DEFINING THE CONCEPT OF “MULTIPLE STRESSES”
Natural, managed, and socioeconomic systems often experience multiple environmental stresses that can come together to cause impacts that are greater than simply additive. Multiple environmental changes can also interact with each other to cause additional unexpected stresses. The term “multiple stresses” is simply a shorthand way of referring to scenarios where multiple environmental influences are at work with various multidimensional interactions among them. Understanding the net impact of a suite of simultaneously occurring environmental modifications (e.g., elevated carbon dioxide; increased oxidants, reactive nitrogen, and acid deposition; decreased stratospheric ozone; increased ultraviolet radiation; higher mean temperature; changes in timing and availability of water; loss of biodiversity, increases in invasive species, rising sea level; coastal development and habitat fragmentation) is essential for developing predictive capabilities and response strategies.
There are no generally agreed upon methodologies for studying complex systems of interconnected environmental influences that can have different impacts in varied and sometimes subtle directions. Understanding multiple stresses almost always requires consideration of multiple variables, nonlinear processes, and a variety of spatial and temporal scales. However, we typically have only a
rudimentary understanding of the dynamics of interactions between different environmental variables in complex systems, making it extremely difficult to predict the combined effects of multiple interacting stresses. Other scientific challenges include understanding the multiple time and space scales on which these interactions take place, developing indicators for “threshold” responses that may lead to sudden and dramatic changes in societal or environmental structure and function, characterizing and quantifying risks and vulnerabilities, and understanding the economic impact of multiple interacting changes. Another critical challenge is proceeding with decision making given the many uncertainties and limited prediction capabilities.
Given the highly interdisciplinary nature of multiple environmental stresses, conducting research on the topic involves a variety of infrastructural and institutional challenges. First, a broad range of observational and experimental/process studies is needed for understanding integrated climate and ecosystem processes. Second, coupled biophysical and biogeochemical models are needed to address the dynamics of exchange of water, energy, carbon, and nitrogen on multiple timescales, with biogeographic models that simulate the effects of climate and other factors on vegetation distribution. New data and information systems may be necessary to enable scientists to integrate knowledge across these disciplines. Further, it appears likely that focusing on natural, managed, and socioeconomic systems on a regional scale may provide a tractable approach to bringing together the diverse researchers and knowledge needed to improve understanding of multiple environmental stresses.
In addition to gaps in the scientific understanding of multiple environmental stresses, there is a lack of understanding of how to devise wise management and policy approaches that address suites of problems and, in particular, options that make sense in the face of uncertainty. These options can be technological, managerial, or institutional and will require much more integrative research in many disciplines. It will also require serious consideration of how we translate information as well as a sense of what we do not know or cannot know, so that it is useful to those who must make decisions. Adaptive management using the best information available, while retaining flexibility to make changes, requires managers to think differently, but it does increase resiliency to risk.
Effective communication between researchers and stakeholders is critical. As research develops more sophisticated understanding of environmental systems and how multiple stresses interact and compound, it becomes even more difficult to translate this understanding to those who must use the information. Although there is no easy answer to this dilemma, the solution appears to require extended interactions, careful attention to including all perspectives, and frankness about the level of certainty and uncertainty of the information.
THE NATURE OF THE PROBLEM
The nature of the environmental issues facing any nation demands a capability that allows us to enhance economic vitality, maintain environmental quality, limit threats to life and property, and strengthen fundamental understanding of Earth. In each case it is the ability to anticipate the future (e.g., forecasting an impending storm, predicting water quality change in response to a new source of pollutant) that makes information about the earth system truly useful. Reliable information about the future (i.e., forecasts or predictions) is essential when addressing environmental issues. Thus, society requires greater access to and greater confidence in both information and forecasts or projections in order to weigh the advantages and risks of alternative courses of action. Such information is a key commodity in enhancing economic vitality and societal well-being. What stands in our way of providing this information?
Many of the driving forces that alter environmental quality are widely recognized and involve primarily weather and climate, patterns of land use and land cover, and resource use with its associated waste products. A key feature of most regions is that more than one driving force is changing simultaneously. Consequently, most locations are characterized by multiple stresses. The effect of a combination of environmental stresses is seldom simply additive. Rather, they often produce amplified or damped responses, unexpected responses, or threshold responses in environmental systems. Multiple, cumulative, and interactive stresses are clearly the most difficult to understand and hence the most difficult to manage.
In contrast to how the real world works, most research and policy focuses on discrete parts of these complex problems. Basically, earth and environmental sciences tend to focus on cause and effect, where we seek to understand how a specific element of the system may respond to a specific change or perturbation (e.g., the impact of acid rain on lake fisheries). The lack of methods to assess the response of the system to multiple stresses limits our ability to assess the impacts of specific human perturbations, to assess advantages and risks, and to enhance economic and societal well-being in the context of global, national, and regional stewardship.
The problem is not limited simply to moving from analysis of discrete parts of complex problems to a more comprehensive analysis. First, economic vitality and societal well-being are increasingly dependent on combining global, regional, and local perspectives. A “place-based” imperative (i.e., site specific) for environmental research stems from the importance of human activities on local and regional scales, the importance of multiple stresses on specific environments, and the nature of the spatial and temporal linkages between physical, biological, chemical, and human systems. We find the strongest intersection between human activity, environmental stresses, earth system interactions, and human decision making in regional analysis coupled to larger spatial scales.
Second, although a decade of research on greenhouse gas emissions, ozone depletion, and deforestation has answered some critical questions, the last decade of effort has also revealed a number of new challenges. The most notable is the challenge of creating integrated global observation capabilities and the computational and scientific limitations inherent in creating a truly integrated, global, coupled system modeling capability suitable for assessing impacts and adaptations. These problems are noteworthy in global change science, but they become intractable at the scales of human decision making. A major part of the problem is simply a matter of scale and the sheer volume of information required to combine physical, biological, chemical, and human systems if the framework is global. For example, whereas a global integrated observing system is challenging but tractable and plays a fundamental role on the scale of a global circulation model, it collapses under its own weight at higher spatial resolutions if we demand a truly comprehensive data system involving the host of observations spanning biology, hydrology, soils, weather, etc., required to address problems at the scales of human decision making.
Recognition of the importance of developing a more integrated approach to environmental research was made abundantly clear in the National Assessment Synthesis Team’s report, Climate Change Impacts on the United States (NAST, 2000). The first recommendation for future research focused on developing a more integrated approach to examining impacts and vulnerabilities to multiple stresses. The report contains many examples where the key limitation to the assessment of potential impacts on climate was a lack of knowledge of other stressors. For example, changes in insect-, tick-, and rodent-borne diseases could be clearly tied to weather and climate, but the number of other environmental factors that could influence the disease vectors (e.g., the importance of land use on disease hosts), transmission dynamics, and population vulnerabilities severely limited our ability to make robust conclusions on how climate change might influence the distribution and occurrence of many infectious diseases in the future.
Why, with so many pressing problems demanding research attention, should the United States give more focus to multiple stresses in environmental research? The driving forces that alter environmental quality and integrity are often well known, but most regions experience multiple simultaneous environmental changes, and the combined effects of these changes are much more difficult to understand and manage than the discrete issues that most research, analysis, and policy focus on. This lack of appreciation for, and understanding of, multiple stresses limits our ability to assess the impacts of specific human perturbations, to assess advantages and risks, and to enhance economic and societal well-being in the context of global, national, and regional stewardship. However, the solution to this problem is not simply increasing the scope of our analysis, but rather developing a more focused and integrated approach to environmental research.
WHAT ASSESSMENTS CONCLUDE ABOUT RESEARCH NEEDS
Many national and international assessments of environmental issues have been conducted in the last two decades, such as the ozone assessments (WMO/ UNEP, 1985, 1989, 1991, 1994, 1998, 2002), the Intergovernmental Panel on Climate Change assessments (IPCC, 1990, 1995, 2001), the U.S. National Assessment (NAST, 2000), and the Millennium Ecosystem Assessment (MEA, 2005). Each of these multiyear efforts involving thousands of scientists, government officials, and stakeholders has made significant contributions by summarizing the state of science and characterizing remaining uncertainties. What is lacking, however, is a strategic research plan that could emanate from these assessments.
After assembling the data that are available, assessment researchers are keenly aware of what was not there and which gaps are most critical to fill to enhance understanding. Taking time at the end of assessments to characterize what is known, what is not known, what is knowable in what time frame, and what is most important to know to assist timely decision making would generate much-needed short-term and long-term research strategies. These strategies would be valuable to governments and researchers in thinking about appropriate budgets across agencies, across disciplines, across space, and over time to answer societal questions.
Especially in times of tight budgets, all desired research cannot be conducted simultaneously. Missing information about scientific processes, technological promise, institutional mechanisms, ecological or socioeconomic thresholds, and behavioral change all need to be developed, but some missing pieces of the puzzle may be more crucial to pursue in order to find robust solutions. And some questions are more tractable than others.
INTRODUCTION TO THE CASE STUDIES
The concept of multiple stresses, taken alone, can seem vague, and thus the steering committee decided to focus on two concrete examples that were selected to provide different perspectives on multiple-stresses scenarios. The first case study selected for examination was drought, a complex environmental condition that both is driven by multiple environmental stresses and leads to multiple stresses across a wide range of time and spatial scales. Drought is a normal climate variation that can vary in magnitude and intensity, and while it is not the only climate-induced generator of multiple stresses, it is a significant one and one that provides clear illustration of the feedbacks involved. The second case study selected for attention focused on a wide range of atmosphere-ecosystem-human interactions that, taken together, reflect characteristics of multiple simultaneous environmental stresses. For each case, workshop participants discussed the current state of understanding, what can be learned from prior unexpected findings, vulnerability and future socioeconomic impacts, and potential response strategies.
Participants then stepped back from the examples in search of common lessons that might be learned.
Drought, in general, originates from a deficiency of precipitation over an extended period, resulting in a water shortage for some activity, group, or environmental sector. Workshop participants discussed the current state of the knowledge base concerning the causes, frequency, intensity, and predictability of drought at multiple spatial scales within the continental United States and how societal changes (e.g., increasing population) affect vulnerability to drought at local and regional scales. The question of how the United States could facilitate development of a risk-based drought management approach directed at increasing resilience and decreasing vulnerability was highlighted.
The second case study looked at atmosphere-ecosystem interactions. In the earth system, both the dynamics and composition of the atmosphere affect the biosphere. In turn, uptake, storage, and emissions by the biosphere affect the composition and dynamics of the atmosphere. Changing atmospheric conditions (e.g., changes in chemical composition and physical characteristics) together constitute multiple stresses to ecosystems, and the resultant ecosystem impacts and atmospheric feedbacks are poorly understood. Workshop participants addressed how global/climate change drivers affect atmospheric composition and dynamics and subsequent atmosphere-ecosystem interactions, as well as the socioeconomic impacts of climate change on agriculture and carbon cycling, capture, and sequestration as regards agriculture and forestry.
For both case studies the steering committee asked participants to explore the historical record and identify unexpected findings that raise concern about future responses to multiple stresses. In addition, the steering committee asked participants to focus on areas where large uncertainties lie, processes that are nonlinear and where predicting the integrated effect of multiple stresses is especially difficult, and on observed and potential thresholds (changes of state) that may be beyond our current ability to predict. The steering committee also recognized important commonalities between the two cases and sought to draw broader lessons about multiple stresses or multiple drivers. The workshop also explored lessons learned from the Millennium Ecosystem Assessment (MEA, 2005) because this document provides a comprehensive summary of the state of the world’s ecosystems and services and how they may change in the future. Finally, workshop participants were asked to engage in a synthesis discussion addressing tools, nonlinearities and thresholds, resilience, and the use of regional studies.
NONLINEARITIES, THRESHOLDS, AND THE VULNERABILITY-RESILIENCE CONTINUUM
In considering multiple stresses with respect to atmosphere-ecosystem interactions and drought, we need to make explicit our conceptions of four criti-
cal concepts: nonlinear responses, thresholds, vulnerability, and resilience. The workshop organizers defined nonlinearity according to the approach adopted by Rial et al. (2004), in which systems so characterized display significant imbalances between inputs and outputs, primarily (although not exclusively) episodic and abrupt rather than slow and gradual change, and multiple equilibria. In such systems small changes in parameters often cause large changes in the behavior of the system. Nonlinearity also often confronts, if not causes, thresholds (i.e., phase changes) in a system that are not easily reversible. The global climate system is inherently nonlinear and quite complex and consequently generates a wide variety of thresholds for natural ecosystems and human social systems. But humans themselves now generate planetary-scale environmental effects, including modification of the global climate system. The interaction of these two drivers multiplies the stresses that ecosystems and human social systems must face. Because the climate system is highly nonlinear and coordination and control of human forcing is weak, thresholds are encountered quite often. These thresholds present severe policy challenges. Thresholds also raise questions about vulnerability and resilience of both ecosystems and human social systems. Vulnerability refers to magnitudes and rates of environmental change that exceed the capacity of ecosystems and human social systems to cope and recover. These systems then either break down or exhibit a variety of pathologies under those conditions. Resilience, on the other hand, refers to a capacity to adapt to and cope with the level of stress imposed, even if damage to the system results.
The application of these concepts can be illustrated with respect to the issue of drought. In Figure 1-1, Wilhite and Buchanan-Smith (2005) illustrate the important differences between societies that are either vulnerable to or resilient in the face of drought. (Box 1-1 defines vulnerability, resilience, and other relevant terms.) The figure shows that societal vulnerability begins with exposure to drought in the absence of either risk-based drought management policies or an early warning system. Vulnerability is then enhanced by multiple-stresses problems in the form of marginalized groups who lack resources, options, and ways of mitigating drought impacts. Vulnerability is also enhanced by previously existing problems of dependence on the overexploitation of natural resources, poverty, and violent conflict. As the drought event intensifies, society resorts to crisis management in the face of potential disaster. How then is resilience to be developed? The two most critical steps represent the development of a culture of prevention by enacting risk-based drought policies and plans combined with an early warning system. Building political capital means solving some of the multiple-stresses problems that impinge on the society and retard adaptation to drought. In addition, once drought mitigation actions have been implemented and specific impacts avoided or reduced, serious attention needs to be paid to lessons learned. This learning can then be fed back into the planning and early warning subsystems. The vulnerability-resilience continuum is considered in more detail in the next chapter.
Adaptation: Adjustment in natural or human systems in response to actual or expected climatic stimuli or their effects, which moderates harm or exploits beneficial opportunities. Various types of adaptation can be distinguished, including anticipatory and reactive adaptation, private and public adaptation, and autonomous and planned adaptation.
Adaptive capacity: The ability of a system to adjust to climate change (including climate variability and extremes) to moderate potential damages, to take advantage of opportunities, or to cope with the consequences.
Resilience: Amount of change a system can undergo without changing state.
Sensitivity: Sensitivity is the degree to which a system is affected, either adversely or beneficially, by climate-related stimuli. The effect may be direct (e.g., a change in crop yield in response to a change in the mean, range, or variability of temperature) or indirect (e.g., damages caused by an increase in the frequency of coastal flooding due to sea level rise).
Vulnerability: The degree to which a system is susceptible to, or unable to cope with, adverse effects of climate change, including climate variability and extremes. Vulnerability is a function of the character, magnitude, and rate of climate variation to which a system is exposed, its sensitivity, and its adaptive capacity.
SOURCE: IPCC (2001).