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Introduction

BACKGROUND

The U.S. Global Change Research Program (USGCRP) was established in 1990 to “assist the Nation and the world to understand, assess, predict, and respond to human-induced and natural processes of global change.”¹ A key responsibility for the program is to conduct National Climate Assessments (NCAs) every 4 years.² These assessments are intended to inform the nation about “observed changes in climate, the current status of the climate, and anticipated trends for the future.” The USGCRP hopes that government entities—from federal agencies to small municipalities, citizens, communities, and businesses—will rely on these assessments of climate-related risks for planning and decision making. The third NCA (NCA3) was published in 2014 and work on the fourth is beginning. The USGCRP provided guidance to the authors of the NCA3 about ways to identify and evaluate key risks. With the fourth NCA (NCA4), the USGCRP hopes to improve its usefulness to decision makers by more clearly addressing the societal risks associated with climate change, rather than primarily cataloging the biophysical effects that have been observed and are projected for the future. The NCA4 will directly address the multiple, interacting factors that influence the risks that changes in climate

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¹ See http://www.globalchange.gov/ for more information [April 2016].

² For more information about NCA reports, see http://nca2014.globalchange.gov/ [May 2016]. As the third NCA was developed, the USGCRP initiated a plan to expand the program beyond the production of reports every 4 years, to provide sustained, ongoing resources as well. See http://nca2014.globalchange.gov/report/response-strategies/sustained-assessment [May 2016].

pose for people and social systems, and it will take into account the fact that many of these factors are themselves changing over time. The developers hope to provide this information in a way that is accessible, useful, and easy to apply in a wide range of contexts.

The USGCRP, after consultation with the Committee to Advise the U.S. Global Change Research Program of the National Academies of Sciences, Engineering, and Medicine, asked the Academies’ Board on Environmental Change and Society to conduct a workshop to explore ways to frame the NCA4 and subsequent NCA reports in terms of risks to society. The workshop was intended to collect experienced views on how to characterize and communicate information about climate-related hazards, risks, and opportunities that will support decision makers in their efforts to reduce greenhouse gas emissions, reduce vulnerability to likely changes in climate, and increase resilience to those changes. The committee’s charge is shown in Box 1-1.

The Planning Committee for the Workshop on Methods for Characterizing Risk in Climate Change Assessments was appointed to plan this workshop, which was held March 23-24, 2016. The workshop included experts on (1) the impacts of climate change on physical, biological, and social systems and the associated uncertainties; (2) analysis of vulnerability and resilience to, and consequences of, climate change; (3) tools and approaches reflecting contemporary insights from the risk and decision sciences related to climate change; (4) planning for and managing climate-related risks in various sectors and U.S. regions; and (5) risk communication. The committee’s role was limited to planning the workshop, and these proceedings have been prepared by a rapporteur

as a factual summary of the presentations and discussions that occurred at the workshop. The views contained in the report are those of individual workshop participants and do not necessarily represent the views of all workshop participants, the planning committee, or the Academies. The workshop agenda and biographical sketches of the committee members and presenters are in Appendixes A and B.

The committee developed a planning document to elaborate the issues it was charged to address and to guide the experts invited to participate in the workshop.³ The document notes that the NCA4, as well as subsequent reports and the sustained assessments of which they are a part, should be useful to a wide range of decision makers in different geographic places, facing different kinds of decisions, and holding different values. The purpose of the workshop was to help the designers of the NCA identify and implement methods for characterizing the risks of climate change that will help these highly varied users understand the range of environmental, social, and economic risks they face, including what is known about these risks and what is uncertain.

As noted in the planning document, previous NCAs have primarily addressed risk in two ways:

1. by making explicit or implicit statements about the probabilities of future changes in key climate variables, such as temperature or precipitation—for example, by showing ranges of future changes in these variables across multiple climate models and emissions scenarios; and
2. by drawing qualitative, narrative connections (as in the chapter on water and agriculture) among a given climate change, associated biophysical impacts of that climate change, and the socioeconomic systems that may be affected.

The committee asked presenters to address five issues as they considered ways to strengthen the NCA4:

1. approaches for framing climate change risk that can guide chapter authors;
2. challenges of representing the range of biophysical consequences of climate change and their interactions with social and economic changes that matter to decision makers;
3. challenges of representing how development pathways (and the vulnerabilities and capacities they influence) could alter the context for decisions and thereby affect their appropriateness and effectiveness;

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³ The document is available at http://sites.nationalacademies.org/DBASSE/BECS/CurrentProjects/DBASSE_168692 [June 2016]. Many of the workshop presentations and an archived Webcast are also posted at this site.

4. the state of knowledge about the likelihood of risks being realized as impacts across spatial and temporal scales under different assumptions of climate and development, and the degree of confidence in scientific understanding; and
5. available methods and processes for making information about the knowledge and uncertainties understandable, credible, and useful to decision makers.

A NOTE ABOUT TERMS

Some of the terms used frequently in the context of climate change and its effects can be confusing because they have both common and specialized meanings. Below are a few such terms used in these proceedings and definitions that reflect their usage in prior Academies reports:

• Adaptation: Actions taken to limit the damage to people, communities, and infrastructure from events resulting from climate change. Adaptation measures include not only taking protective steps (building sea walls and the like) but also preventing human settlement in vulnerable areas or moving people away from such areas. In some cases, the term also includes compensating people for loss.
• Consequence: The magnitude of damage that would result from a hazard.
• Hazard: A potentially damaging event, such as a fire or flood.
• Impact: An effect of physical climatic events, such as an increase in droughts or wildfires or a rise in sea levels.
• Mitigation: Steps taken to reduce the rate of climate change, for example, using changes in technology and human behavior to reduce emissions of greenhouse gases.
• Probability: The likelihood that a specific event, such as a hazard, will occur within a specified timeframe.
• Resilience: The capacity to anticipate, prepare for, respond to, and recover from a significant hazard or multihazard threats with minimum damage to social well-being, the economy, and the environment.
• Risk: A combination of the magnitude of a potential consequence of a hazard or hazards attributable to climate change and the likelihood that the consequence will occur. Risk may refer to physical, biological, or socioeconomic consequences.
• Vulnerability: The degree to which a system is susceptible to, or unable to cope with, adverse effects of climate change, including climate variability and extreme events. Vulnerability is a function of the character, magnitude, and rate of climate variation to which a system

OPENING REMARKS

Planning committee chair Joseph Arvai noted in opening the workshop that uncertainty cannot be eliminated from scientific projections but that the NCA4 is an “opportunity to address risk even in a climate of uncertainty.” John Holdren of the White House Office of Science and Technology Policy and Thomas Karl of the USGCRP provided an overview of the primary objectives they hoped the workshop would accomplish, which was followed by discussion of some of the most important challenges for the NCA4.

The assessment and characterization of risk, in Holdren’s view, should address the needs of those who will use the information: the makers and implementers of policy, firms and businesses, and individuals who make decisions about mitigation and adaptation, as well as voters. Up to now, he suggested, risk has been defined in terms of physical and biological events that can follow from climate change as “the sum over all possible events of probability times consequences.” Less attention has been given to the consequences of these events for human well-being, that is, to characterizing the probabilities of their occurrence, as well as the character and quality of the consequences. To prepare for the future, he said, it is critical that people have a much clearer picture of how likely different possible consequences are, understand the strength of the available evidence, and have a realistic understanding of what it will mean for society if “the worst is true.”

Many people assume that the uncertainty in climate change projections means it is just as likely that the outcomes could turn out to be favorable as not, Holdren noted. In his view, it is actually the case that “there is a larger chance that things will be worse than we currently expect than better.” Because of this gap in understanding of the risks, he said, it is critical that the NCA4 be very clear not only about what is known, but also about how it is known, and that it clearly explains the implications of what is known for mitigation and adaptation. He suggested that the NCA4 could move in this direction by providing the following:

• Disaggregation of information by geographic region and by sectors of economic activity and other influences on human life, such as the water cycle. He said the report should be directly relevant to the needs of many types of users (including makers and implementers of government policy, businesses, and individuals).
• Characterization of vulnerabilities. The report should not only explain how climate will or may change things (e.g., that there is likely to be an increase in stormy weather or a rise in sea level) but also offer specific

• information on how those changes will affect people, infrastructure, local weather patterns, and so on.

• Measures of the costs associated with the risks discussed, in dollars and in other terms. The report should compare the costs of taking action to mitigate or adapt to known risks with the costs of taking no action. Drawing reasonable comparisons may require “creative use of yardsticks,” Holdren acknowledged.
• Perspectives developed through partnerships across government and the private sector to illuminate specific vulnerabilities and how they work.

Karl focused on what leaders and policy makers need to understand about the risks changes in climate bring, noting that these individuals are a diverse group that includes not only public officials at the local, state, and federal levels, but also decision makers within major corporations, international agencies, and intergovernmental bodies. Recent reports, including Risky Business: A Climate Risk Assessment for the United States (Risky Business Project, 2014) and Climate Change in the United States: Benefits of Global Action (U.S. Environmental Protection Agency [EPA], 2015), have demonstrated progress in effectively presenting the economic impacts of climate change for a policy audience, Karl noted.

Looking in detail at one example from the 2015 EPA report, which discusses many sectors, Karl noted that it includes a thorough documentation of the cascading impacts of changes in climate on water quality in the coming decades. The authors fed information about expected changes into models for areas related to water supply, such as river flow and water demand, also factoring in weather patterns, and then used those models to work out a water quality index to help readers understand the range of possible impacts and their costs. Other issues are addressed in a similar way.

It is also important, Karl added, to be clear about areas where it is more difficult to outline the potential cascading effects. The interactions among and within complex systems—such as the way energy, agriculture, water, transportation, drought, wildfires, and rising temperatures interact—are difficult to map. But it is these “complex cascades” that bring the greatest potential risks to society, so it is critical that the NCA4 communicate effectively about these risks, in his view.

DISCUSSION

Participants elaborated on several of these opening points. One observed that the frameworks scientists have customarily used to characterize risk do not apply well to climate change. For example, the assumption that the relationships between exposure to a risk factor and response to it are constant over time—which may hold in many situations—is not true for climate. In order to

convey the cascading nature of risks, this person suggested, it is important to factor in varying degrees of vulnerability to particular risks. Another participant also called attention to the significant variation in circumstances, vulnerabilities, and risk that face people and systems across geographic regions and sectors.

Nonmonetary measures are needed to help people understand the real risks, a participant noted, and several measures were identified. In the domain of human health, for example, lost life expectancy and days lost to illness and disability can help quantify some effects. It is a big challenge, however, to educate people about how dependent humans are on ecosystems, a participant noted. As the importance of ecosystems to food and water supplies, for example, becomes better understood, measures of how badly ecosystems are being disrupted by climate change will become effective tools.

Another issue raised was that the time trajectories for different elements of the discussion are so different. As one participant noted, three separate timelines are key: the time required for climate science to reduce the uncertainties in risk forecasts, the time it will take for impacts to be felt and for society to identify and execute responses, and the length of time many of the impacts will last. These timelines do not align well, so people may fail to fully understand the consequences of their actions until it is too late to avert those consequences. By one estimate, another participant pointed out, sea-level rise will be as much as 25 meters over the next 10,000 years, even if all greenhouse gas emissions related to human activity stop by the year 2100. In that context, the time it will take to transform the world’s energy system may be the most critical timescale. The $25 trillion energy infrastructure, which is currently 80 percent fossil fuel-based, cannot be transformed in a time span shorter than several decades, in that person’s view.

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