated understanding of how the landscape is evolving in response to the deglaciation. Likewise, coastal erosion is still influenced by uplift and subsidence in response to shifts in ice loading over Holocene time. The fate of global deltas on which hundreds of millions of people depend in turn depends on the delicate interplay of subsidence and sedimentation developed over geologic time. In upland regions, the general importance of tectonic history is obvious; more subtle are the possible effects of variations in uplift rate and climate in influencing the balance of sediment storage and release to downstream river systems. Soils are among the clearest examples of the influence of past time—the soils that support global agriculture represent the integrated effects of tens of thousands of years of biogeochemical processes. The evolutionary road from the prebiotic Earth and an atmosphere devoid of oxygen to the human-dominated conditions of today has been well documented in the popular
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface press: for example, extreme variations in global ice cover from nearly ice-free to (some believe) near-complete cover; meteorite impacts; sea-level variations of hundreds of meters; supercontinent assembly and breakup; mass extinctions; the growth and decay of mountain ranges; and the rich variety of life forms that preceded human existence. The sedimentary record of Earth’s colorful past is a composite of self-recording landscapes. Beyond the drama and fascination of Earth history, the record in landscapes and sediments gives us an archive of natural experiments, performed at full planetary length and time scales, from which to reconstruct planetary dynamics. This deep-time record of landscape evolution also opens an avenue to explore the connections between the Earth’s interior and its surface boundary. The morphology and properties of Earth’s surface constitute fundamental, directly observable attributes that help constrain the long-term behavior of the Earth’s convecting interior and the tectonic plates that gradually move across its surface. Landscapes on other worlds also provide a chance to test and expand our understanding of the Earth’s surface processes by applying them to materials and basic surface conditions (e.g., gravity, atmospheric pressure) very different from the ones with which we are familiar. Practical reasons also exist for studying the record of surface evolution. Nearly every aspect of the present form, composition, and function of Earth’s surface reflects its evolution over geologic time; thus, the surface environment—including the services it provides and the hazards it poses—cannot be understood outside the context of its history. Second, the subsurface heterogeneity that controls the availability of resources such as water, hydrocarbons, and minerals is in effect a three-dimensional tapestry of fossilized surface dynamics created via crustal subsidence and burial of paleo-landscapes in sedimentary basins. Lastly, the history of surface evolution in landscapes and sedimentary basins provides us with a rich archive from which to extract information on extreme events, natural variability in space and time, and how the surface responds to change. This archive is sometimes the only means we have to observe critical thresholds and other forms of nonlinear response to our modifications of the Planet’s surface systems, such as those induced by human activity. The past as the context for the current environment. Although “sustainability” has been defined in different ways, at its core it connotes management with a long, perhaps indefinite, time horizon. The further ahead we would like to forecast, the further back must we look to understand how the system arrived at its current state and how it is likely to evolve. For example, consider two contrasting scenarios for beach retreat, an issue that has consumed billions of dollars in the United States. One is a case of local erosion caused by interruption of longshore sediment flow by jetties. In the other, the retreat is the result of long-term land sinking caused by adjustment from the last glaciation. Although the short-term responses to both cases might appear similar, their contexts and hence strategies for sustainable management may be entirely different.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface Scenarios such as this, in which long-term processes strongly influence the modern environment, can be found across the Earth’s surface. Centennial-scale or longer variations in seismicity and climate may control landslide frequency in steeplands, altering the supply of sediment to river systems, with a broad spectrum of environmental consequences. Over much of the Northern Hemisphere the strong imprint of glacial history persists in the distribution of soils, surface biota, river courses, and groundwater. The extraordinary fertility of the breadbasket of the United States is related to soils developed on sediments deposited in the wake of glacial retreat; now human activities are changing the physical structure and geochemistry of these soils and locally stripping them rapidly from the landscape (Box 1.1; see also Sections 2.4, 2.8, and 2.9). As long as we thought we could simply control the landscape for the convenience of humankind, temporal trends such as these were primarily of academic interest. Now momentum is growing around the world to move away from this type of “hard” engineering to a more sustainable approach based on working with natural tendencies rather than against them. This cannot be done unless we know what the natural tendencies are—a task that requires a new synthesis of process understanding with research to reconstruct past history quantitatively. Subsurface architecture. Buried surface features such as channels and beaches serve as conduits and reservoirs for water as well as for oil and natural gas (Figure 2.2). In addition, the new field of carbon sequestration focuses on attempts to remove carbon dioxide (CO2) from the atmosphere and store it indefinitely in subsurface reservoirs—often ones from which hydrocarbons have been extracted. Again, this cannot be done safely and sustainably without detailed knowledge of the subsurface plumbing system. The petroleum industry spends billions of dollars creating extremely detailed images of the subsurface architecture created as basin subsidence and sediment input build depositional sequences in three dimensions (Figure 2.3). These images are used to characterize hydrocarbon reservoirs, but as records of geomorphic evolution, their potential value for understanding surface dynamics is enormous and has hardly been tapped. Four billion years of natural experiments. Given the complexity of the surface environment, with its interwoven and highly nonlinear physical, geochemical, and biotic systems, it is not surprising that theoretical methods for predicting its evolution are only in their early stages. In addition to hypotheses, we must learn how the Earth’s surface works through field observation and experimentation. Intentional scientific experiments in the field are common, but necessarily involve short length and time scales; reduced-scale laboratory experiments are proving to be useful, but they cannot capture all aspects of surface dynamics. Humankind seems determined to carry out full-scale unplanned “experiments,” through activities such as changing land use, reconfiguring and regimenting natural landforms, and
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface FIGURE 2.2 Buried channels such as these in the Ebro Basin, Spain, continue to serve as preferred conduits and reservoirs for fluid flow (oil, gas, and/or water) in the subsurface. SOURCE: Photo courtesy of Christopher Paola, University of Minnesota. FIGURE 2.3 High-resolution seismic reflection has opened the way to “seismic geomorphology,” including temporal evolution of features like the submarine landscapes visualized here. These topographic maps (field of view 8 × 12 kilometers, relief of approximately 600 meters, with 4 times vertical exaggeration) represent a time series of Pliocene (circa 3.5 million years ago) seascapes (the oldest is A, the youngest is D) now buried at depth offshore of the Ebro Delta in the northwest Mediterranean Sea. Reflections from deeply buried surfaces were extracted from a three-dimensional seismic volume to produce these maps of ancient seascapes. SOURCE: Bertoni and Cartwright (2005). Reproduced with permission of Blackwell Publishing Ltd.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface altering the climate. We will learn from these experimental results even as we try to predict and perhaps influence them. Yet the increasing influence of humans at global scales impels us to make better use of the roughly 4-billion-year record of natural experiments that have already occurred on Earth. The record of natural experiments includes extreme events such as meteor impacts and rapid climate changes, as well as states of the Earth—for example, one that is nearly ice-free—that are quite different from the one we know. Although we have much to learn, we do know that the world of direct human experience represents only a small fraction of our planet’s highly varied history and of its potential future. Now as we put increasing pressure on environmental services, urbanize hazard-prone landscapes, and become predominant geologic agents ourselves, understanding the full range of planetary behavior has become crucial. Numerous other examples of insights gained from studies of surface evolution on a range of time scales are included in other sections of this report. The archive of Earth history records a far richer range of states and behaviors of the landscape than we can observe directly in the tiny slice of time we occupy today. As such, sedimentary and other landscape records can provide critical information about how the landscape could change in the future. Three primary research opportunities to understand Earth’s surface and its future through examination of its past include (1) quantitative reconstruction of landscape records, (2) application of dating and imaging tools to understand landscape history, and (3) linking studies of the surface and subsurface. Quantitative Reconstruction Interpretation of landscape records by Earth surface scientists to this point has been mostly qualitative. The crucial next step is to accelerate the process of learning to read these often fragmentary records quantitatively, in order to understand long-term dynamics and reconstruct extreme events and variability and to provide the results in a form that can be used by decision makers. Recent examples of reconstruction include the spectacular findings from ice-core analysis showing that climate can change dramatically (see also Section 2.7)—over time scales measured in years—and from quantitative estimates of storm and earthquake frequency from sediment studies. These inspiring results represent a first step to unlocking the surface and sedimentary archive. One way to focus this type of reconstruction work is to seek “distant mirrors”2: times in Earth’s past with high potential to shed light on problems of prediction facing us today, such as accelerated climate change. Initiatives to study the Paleocene-Eocene Thermal Maximum (approximately 55 million years ago), an ancient warm interval potentially analogous to an anthropogenically warmed Earth, provide a good example of this approach. 2 Quoted material credited to B.W. Tuchman, 1978. A Distant Mirror: The Calamitous 14th Century. New York: Random House.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface Landscape History New imaging methods allow us to study the present morphology of the landscape in unprecedented detail (Box 2.1; see also Sections 2.2, 2.3, and 2.6; Appendix C). Equally importantly, new radiometric and other surface-dating methods (Box 1.2; see Sections 2.2 and 2.3; Appendix C) allow us to measure rates of surface change and add temporal evolution to our snapshot of the current state of the landscape. These tools are precisely those needed to quantify landscape evolution and to reconstruct the path that has led to the present state. In tectonically active areas, analysis that includes data from these tools can, for instance, extend seismic records to time spans beyond those of human records (see also Section 2.3). In any area, reading geomorphic history quantitatively will allow us to measure fluctuations in key environmental quantities, such as fluxes of sediment and nutrients through Holocene time (the last 11,700 years of Earth history), and to constrain the magnitudes of extreme events (see also Section 2.7). These essential data provide the natural context for the observed acceleration of anthropogenic changes to the surface environment. BOX 2.1 Digital Landscapes: Documenting Topography at the Scale of Transport and Erosion Processes Airborne laser mapping began only in the mid 1990s, and the technology has improved rapidly since then. In 2003 the National Science Foundation (NSF) supported the founding of the National Center for Airborne Laser Mapping (NCALM) to provide research-grade airborne laser swath data to the national research community, to advance the technology, and to provide education and training for students. By January 2009, more than 60 projects covering some 13,000 km2 had been flown. For the GeoEarthScope program, all of the San Andreas Fault System (~3,000 km2) was surveyed, and those data (plus other fault zones flown by NCALM) are freely available via the GEON OpenTopography Portal (http://www.geongrid.org/index.php/topography/opentopo). NCALM makes available the data for all its surveys through its web page (http://calm.geo.berkeley.edu/ncalm/ddc.html). By the end of its first 10 years, NCALM will have flown about 80 seed projects in support of graduate research, providing key data and contributing to a new generation of researchers advancing the field of Earth surface processes through use of high-resolution topography. In 2008, NSF supported an international workshop on High-Resolution Topographic Data and Earth Surface Processes to explore how high-resolution topographic data can advance understanding of Earth surface processes (Merritts et al., 2009). Nearly half of the more than 60 participants were graduate students or postdoctoral researchers who had received a doctorate within the past three years.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface Connecting Surface and Subsurface For too long the closely related sciences of geomorphology and sedimentary geology have been pursued separately. The confluence of the need to understand and predict the evolution of the surface environment and the availability of large volumes of high-resolution information on the subsurface archive—much of it from the oil and gas industry—provides a new level of motivation to bring these two fields together both to improve prediction in the subsurface and to use the archive to understand how the surface system works. Concerted effort to understand the links between the Earth’s surface and its deep interior also provide an opportunity to meet this challenge (see also Section 2.3). Another key element is to develop observations and methods that link processes across the range of time scales from the present day to deep time. Laboratory experiments that, in effect, speed up time are one way of approaching this. Another bridge to deep time is the set of natural climate experiments constituting the current “ice house” (glacially influenced) period. These experiments are being investigated intensively for their value in understanding climate dynamics and testing numerical climate models. 2.2 HOW DO GEOPATTERNS ON EARTH’S SURFACE ARISE AND WHAT DO THEY TELL US ABOUT PROCESSES? A glance out an airplane window on a clear day is enough to remind us of the remarkable capacity of landscape processes to create spatial patterns. These geopatterns comprise a diversity of scales and forms, and most show a fascinating mix of order and disorder. Familiar examples include the treelike, branching patterns of stream networks that create erosional and depositional landscapes; river channels, with their ornate meanders and braids; sand dunes; glacial valleys and landforms; deltas; barrier islands; and the zones and fabrics of soils (Figures 2.4-2.6). Physical landscape patterns are often closely associated with biotic ones, ranging from the variation in forest type with upland elevation to riparian ecosystems tied to stream channels to the exquisite control of marsh vegetation by small changes in land elevation and wetting frequency. In inhabited landscapes, we notice the human footprint, visible as clearly “unnatural” patterns (Figure 2.7; see also Section 2.8). These include cultivated areas with simple geometric boundaries quite unlike the intricate patterns of natural landscapes, as well as cities and towns that may exhibit locally regular spatial structures. New kinds of surface geopatterns appear as we explore landscapes that are new or are on unfamiliar scales. For example, advances in sonar and other underwater imaging techniques have enabled us to visualize underwater landscapes as if from an airplane, revealing spatial patterns on the seafloor that often appear to be scaled-up cousins to their terrestrial counterparts (Figure 2.8).
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface FIGURE 2.4 These two images show that repeating patterns apply in different landscape types. These and many other geopatterns arise through local interactions and structure the landscape. What do they tell us? The image on the left shows patterns in a mountain and valley landscape on the border of China and Myanmar, while the image on the right shows the sinuous and branching patterns present at the mouth of the Kayan River, Indonesia. SOURCE: SPOT satellite image © CNES (2009), acquired by CRISP, NUS. Microscopic imaging has also revealed patterns of mineral dissolution at micron scales that are similar to those developed on the scale of landscapes. Measurement of molecular biological signatures has revealed the details of spatial patterns, recognized for decades, in the distribution of biota as a function of depth and position in soils and sediments (see also Section 2.4). Satellite images of other bodies in the solar system also reveal geopatterns that show familiar forms developed in unusual materials and under conditions different from those on Earth. Most of the natural geopatterns we see are self-organized—they emerge spontaneously from local interactions as opposed to being imposed by some outside influence. Patterns that arise from the internal dynamics of landscapes are autogenic, a term that encompasses both spatial and temporal variation. Most geopatterns, regardless of scale, are also dynamic—they develop over time and in many cases remain dynamic even when statistically in steady state. Natural geopatterns are also resilient. A kind of “geomorphic natural selection” weeds out unstable patterns and favors those that are resilient in the face of natural forces and fluctuations (see also Section 2.7). Where do these patterns come from? What do they tell us? How can we use them? As old as these questions are, new observational and analytical methods are now available to yield deeper answers to them.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface FIGURE 2.5 The cuspate capes of North Carolina, examples of a feature common on sandy coasts, illustrate the emergence of a large-scale structure from the interplay of waves and beach sand. Their characteristic scale is much greater than that of the waves that create them. SOURCE: Jeff Schmaltz, MODIS Rapid Response Team, NASA/GSFC. FIGURE 2.6 Biogeochemical reactions can cause pattern formation such as these soil bands. The striping results when biogeochemical processes cause electrons to be transferred to metal oxides containing iron and manganese. The bars represent downward depth in the soil profile. The top bar is at 1.3-meter depth and bottom bar is at 1.5-meter depth; thus the distance between the two bars is 20 centimeters. SOURCE: Fimmen et al. (2007). With kind permission from Springer Science+Business Media.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface FIGURE 2.7 The unmistakable human footprint dominates many landscapes. Even someone unfamiliar with central-pivot irrigation systems would have no trouble identifying this pattern as unnatural. Central-pivot irrigation systems tap subsurface groundwater. Each circle can be very large—51 hectares, or 126 acres or more in some places. SOURCE: NASA/GSFC/METI/ERSDAC/JAROS and U.S.-Japan ASTER Science Team. FIGURE 2.8 The submarine landscape off California revealed by high-resolution sonar. SOURCE: Image courtesy of Lincoln Pratson, Duke University.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface quantitative data about human impacts on processes and process rates. Our capability to collect large datasets through surveys (for example, with airborne laser swath mapping [ALSM]) or field monitoring (using networks of wireless devices) will improve our ability to address the long-term human impacts on landscapes over the various time scales involved; however, significant challenges remain in data analysis and interpretation. A need exists for increased collaboration between Earth surface scientists and geospatial scientists with expertise in the application of existing and emerging geospatial and remote sensing technologies. Lack of understanding of the human responses to anthropogenic landscape change also limits development of the integrated models needed by society. This limitation points to the need to strengthen interactions between the natural and human sciences. Complex Interactions within Anthropogenic Landscapes Interactions in human-impacted landscapes have been examined recently within a framework for complex environmental systems (Pfirman and the AC-ERE, 2003). This framework recognizes the interconnectedness among Earth’s surface components, the nonlinearity of the relationships between them, the historical conditions and inheritance that relate to local settings, and the range of spatial and temporal scales for examining these systems. For example, although the installation and operation of dams provide many economic benefits, these structures also induce physical, biological, and chemical interactions in rivers that may not have been anticipated (Box 2.11). These interactions often intertwine with social processes, as in the case of the Klamath River of California and Oregon, where dikes and levees have increased water supplies since 1905 but adversely impacted threatened and endangered fish (NRC, 2008). Although management actions undertaken during the dry year of 2001 protected the basin’s declining fish population, they reduced the water available in the system and triggered further physical and biological reactions, as well as social conflicts related to agriculture. Although singular impacts and responses are known in many cases, multiple stressors, their impacts, and consequent interactions make human-influenced landscapes difficult to model or predict. How do social processes influence these interactions? How will the mutual interactions adjust to global climate change? Which environmental processes are most vulnerable to change? How will these changes affect the sustainability of water and biotic resources? To improve predictive capabilities for these complex interactions, focused interdisciplinary field studies that link multiple processes are needed as well as experimental studies and theoretical modeling (see also Section 2.6). Incorporating decision making and social processes in some of these interactions also requires strengthening ties between the natural and social sciences.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface Coupled Human-Landscape Dynamics Traditionally, Earth and environmental scientists have treated humans as external drivers of change in examining human impacts on the environment. For river landscapes, for example, studies have emphasized adjustments in fluvial forms and processes following deforestation, mining, impoundment by dams, and urban development. Knowledge of how landscapes have changed after human disturbances has been important in formulating theories of landscape change and assisting management. Yet the traditional approach is insufficient to capture the interrelationships and feedbacks that characterize landscapes affected by increasing and multiple human-caused stressors. Examining coupled natural and human systems enables integrated understanding of how humans both influence and are affected by natural patterns and processes (Pfirman and the AC-ERE, 2003). The reciprocal relationship between natural and human systems may exhibit positive or negative feedbacks. In Kenya, for example, where conversion of forests into cropland has degraded soils and decreased crop yield, greater food insecurity has hastened the conversion of remaining forests to agriculture, illustrating a positive feedback that accelerates environmental degradation. Negative feedbacks often involve modification of human behavior and policy that potentially slows or halts the impact. For example, flow regulation in Cypress Creek, Texas, eliminated the variability in flows and eventually threatened the culturally and aesthetically valuable Bald Cypress tree, prompting local stakeholders to reevaluate and adjust policies for dam releases. Historically, degradation of Earth’s surface by humans has been accompanied by only weak negative feedbacks on human behavior because humans have proven innovative in their ability to endure their own continued alterations to an environment with little regard to its original or “sustainable” function. Weak feedbacks also result where the causes of the impact originate at different scales and faraway places, as in the case of the impacts of global climate change on Arctic residents. Earth surface science plays an important role in public policy by recognizing these diffuse, and often subtle, influences while there is still time to mitigate them. Understanding coupled human-landscape dynamics is among the epochal challenges for predicting the evolution of Earth’s surface in the presence of the growing human population and for developing innovative solutions for environmental management. The study of coupled natural and human systems has thus far emphasized ecological phenomena, producing an emerging science for social-ecological systems that explicitly incorporates human decisions, cultural institutions, and economic and political systems. Enormous gaps in knowledge remain, however, especially with respect to understanding the impact-feedback loops within geomorphic systems. Social processes influence both the original human impact and the potential responses and feedbacks within landscape systems—in other words, human systems influence whether crops are grown on particular soils or how a city is developed, and human systems need to manage and adapt to the
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface BOX 2.11 Constructing and Removing Dams Of all the human alterations of the Earth’s surface, the construction of dams and the acceleration of erosion through agricultural and other land clearance activity are among those having the most profound effects on river sedimentation and morphology. Nearly two-thirds of the world’s rivers are regulated by dams, levees, and other structures. In the United States alone, approximately 80,000 dams are registered in the National Inventory of Dams, many built in the 1960s. Millions more small impoundments dot the landscape. These structures can collectively store a volume of water equaling almost one year’s runoff. The large ones alter the timing and magnitude of river flows and dissolved and particulate material, as well as the locations and pathways of chemical reactions. The dams also compartmentalize our nation’s rivers, fragment landscapes and aquatic habitats, and create new reservoirs for sediment and carbon. Globally, dams have reduced the flux of sediment reaching the world’s coasts by 1.4 ± 0.3 billion metric tons annually, despite increased quantities of sediment introduced through soil erosion and transported by rivers (2.3 ± 0.6 billion metric tons per year) (see figure below). The diminished transfer of sediment from land by rivers is increasingly important with respect to global denudation and geochemical cycles, the functioning and loss of coastal ecosystems, and the stability and evolution of deltas and depositional environments (see also Box 2.2). Climate change is expected to accelerate and confound these impacts, as melting glaciers (see also Section 2.7) increase the potential need for water storage by dams, and sea-level rise further threatens the viability of coastal living space. Quantifying these potential impacts is hampered by the limited availability of data for sediment loads. Since many reservoirs are now approaching their limits of usefulness because of sedimentation, and as environmental impacts and the associated loss of ecosystem services are increasingly recognized, decommissioning and removing dams and other structures also bring new opportunities for ecosystem restoration (see Section 2.9). environmental changes, such as accelerated erosion, that may be caused by these activities. Thus, a primary need in understanding the coupled system is the exchange of analytical perspectives between Earth surface scientists and social scientists, including cognitive-behavioral scientists, economists, political scientists, sociologists, and human geographers. Anthropogenic Earth surface changes also need to be quantified in terms of how they, in turn, affect humans directly or indirectly. For example, accelerated soil erosion (Box 1.1) can be quantified in terms of how it affects the water-holding capacity of the soil profile, and thus plant and crop production, and predicted over time scales that can be expected to be valued by humans. These changes can also be translated into the language of ecosystem services, human perceptions and valuation, and the extent to which predictable thresholds exist that would trigger feedback responses to accelerate or decelerate the original impact. Because policy and institutional processes are the key instruments that society has for effect-
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface Comparison between pre-“Anthropocene” and modern (“Anthropocene”) sediment loads using data from 217 rivers before and after dams were constructed. Data are presented as cumulative curves ranked by decreasing river discharge. Two curves above the 1:1 line (representing no human influence) indicate the increased sediment yield caused by deforestation (soil erosion) and other human activity. Two curves below account for the impact of sediment sequestering in reservoirs. Inserts show the basinwide trapping of sediment by large reservoirs, reported in Vorosmarty et al. (2003). SOURCE: Syvitstki et al. (2005); reprinted with permission from AAAS. ing necessary feedbacks, scientific frameworks for coupled human-landscape systems would also require linkage to policy mechanisms. Successful transmission of scientific information is required to guide policies for mitigation and adaptation, pointing to the need for research findings to be articulated in forms that are useful for policy making, as well as the identification of pathways for transmission of this information to policy makers. Science is far from developing a general theory of coupled human-natural systems, even though such a theory may offer the potential to slow or reverse environmental degradation and improve human well-being. Because such a theory would include knowledge of societal perceptions of environmental impacts and the ability and willingness of societies to react to these changes, much focused inductive and empirical work is required to investigate these interacting processes in a range of Earth surface environments, and in societies at various levels of institutional development and types of organization.
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface 2.9 HOW CAN EARTH SURFACE SCIENCE CONTRIBUTE TOWARD A SUSTAINABLE EARTH SURFACE? As human activity became widespread and intensive some 5,000 years ago, the surface of Earth was—as it continues to be—altered in many ways (Section 2.8, Figure 1.3). With increasing scientific understanding of the causes and cumulative, long-term effects of human-induced changes, a consensus has emerged that at least some of these disrupted or degraded landscapes can and should be “restored” or “redesigned.” The meaning of the term “restored” is not agreed on with precision, but it generally refers to a set of strategies to reestablish processes that functioned before the most recent and intense anthropogenic disruption. Where this is not possible, a more limited set of natural processes may be reestablished, or a fuller set may be reestablished at a smaller scale than the original case. Restoration is often incorporated into engineering strategies for making landscapes safer for human habitation (such as coastal barrier reinforcement or flood risk reduction). Landscape restoration may involve river channels and floodplains (river restoration), entire ensembles of hillslopes and small channels (watershed restoration), or coastal wetlands and waterways. Landscape restoration is typically managed by coalitions of engineers, biologists, and planners, but as restoration activities become more sophisticated they will include a broader range of Earth surface scientists. They will also involve social scientists, planners, and practitioners, as in the development of integrated assessment tools to guide decision making. The design of landforms (in the case of rivers, floodplains, and tidal marshes) or land conditions (in the case of watershed restoration) that will function within a predictable, preferred range has had some success. The real challenges emerge in designing landforms that interact with the desired biological processes and communities required to establish sustainable ecosystem complexity (Box 2.12). In fact, biotic processes are central to the evolution of many landforms (see also Section 2.6). Hence, the design and monitoring of restored landscapes and their ecosystems should both rely on and add to emerging models of interacting biological, physical, and chemical processes in landscape evolution. Furthermore, the long-term cost of a project will be minimized and the likelihood of its sustainability will be maximized if the original design lies within the stable range of landscape fluctuation and if it respects long-term trends in landscape evolution. To this activity, research in Earth surface processes brings knowledge of how major environmental drivers (such as water regime or sediment supply) scale the size and shape of landforms that are treated as the “reference conditions” for restored features. Thus, specific research challenges arise about controls on the size and geometry of redesigned rivers, tidal creeks and inlets, deltas, and beaches and how they are controlled by the expenditure of flow energy and by sediment supplies. In other cases, where landforms originally existed in sediment-starved environments or were dominated by intense biogenic drivers such as large wood or riparian vegetation, little theory exists at the present time for how such landscapes
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface might be recreated or what the alternative sustainable states are. In these cases, the need is for extensive field investigations to support new theories of the nature and functioning of biogenically reinforced landscapes that would retain sediment and chemicals and release them slowly into water bodies. Research in Earth surface processes also focuses on the transport processes that redistribute sediment and chemicals, and interact with biotic processes such as seed dispersal, plant survival, and colonization of aquatic substrates by woody plants, algae, and macrophytes. Attempts to modify the sediment and carbon storage in marshlands and soils require new field studies of the accumulation of organic sediments. Where human effects involve the disruption of sediment supplies, restoration entails reducing excessive sediment supplies from disturbed areas using principles of hydrology, geomorphology, plant community regeneration, soil profile enhancement, and engineering. In other cases, perturbations such as the removal of dams or the augmentation of beaches involve increases in sediment supplies to landforms, and the effects of these perturbations need to be predicted through advances in the study of mixed-grain-size sediment transport that may transform or even engulf the landform itself. The nation’s gradually expanding program of dam removal from rivers (now ~50 projects per year; see also Box 2.11) adds urgency to the need for research into sediment management along rivers. In most dam removals, the major problem to be solved is not the removal of the dam or the restoration of flows, but the management of impounded sediment. Strategies for stabilization, removal and disposal, or release of this sediment involve large costs and ecological and societal consequences, and pose research challenges for Earth surface scientists and engineers involved with sediment transport and channel mechanics (Box 1.1). Earth surface processes research also illuminates the long-term context of restoration plans. Restoration of landscapes requires designs that will survive long-term conditions such as highly stochastic sediment supplies, regional land-use changes, the relaxation of some landscapes from deglaciation, sediment starvation, crustal deformation, and sea-level change. Studies of these phenomena clarify the limitations on restoration and, especially, the role of time in allowing ecosystem complexity to develop. In other situations, the disruption of flows and sediment supplies or the deepening of coastal waters through land surface subsidence renders restoration to a preferred state with desired biological characteristics impossible on time scales of human interest. Research into the Late Cenozoic evolution (the last several million years) of sedimentary landforms and their ecology and biogeochemistry provides a basis for understanding this long-term context and sustainable environmental range of landscape restoration (see also Section 2.1). At the same time, restoration projects provide field-scale experiments within which the evolution of landforms or soil profiles can be studied with a resolution and degree of simplicity that are rarely possible in nature. Initial conditions are well known and usually simpler than natural landscapes, and processes occur at a natural scale as opposed to miniaturized laboratory conditions. Often, variables such as sediment supplies to beaches,
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface BOX 2.12 Coastal Wetland Restoration Many tidal marshes have been converted to non-wetland habitat by dredge-and-fill activities, nutrient loading, groundwater extraction, contaminated runoff, fire management, and invasive species. Coastal wetlands have declined worldwide by 0.5-1.5 percent per year over the past few decades. In Louisiana, losses in coastal wetlands have risen to ~60 km2 per year or nearly 18 hectares (~45 acres) per day (Barras et al., 2003). Sea-level rise and crustal subsidence also lead to loss of wetlands, but the destruction of wetlands by direct and indirect human activities is of much greater consequence (see also Box 2.2). The rate at which coastal wetlands continue to be filled and drained belies their ecological and economic importance as wetlands. Wetlands are among the world’s most productive ecosystems. They provide habitat, food, nurseries, and refuge for fish and shellfish. Wetlands store and cleanse water, prevent coastal erosion, and reduce storm surges. In the 1960s, public awareness of the importance of wetlands led to policy changes intended to slow wetland loss by encouraging wetland mitigation and restoration (e.g., Section 404 of the Clean Water Act in 1972). Coastal wetlands develop at the interface between land and water. Their very existence results from a tenuous, but enduring mass balance of mineral and organic sediment that is imported, exported, and generated in situ in locations that lie approximately at mean sea level and are simultaneously subject to the eustatic rising of sea level, the warping of Earth’s crust as a result of the last deglaciation, tectonics, and sediment loading (often altered by human activity), and anthropogenic effects such as groundwater extraction. Many coastal wetlands have very low topographic gradients (see figure below), so that differences of only several centimeters in salt marsh elevation strongly influence vegetation patterns and ecological and biogeochemical processes. Restoration of coastal wetland ecosystems requires the land surface to be sufficiently high in the tidal range that vegetation is not flooded constantly, but also low enough that the energy and sediment supply provided by regular flooding is achieved. Vegetation exerts strong controls on water movement and sediment deposition and erosion that shape the marshes and tidal creek networks. Until recently, only the effects of physical processes on biotic processes were considered in restoration of coastal marshes. This approach focused on contouring sites, planting native species, redirecting sediment dispersal at river mouths, and reintroducing tidal flushing. The outcomes of such efforts, however, were natural vegetation patterns, but not functionally diverse marshes (e.g., Tijuana River Estuary, California; Barn Island Wildlife Management Area, Connecticut). Reestablishing functional, as well as structural, characteristics of coastal wetlands will require fundamental advances in understanding the coupling of ecological and morphodynamic processes at a range of scales (Fagherazzi et al., 2004). Within individual marshes, emphasis needs to be placed on incorporating base levels of wetlands, and flows to rivers are manipulated in ways that allow quantitative expectations to be developed (Box 2.13). Observations in the field of the effects of real trees, root zones, material properties, and sediment supplies, for example, cannot easily be reproduced indoors. In other cases, animals are excluded from large areas or small plots, or climate is controlled through watering or soil profile warming and fertilization. Field experiments
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface topographic and hydrologic spatial heterogeneity and temporal variability for distributing water and sediments in ways that support compositionally and functionally diverse wetlands. At the watershed and landscape scales, there is a need for greater knowledge about wetland diversity within watersheds, the proportion of uplands to wetlands, and the physical and ecological processes connecting wetland and upland ecosystems (NRC, 2001b). Increasing rates of sea-level rise and projected increases in the frequency and intensity of tropical and extratropical storms will lead to greater threats to coastal shorelines, wetlands, and developed areas. Management and restoration of coastal landscapes will be dramatically complicated by these effects because more than half of the U.S. population lives on coastal landscapes. Accurate predictions of how coastal landscapes will respond to sea-level rise and disturbance due to storms will provide coastal managers with information that allows them to reduce the risk of potential hazards rather than responding to destruction that has already occurred (e.g., Hurricane Katrina). Composite wetlands transect for Charleston illustrating the approximate percentage occurrence and modal elevation for key indicator species or habitats based on results of 12 surveyed transects. Minor species have been omitted. Elevations are with respect to 1929 NGVD (National Geodetic Vertical Datum), which is about 15 centimeters lower than current sea level. Current tidal ranges are shown at right. SOURCE: Titus (1988). with replication are often possible within the confines of restoration projects. Research opportunities include the high-resolution testing of predictions of the stable scale or range of some landforms, as well the trajectory of other features that may not equilibrate. Recently developed geochronological and geophysical tools and geochemical tracers enhance the nature and level of detail of studies that are now possible of both the mechanisms and the
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface BOX 2.13 River Restoration What shape and gradient should a river have in order to behave in a more or less predictable manner consistent with human designs? How much water should flow through it? What plants and animals should live in its waters and on its banks? In the United States, at least $1 billion is spent on river restoration annually, and the number of restoration projects is growing exponentially (Bernhardt et al., 2005). The environmental and economic motives for restoration vary with geographic region and history of environmental impact. Restoration practice has outpaced scientific investigation, in that many of the basic scientific questions relevant to stream restoration have yet to be answered or even investigated. Better understanding of the processes that determine river evolution and response to environmental change, for example, could improve our ability to design sustainable river systems that provide ecosystem services that people desire. The term “river restoration” begs the question: Restore to what state? “Restoring” a river usually means returning it to a close approximation of its condition prior to a disturbance. Yet understanding a river’s past and present states is often complicated, requiring the use of a broad set of tools and disciplinary approaches that include many Earth surface processes. Rivers are dynamic features that adjust to changes in climate, hydrology, vegetation, and human activities. Increasing sediment input to a river—by logging of neighboring forests, for example—may increase erosion of stream banks. Left unchecked, stream bank erosion can cause the river to become wider and shallower and—in severe cases—to migrate erratically and flood more frequently. Wastewater discharge or runoff from agricultural fields can change the chemical load of rivers as well as their habitability for different species. River restoration projects not only take stock of past and current processes that control the long-term stability of streams, but also attempt to predict what modifications will best achieve the project’s goals. In addition to the scientific framework, rivers are restored within particular constraints of regulatory goals, economic needs, and public preferences that vary geographically. Projects are guided by a variety of federal, state, and local regulations that oversee land use, water quality, and flood control, for example. Regulations also often interact with economic incentives that drive decision making. In one case, after determining that fish passage upgrades and maintenance costs would exceed the value of Oregon’s power-generating Marmot Dam (see figure below), the dam was decommissioned and removed in 2007. The dam breaching released 100,000 m3 of accumulated gravel and sand into the river downstream within the first 48 hours (Major et al., 2008). Although the full consequence of this added sediment load downstream over the coming decades cannot yet be predicted quantitatively, the decade of research that preceded dam removal made accurate predictions of some of the impacts of sediment release and aided the decision-making process for decommissioning and removing the dam. rates of change to be expected from managed or reconstructed landscapes (Boxes 1.2, 2.1, 2.4, 2.6; Appendix C). Thus, landscape restoration provides an opportunity both for learning and for service by the Earth surface processes research community. In addition to restoring and redesigning landscapes, society needs Earth surface scientists to guide decisions regarding the functioning and evolution of the surface, while
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface Aging infrastructure, combined with shifting technological practices and environmental values, has contributed to environmental restoration efforts that include dam removal and stream restoration. Here, the 15-meter-high Marmot Dam on the Sandy River is destroyed in July 2007. This hydroelectric dam was built in the early twentieth century, but was no longer in operation. Sediment stored in the reservoir to the level of the dam breast was rapidly mobilized once the dam was breached, leading to a sudden pulse of sediment downstream. Nevertheless, salmon migrated upstream past the dam site within several days of its breaching. SOURCE: Portland General Electric. recognizing uncertainty and gaps in scientific knowledge. In human-dominated regions, for example, landscape-modifying processes are often simplified by engineering and other forms of resource management, but re-naturalizing the processes is not simple. Complex historical cascades of decisions have often been made about rights to resources of land,
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Landscapes on the Edge: New Horizons for Research on Earth’s Surface water, and minerals and multiple layers of interests have solidified in the status quo. If this history has resulted in the degradation of resources or ecosystem services, or even the viability of human living space, the complexity of human interests creates resistance to unraveling the damage. This difficulty is compounded by the complexity of the physical, chemical, and biological processes themselves, which makes it difficult to predict the results of changing resource utilization or other societal action. Yet, society needs Earth surface scientists to construct models for assessing the probable effects of various possible restoration actions. The models may initially be crude and may require sequential elaboration and refinement. Learning how to develop such assessment tools in a politically sensitive, useful way requires that natural scientists studying Earth’s surface processes collaborate with social scientists who study all of the other processes that affect the surface, as well as with practitioners in industry, engineers, and planners. Landscape restoration is a complex, high-priority goal for many researchers, practitioners, policy makers, and the public, but only recently have these different communities begun to examine together the legacy of past restoration efforts in the context of new data from specific regions of the Earth’s surface. Earth surface scientists have vital contributions to offer as restoration activities are carried toward quantitative models and predictions. Just as importantly, Earth surface scientists are in position to guide future decisions through inclusion of understanding of human-process interactions, even while recognizing uncertainty and limits of predictability. Such guidance may enhance the goods and services that Earth’s surface provides to society and thereby produce enormous economic benefits, as well as aid in developing a sustainable living surface for the next generation.