Climate Change 2001:
Working Group II: Impacts, Adaptation and Vulnerability
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7.2.2 Sensitivity and Vulnerability of Human Settlements to Direct and Indirect Impacts of Climate Change

This chapter highlights some of the key processes through which climate impacts could occur; individual regional chapters categorize settlements based on size, location, or complete coverage of the population.

As a result of research that has been done on settlements since the SAR and RICC, as well as additional interpretation of older research, it is becoming clearer where many of the key vulnerabilities of human settlements, energy, and industry occur, although it is still very difficult to provide more than qualitative guidance. Table 7-1 provides an overview of these vulnerabilities for the years between approximately 2050 and 2080; much of the available literature concentrates on the effects of climate change of a magnitude roughly corresponding to that time period. The table divides human settlements into general size categories and economic function in a hierarchy of settlements. The table emphasizes the most salient effects that appear to be characteristic of certain types of settlements and mechanisms that might make the settlements more or less sensitive to climate change.

Implications of climate change for development of settlements, energy, and industry are highly location-specific. For instance, as shown in Table 7-1, climate change is more likely to have important impacts on the development of settlements in resource-dependent regions or coastal or riverine locations. Most of the concerns are about possible negative impacts on development (e.g., on the comparative advantage of a settlement for economic growth compared with other locations), although impacts on some areas are likely to be positive. Impacts on sustainability depend very largely on how climate change interacts with other processes related to multiple stresses and opportunities-such as economic, demographic, and technological change-except in low-lying areas that may be subject to sea-level rise or polar regions whose physical conditions will be more directly affected by global warming. Equity effects are of considerable concern because the ability to cope with negative impacts or to take advantage of positive impacts is likely to be greater among advantaged groups than among disadvantaged groups, within regions and between regions. As a result, climate change has the potential to enlarge equity-related gaps in human settlements and systems.

In general, country studies that have been completed since the SAR was published have provided more specific regional details concerning sensitivities and vulnerabilities to climate change (e.g., IPCC, 1998; see Chapters 10-17). Because of variability in settlements across the world, it is virtually impossible to create rankings of impacts that do not contain numerous exceptions. However, the impact ratings in Table 7-1 provide a framework that can be adapted to local circumstances. Table 7-1 shows the author team's judgments, based on the available literature, about the vulnerability of different types of settlement to various aspects of climate change. The horizontal axis differentiates vulnerability according to type of settlement, capacity to adapt, and the mechanism through which the settlement is affected by climate change. For example, the resource base of settlements that are economically dependent on activities such as agriculture, forestry, fishing, hunting and gathering, or tourism may be affected; housing and infrastructure may be affected in coastal areas, riverine floodplains, islands that are sensitive to flooding, steeplands that are sensitive to landslides, and urban/wildland boundaries that are sensitive to fires; and the health and productivity of urban populations may be affected directly through air pollution, heat waves, and heat island effects. The vertical axis identifies 12 different types of climate change impact in descending order of global importance. Vulnerabilities are rated as low, medium, or high magnitude as described in Box 7-1. The information in Table 7-1 generally is presented as a range, reflecting the diversity of settlements within each broad class. The final column shows the level of confidence that the author team assigns to each type of climate impact. Table 7-1 depicts vulnerabilities for the years between approximately 2050 and 2080. Much of the available human settlements literature is silent on the timing of impacts; the choice of the years 2050-2080 in Table 7-1 is based on the size of the impacts or amount of climate change addressed in the literature reviewed by the author team. Table 7-1 takes into account the number and type of settlements affected worldwide and the likely strength of these effects by mid-to-late 21st century, as well as the financial, technical, and institutional capacity of settlements to respond. Figure 7-2 provides confidence scores for the impacts on individual scales described more fully in Box 7-1 (see also Moss and Schneider, 2000).

The negative impacts in Table 7-1 generally would be less negative or even positive in some regions before 2050 but greater than shown and becoming more negative in more regions after 2100. The table is not intended to show that only specific types of settlements would be harmed (or helped) in certain ways by certain changes; it is intended to show that settlements of certain types probably are likely to be affected by certain impact mechanisms and are likely to be particularly vulnerable to certain types of climate changes or conditions.

Many of the effects in Table 7-1 are quite likely for some communities in some places; other effects are extremely uncertain, controversial, or inapplicable. Key articles that underlie the ratings are provided as footnotes to Table 7-1.

Confidence in the main conclusions of this chapter in Table 7-1 is rated in Figure 7-2 from very high (5) to very low (1) in four dimensions: support from theory, support from model results, support from data or trends in the existing environment, and the degree of consensus in expert opinion. Although these ratings reflect the subjective judgments of the chapter's authors concerning the weight that can be given to each element that increases confidence in the findings, the figure is useful in depicting the dimensions of the underlying literature that are particularly strong or weak in support of the chapter's conclusions. Confidence levels vary widely:

Box 7-1. Development of Scales for Assessing Potential Vulnerability of Human Settlements to Effects of Climate Change and Confidence in the Certainty of Impacts

Climate affects the stability of resources that support human systems. One way to assess the potential impact of climate change on human systems is by using a qualitative scale that expresses the vulnerability of settlements to various kinds of climate effects (e.g., floods) in terms of how potentially disruptive these climate effects are expected to be for various types of human settlements (based on differences in their economic base, location, size, and adaptability). The definitions in the rating system below are derived from standard environmental impact assessment language and are intended to apply to local climate impacts. However, the scale may be used nationally if the nation is small and homogeneous or if most of the population lives in settlements of a certain type.

Magnitude Ratings (Size of Impacts)

  • Low: Impacts of changed climate are not distinguishable from normal background variability in weather impacts or there is little noticeable effect.
  • Moderate: Resources or sectors are affected noticeably, even substantially, but the effect is not destabilizing and recovery is rapid.
  • High: Impacts are large and sometimes catastrophic. Resources or settlements are destabilized, with little hope for near-term recovery.

A semi-quantitative approach is used with a 5-point confidence scale to indicate the certainty of the effects of climate change. The author team subjectively rated confidence on the basis of the literature in four dimensions: consensus among experts (consensus), the extent to which underlying theory and data is developed (theory), the quality of model results (model results), and the consistency of observational evidence (observations). The scores were used to create a four-sided polygon, as shown in Figure 7-2 on the facing page. All four dimensions were weighted equally to determine the area of the polygon and an overall confidence score.

Polygon Area = 0.5 . (Theory . Observations + Observations . Model Results + Model Results . Consensus + Consensus . Theory)

The overall confidence score assigned was based on the area of the polygon. For example, to rate a 4 for "high confidence," the polygon had to have an area between 16 and 25-the area of a polygon with ratings of greater than 4 but less than 5 on all four dimensions.

Confidence Rating (Certainty of Impacts)

  1. Very Low: Impacts are extremely difficult to predict (confidence < 5%) (Polygon Area = 0-8).
  2. Low: Impacts are regularly much greater or less than the median value (confidence < 33%) (Polygon Area = 8-18).
  3. Medium: Impacts are regularly greater or less than the median value (confidence Z 33%) (Polygon Area = 18-32).
  4. High: There is noticeable variation in the size of impacts (confidence Z 67%) (Polygon Area = 32-50).
  5. Very High: There is little variation in impact among scenarios, within a settlement type (confidence = 95%) (Polygon Area = 50).


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