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APPENDIX D Explanation of the Rationale for Reasons of Concern T his appendix provides the detailed assumptions and explanation for Figure 2.9 in Chapter 2. For the United States, assessments of droughts, wildfires, heat waves, extreme precipitation events, and pervasive outbreaks of pests (reported in both IPCC [2007b] and USGCRP [2009]) support the first column for the risks of weather extreme events. For each of these extreme events, both assessments report an increase in frequency over the past few decades that can be attributed to warm- ing and precipitation trends. This column therefore begins yellow and turns orange at around 3.6ºF (2°C) to reflect the growing range of pine-beetle destruction across the western states and into southern Alaska and the associated heightened threat of extraordinarily dangerous wildfires (Westerling et al., 2006), as well as the anticipated acceleration of other effects. These high-risk impacts will be encountered roughly at the middle of the midcentury temperature range for the A1b SRES scenario upon which these projections were based. The second column in the graphic relates to climate-borne risks to unique and threat- ened (human and natural) systems across the United States, which were reported by Fischlin et al. (2007) and the National Science and Technology Council (NSTC, 2008). The links with climate are complex and diverse, but it is possible to detect a common thread. Rosenzweig et al. (2008) showed a concentration of statistically significant ob- served impacts in the West, providing evidence that this reason for concern should be- gin yellow or perhaps orange. Threats facing Arctic communities from eroding coasts are the result of coastal storms superimposed upon rising seas. Corals face death and eventual collapse from bleaching episodes that are the result of short periods of unusually high ocean temperatures in combination with other factors such as eutro- phication and ocean acidification. Coastal wetlands, and the protection that they pro- vide, can be completely destroyed by the storm surges of high-intensity storms. Even without expanding this list of examples, it is clear that a wide range of climate-related risks to unique and threatened natural systems are frequently driven by (changes in) climate variability that manifests itself in the form of extreme weather events. Com- bining this with observed and anticipated impacts on unique and threatened human settlements, particularly for the Arctic region of the United States, it follows that this 

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APPENDIX D column should, in its progression from yellow to red, at best parallel that for risks of extreme weather events. The color scheme for aggregate net damages (column 4) is informed by at least two aggregate economic metrics. The first, illustrated by the calibration of the RICE inte- grated assessment model portrayed by Nordhaus and Boyer (2000), places the net economic cost of climate change for the United States associated with a 4.5ºF (2.5°C) warming relative to 1990 levels at 0.45 percent of market gross domestic product (GDP). This estimate includes a willingness to pay of 0.44 percent of market GDP to eliminate a 1.2 percent chance that a permanent loss of 25 percent of global eco- nomic income might occur (a reflection of the damage associated with uncertain catastrophic loss) as well as more modest net costs in agriculture and energy. The second metric reflects calculations of annual contributions to the social cost of car- bon1 for the United States alone along alternative baselines with a range of assumed climate sensitivity. Tol and Anthoff (2008) supplied such estimates—derived from the FUND integrated assessment model for the aggregate and for a collection of sec- tors—to the Environmental Protection Agency. Even for trajectories characterized by high climate sensitivities, none of their (undiscounted) aggregate estimates peak above $3.50 (2000$) per ton of carbon, and most fall short of $1.00 per ton of carbon. Meanwhile, the few sectoral contributions to the aggregate that climb over time do not accelerate until late in the century. Because this time frame would put the increase in global mean temperature above 1990 levels in the 2.7°F–9.9°F (1.5°C–5.5°C) range, the column for aggregate net damages depicted in Figure 2.9 turns from white to light yellow around 3.6°F (2°C). It also reflects both the Nordhaus and Boyer aggre- gate estimates and the Tol and Anthoff trajectories by not turning orange until global mean temperatures increase by nearly 5.4°F (3°C) and not turning red they pass above 5.4°F (3°C). Many, if not all, of the risks associated with extreme weather events have asymmetric impacts; this asymmetry is clearly the source of unevenly distributed vulnerabilities that are reflected in column 4, devoted to the distribution of impacts. Even though it will be argued that aggregate economic indicators do not appear to be very sensi- tive to increases in global mean temperature below 5.4°F (3°C) or so, this diversity of impacts is reflected in the color shading of this column. It is important to recognize 1 The social cost of carbon estimates the discounted economic damages associated with the emission of an extra tonne of carbon at any point in time. It is highly dependent on the future scenario of emissions and development, on a variety of preference parameters like the time preference and aversion to inequality or risk, and a collection of scientific parameters like climate sensitivity. It must be emphasized, as well, that the estimates quoted here are for the United States alone; that is, they do NOT include estimates of economic contributions to the social cost of carbon from damages felt beyond our borders. 

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Appendix D that vulnerability to sea level rise is, for example, hardly ever generated by sea level rise itself. Coastal vulnerability is, instead, increased by even modest amounts of sea level rise due to the character of coastal storms whose impacts are very local and dis- tributed differently along the coast, and it is this characteristic that carries an impor- tant lesson about the sources of risk. New York City has begun to consider the threat that coastal storms might pose to its vital infrastructure (see Chapter 4). Planners there are beginning to consider investing billions of dollars in projects that are de- signed primarily to protect vital infrastructure from flooding associated with extreme weather (coastal storms and simply extreme precipitation events). On the opposite coast, planners in California have already engineered a $13 billion protection project for San Francisco Bay (Moser et al., 2009). They have not implemented the requisite investment, but it is estimated that it would involve annual maintenance expenditures in excess of $1 billion per year after construction. If implemented in time, it is likely to save infrastructure worth 10 times the original investment (Moser et al., 2009). The fundamental point illustrated here is that the distributional impacts of climate change that are buried in the aggregation required to produce national economic estimates are driven to a large degree by the incidence of extreme events and the capacity of specific communities, subcommunities, or systems to respond. Because the column for extreme events misses this adaptation component, the color progres- sion here proceeds less rapidly. It begins at yellow because dramatically asymmetric distributional impacts have already been observed, and it changes quickly to orange because the implicit equity implications of extreme events must also be recognized. As demonstrated by Kates et al. (2006), the poor, the elderly, and perhaps the ethnically disadvantaged are the most vulnerable because of high exposure and high sensitivity. Red shading begins to appear around 3.6°F (2°C). Many of these risks may not appear in the aggregate economic estimates, but they begin to pile up below 3.6°F (2°C). By virtue of their diversity and collective coverage, turning to red then reflects the view that disparate regional indicators of concern should perhaps be used as a national indicator of risk calibrated in noneconomic metrics. IPCC (2007a) reported on a number of potential futures that would involve large-scale and possibly abrupt climate change. NSTC (2008) briefly discussed ice-sheet contribu- tions to global sea level rise and the chance of significant weakening of the meridional overturning circulation (MOC). Smith et al. (2009a) amplified these assessments by reporting that the risk of additional contributions to sea level rise from both the Greenland and possibly the Antarctic ice sheets may be larger than projected by ice- sheet models and could occur over shorter time scales. This could cause an additional contribution to sea level rise of more than 4 meters, and the climate system could be committed to that future with an increase in global mean temperature of about 3.6°F 

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APPENDIX D (2°C) above 1990 levels. Smith et al. (2009a) also noted increased confidence in projec- tions of carbon cycle feedbacks with potentially far-reaching consequences. In addi- tion, these sources report results from Challenor et al. (2006) that some newer models suggest it is possible that as little as 4.5°F (2.5°C) of additional warming could possibly commit the planet to a significant MOC weakening and/or collapse. Because any of these sources of abrupt change would affect the United States as much as anywhere else, the column for risks of large-scale discontinuities (column 5) depicted in Figure 2.9 duplicates the reasons for concern for the globe depicted by Smith et al. (2009a). The last column of the figure refers to the National Security Concern for the United States. A report released by the Military Advisory Board (MAB, 2007) summarizes the results of a relatively thorough review of security concerns for the United States that are derived from observed and prospective manifestations of climate change around the world; it is illustrative of the documents from the military and intelligence commu- nities summarized in Chapter 6. Specific impacts extracted largely from IPCC (2007b) for Asia, Africa, South America, Europe, and the Arctic attracted the MAB’s attention. The MAB viewed these impacts and vulnerabilities through a risk-management lens that revealed significant risks of social upheaval around the world (e.g., from migration pressures and humanitarian crises in the wake of extreme events like floods, droughts, and severe coastal storms). Because most of the evidence that supported the report’s findings was derived from “Risks of Extreme Weather Events” distributed across the globe and well into the long-term future, the global column from Smith et al. (2009a) is replicated in Figure 2.9 as a representation of the sensitivity of national security concerns to changes in global mean temperature. 0