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CHAPTER ONE
Introduction: Science for Understanding and Responding to Climate Change

Humans have always been influenced by climate. Despite the wealth and technology of modern industrial societies, climate still affects human well-being in fundamental ways. Climate influences, for example, where people live, what they eat, how they earn their livings, how they move around, and what they do for recreation. Climate regulates food production and water resources and influences energy use, disease transmission, and other aspects of human health and well-being. It also influences the health of ecosystems that provide goods and services for humans and for the other species with which we share the planet.


In turn, human activities are influencing climate. As discussed in the following chapters, scientific evidence that the Earth is warming is now overwhelming. There is also a multitude of evidence that this warming results primarily from human activities, especially burning fossil fuels and other activities that release heat-trapping greenhouse gases (GHGs) into the atmosphere. Projections of future climate change indicate that Earth will continue to warm unless significant and sustained actions are taken to limit emissions of GHGs. Increasing temperatures and GHG concentrations are driving a multitude of related and interacting changes in the Earth system, including decreases in the amounts of ice stored in mountain glaciers and polar regions, increases in sea level, changes in ocean chemistry, and changes in the frequency and intensity of heat waves, precipitation events, and droughts. These changes in turn pose significant risks to both human and ecological systems. Although the details of how the future impacts of climate change will unfold are not as well understood as the basic causes and mechanisms of climate change, we can reasonably expect that the consequences of climate change will be more severe if actions are not taken to limit its magnitude and adapt to its impacts.


Scientific research will never completely eliminate uncertainties about climate change and its risks to human health and well-being, but it can provide information that can be helpful to decision makers who must make choices in the face of risks. In 2008, the U.S. Congress asked the National Academy of Sciences to “investigate and study the serious and sweeping issues relating to global climate change and make recommen-



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CHAPTER ONE Introduction: Science for Understanding and Responding to Climate Change H umans have always been influenced by climate. Despite the wealth and tech- nology of modern industrial societies, climate still affects human well-being in fundamental ways. Climate influences, for example, where people live, what they eat, how they earn their livings, how they move around, and what they do for rec- reation. Climate regulates food production and water resources and influences energy use, disease transmission, and other aspects of human health and well-being. It also influences the health of ecosystems that provide goods and services for humans and for the other species with which we share the planet. In turn, human activities are influencing climate. As discussed in the following chap- ters, scientific evidence that the Earth is warming is now overwhelming. There is also a multitude of evidence that this warming results primarily from human activities, es- pecially burning fossil fuels and other activities that release heat-trapping greenhouse gases (GHGs) into the atmosphere. Projections of future climate change indicate that Earth will continue to warm unless significant and sustained actions are taken to limit emissions of GHGs. Increasing temperatures and GHG concentrations are driving a multitude of related and interacting changes in the Earth system, including decreases in the amounts of ice stored in mountain glaciers and polar regions, increases in sea level, changes in ocean chemistry, and changes in the frequency and intensity of heat waves, precipitation events, and droughts. These changes in turn pose significant risks to both human and ecological systems. Although the details of how the future im- pacts of climate change will unfold are not as well understood as the basic causes and mechanisms of climate change, we can reasonably expect that the consequences of climate change will be more severe if actions are not taken to limit its magnitude and adapt to its impacts. Scientific research will never completely eliminate uncertainties about climate change and its risks to human health and well-being, but it can provide information that can be helpful to decision makers who must make choices in the face of risks. In 2008, the U.S. Congress asked the National Academy of Sciences to “investigate and study the serious and sweeping issues relating to global climate change and make recommen- 

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A D VA N C I N G T H E S C I E N C E O F C L I M AT E C H A N G E dations regarding what steps must be taken and what strategies must be adopted in response . . . including the science and technology challenges thereof.” This report is part of the resulting study, called America’s Climate Choices (see Foreword). In the chap- ters that follow, this report reviews what science has learned about climate change and its causes and consequences across a variety of sectors. The report also identifies scientific advances that could improve understanding of climate change and the ef- fectiveness of actions taken to limit the magnitude of future climate change or adapt to its impacts. Finally, the report identifies the activities and tools needed to make these scientific advances and the physical and human assets needed to support these activities (see Appendix B for the detailed statement of task). Companion reports provide information and advice on Limiting the Magnitude of Future Climate Change (NRC, 2010c), Adapting to the Impacts of Climate Change (NRC, 2010a), and Informing an Effective Response to Climate Change (NRC, 2010b). SCIENTIFIC LEARNING ABOUT CLIMATE CHANGE Climate science, like all science, is a process of collective learning that proceeds through the accumulation of data; the formulation, testing, and refinement of hypoth- eses; the construction of theories and models to synthesize understanding and gener- ate new predictions; and the testing of hypotheses, theories, and models through experiments or other observations. Scientific knowledge builds over time as theories are refined and expanded and as new observations and data confirm or refute the predictions of current theories and models. Confidence in a theory grows if it survives this rigorous testing process, if multiple lines of evidence lead to the same conclusion, or if competing explanations can be ruled out. In the case of climate science, this process of learning extends back more than 150 years, to mid-19th-century attempts to explain what caused the ice ages, which had only recently been discovered. Several hypotheses were proposed to explain how thick blankets of ice could have once covered much of the Northern Hemisphere, in- cluding changes in solar radiation, atmospheric composition, the placement of moun- tain ranges, and volcanic activity. These and other ideas were tested and debated by the scientific community, eventually leading to an understanding (discussed in detail in Chapter 6) that ice ages are initiated by small recurring variations in Earth’s orbit around the Sun. This early scientific interest in climate eventually led scientists work- ing in the late 19th century to recognize that carbon dioxide (CO2) and other GHGs have a profound effect on the Earth’s temperature. A Swedish scientist named Svante Arrhenius was the first to hypothesize that the burning of fossil fuels, which releases CO2, would eventually lead to global warming. This was the beginning of a more than 0

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Introduction 100-year history of ever more careful measurements and calculations to pin down exactly how GHG emissions and other factors influence Earth’s climate (Weart, 2008). Progress in scientific understanding, of course, does not proceed in a simple straight line. For example, calculations performed during the first decades of the 20th century, before the behavior of GHGs in the atmosphere was understood in detail, suggested that the amount of warming from elevated CO2 levels would be small. More precise experiments and observations in the mid-20th century showed that this was not the case, and that increases in CO2 or other GHGs could indeed cause significant warming. Similarly, a scientific debate in the 1970s briefly considered the possibility that human emissions of aerosols—small particles that reflect sunlight back to space—might lead to a long-term cooling of the Earth’s surface. Although prominently reported in a few news magazines at the time, this speculation did not gain widespread scientific accep- tance and was soon overtaken by new evidence and refined calculations showing that warming from emissions of CO2 and other GHGs represented a larger long-term effect on climate. Thus, scientists have understood for a long time that the basic principles of chemistry and physics predict that burning fossil fuels will lead to increases in the Earth’s average surface temperature. Decades of observations and research have tested, refined, and extended that understanding, for example, by identifying other factors that influence climate, such as changes in land use, and by identifying modes of natural variability that modulate the long-term warming trend. Detailed process studies and models of the climate system have also allowed scientists to project future climate changes. These projections are based on scenarios of future GHG emissions from energy use and other human activities, each of which represents a different set of choices that societies around the world might make. Finally, research across a broad range of scientific disciplines has improved our understanding of how the climate system interacts with other environmental systems and with human systems, including water resources, agricultural systems, ecosystems, and built environments. Uncertainty in Scientific Knowledge From a philosophical perspective, science never proves anything—in the manner that mathematics or other formal logical systems prove things—because science is funda- mentally based on observations. Any scientific theory is thus, in principle, subject to being refined or overturned by new observations. In practical terms, however, scientific uncertainties are not all the same. Some scientific conclusions or theories have been so thoroughly examined and tested, and supported by so many independent observa- 

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A D VA N C I N G T H E S C I E N C E O F C L I M AT E C H A N G E tions and results, that their likelihood of subsequently being found to be wrong is van- ishingly small. Such conclusions and theories are then regarded as settled facts. This is the case for the conclusions that the Earth system is warming and that much of this warming is very likely due to human activities. In other cases, particularly for matters that are at the leading edge of active research, uncertainties may be substantial and important. In these cases, care must be taken not to draw stronger conclusions than warranted by the available evidence. The characterization of uncertainty is thus an important part of the scientific enter- prise. In some areas of inquiry, uncertainties can be quantified through a long se- quence of repeated observations, trials, or model runs. For other areas, including many aspects of climate change research, precise quantification of uncertainty is not always possible due to the complexity or uniqueness of the system being studied. In these cases, researchers adopt various approaches to subjectively but rigorously assess their degree of confidence in particular results or theories, given available observations, analyses, and model results. These approaches include estimated uncertainty ranges (or error bars) for measured quantities and the estimated likelihood of a particular result having arisen by chance rather than as a result of the theory or phenomenon being tested. These scientific characterizations of uncertainty can be misunderstood, however, because for many people “uncertainty” means that little or nothing is known, whereas in scientific parlance uncertainty is a way of describing how precisely or how confidently something is known. To reduce such misunderstandings, scientists have developed explicit techniques for conveying the precision in a particular result or the confidence in a particular theory or conclusion to policy makers (see Box 1.1). A NEW ERA OF CLIMATE CHANGE SCIENCE: RESEARCH FOR UNDERSTANDING AND RESPONDING TO CLIMATE CHANGE In the process of scientific learning about climate change, it has become evident that climate change holds significant risks for people and the natural resources and ecosystems on which they depend. In some ways, climate change risks are different from many other risks with which people normally deal. For example, as discussed in Chapters 2 and 3, climate change processes have considerable inertia and long time lags. The actions of today, therefore, will be reflected in climate system changes several decades to centuries from now. Future generations will be exposed to risks, some potentially severe, because of today’s actions, and in some cases these changes will be irreversible. Likewise, climate changes can be abrupt—they have the potential to cross tipping points or thresholds that result in large changes or impacts. The likelihood of such abrupt changes is not well known, however, which makes it difficult to quantify 

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Introduction BOX 1.1 Uncertainty Terminology In assessing and reporting the state of knowledge about climate change, scientists have devoted serious debate and discussion to appropriate ways of expressing uncertainty to policy makers (Moss and Schneider, 2000). Recent climate change assessment reports have adopted specific procedures and terminology to describe the degree of confidence in specific conclusions or the estimated likelihood of a certain outcome (see, e.g., Manning et al., 2004). For example, a statement that something is “very likely” in the assessments by the Intergovernmental Panel on Climate Change indicates an estimated 9 out of 10 or better chance that a certain outcome will occur (see Appendix D). In estimating confidence,scientific assessment teams draw on information about“the strength and consistency of the observed evidence, the range and consistency of model projections, the reliability of particular models as tested by various methods, and, most importantly, the body of work addressed in earlier synthesis and assessment reports” (USGCRP, 2009a). Teams are also encouraged to provide “traceable accounts” of how these estimates were constructed, including important lines of evidence used, standards of evidence applied, approaches taken to combining and reconciling multiple lines of evidence, explicit explanations of any statistical or other methods used, and identification of critical uncertainties. In general, statements about the future are more uncertain than statements about observed changes or current trends, and it is easier to employ precise uncertainty language in situations where conclusions are based on extensive quantitative data or models than in areas where data are less extensive, important research is qualitative, or models are in an earlier stage of development. In this report, Advancing the Science of Climate Change, when we draw directly on the state- ments of the formal national and international assessments, we adopt their terminology to describe uncertainty. However, because of the more concise nature and intent of this report, we do not attempt to quantify confidence and certainty about every statement of the science. the risks posed by such changes. Climate change also interacts in complex ways with other ongoing changes in human and environmental systems. Society’s decisions about land use and food production, for example, both affect and are affected by climate change. On the basis of decades of scientific progress in understanding changes in the physi- cal climate system and the growing evidence of the risks posed by climate change, many decision makers—including individuals, businesses, and governments at all levels—are either taking actions to respond to climate change or asking what ac- tions they might take to respond effectively. Many of these questions center on what specific actions might to be taken to limit climate change by reducing emissions of 

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A D VA N C I N G T H E S C I E N C E O F C L I M AT E C H A N G E GHGs: what gases, from what sources, when and where, through what specific technol- ogy investments or changes in management practices, motivated and coordinated by what policies, with what co-benefits1 or unintended consequences, and monitored and verified through what means? Other questions focus on the specific impacts that are expected and the actions that can be taken to prepare for and adapt to them, such as reducing vulnerabilities or improving society’s coping and adaptive capacity. This report explores what these emerging questions and decision needs imply for future scientific learning about climate change and for the scientific research enter- prise. As the need for science expands to include both improving understanding and informing and supporting decision making, the production, synthesis, and translation of scientific knowledge into forms that are useful to decision makers becomes increas- ingly important. It may also imply a need to change scientific practices, with scientists working more closely with decision makers to improve the scientific decision support that researchers can offer. However, even with this decision focus, scientific knowl- edge cannot by itself specify or determine any choice. It cannot tell decision makers what they should do; their responsibilities, preferences, and values also influence their decisions. Science can inform decisions by describing the potential consequences of different choices, and it can contribute by improving or expanding available options, but it cannot say what actions are required or preferred. REPORT OVERVIEW This report describes what has been learned about climate change. It then identifies the most critical current research needs, including research needed to improve our understanding of climate change and its impacts and research related to informing decision makers and allowing them to respond more effectively to the challenges of climate change. As directed by the charge to the panel (see Appendix B), this report covers the broad scientific territory of understanding climate change and its interac- tions with humans and ecosystems, including responses to climate change. Thus, it spans the breadth of “climate change science,” which in this report is defined to in- clude research in the physical, social, ecological, environmental, health, and engineer- ing sciences, as well as research that integrates these and other disciplines. The following chapters, which are broken into two parts, discuss the contributions that climate change science has made and can make in advancing our understanding of climate change and in supporting climate-related decisions. The five chapters in Part 1 A co-benefit refers to an additional benefit resulting from an action undertaken to achieve a particular purpose, but which is not directly related to that purpose. 

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Introduction I include the panel’s conclusions, recommendations, and supporting analysis. Chapter 2 provides an overview of available scientific knowledge about climate change. This overview is drawn from the 12 technical chapters in Part II of the report, which provide more detailed and extensively referenced information on what science has learned about climate change and its interactions with key human and environmental sys- tems. Chapter 3 examines some of the complexities and risks associated with climate change that emerge from what has been learned and discusses the role that scientific research can play in helping decision makers manage those risks. Chapter 4 describes seven crosscutting and integrative research themes that emerge from the panel’s review of key scientific research needs (the details of which can be found in the final section of each of the chapters in Part II). Chapter 5, the final chapter in Part I, provides the panel’s recommendations for advancing the science of climate change, including priority-setting, infrastructural, and organizational issues. Broadly speaking, the report concludes that the causes and many of the consequences of climate change are becoming increasingly clear, and that additional research is needed both to continue to improve understanding of climate change and to sup- port effective responses to it. This expanded research enterprise needs to be more integrative and interdisciplinary, will demand improved infrastructural support and intellectual capacity, and will need to be tightly linked to efforts to limit and adapt to climate change at all scales. In short, the report concludes that we are entering a new era of climate change research, one in which research is needed to understand not just where the world is headed, but also how the risks posed by climate change can be managed effectively. 

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