|Working Group II: Impacts, Adaptation and Vulnerability|
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19.3. Impacts on Unique and Threatened Systems
19.3.1. What are Unique and Threatened Systems?
Unique systems are restricted to a relatively narrow geographical range but can affect other entities beyond their range. Indeed, many unique systems have global significance. The fact that these unique entities are restricted geographically points to their sensitivity to environmental variables, including climate, and therefore attests to their potential vulnerability to climate change.
Identification of these unique entities provides the first reason for concern
regarding vulnerability to climate change. In this section, we provide examples
of unique entities that are likely to be threatened by future changes in climate.
From those treated by WGII, we address physical, biological, and human systems.
We offer a few examples in each system: tropical glaciers, coral reefs, mangrove
ecosystems, biodiversity "hot spots," ecotones, and indigenous communities.
These are meant only as illustrative examples; there are many unique and threatened
entities. Table 19-1 lists some unique and potentially
threatened systems in relation to climate change thresholds that may cause adverse
effects. Table 19-2 lists some of the unique and
threatened systems that are discussed elsewhere in the TAR.
A number of physical systems are threatened by climate change. Among the most prominent are those in regions dominated by cold temperatures, such as glaciers. Many glaciers already are receding, and many are threatened by climate change. Other physical systems, such as small lakes in areas that will become drier (see Chapter 4), also are threatened by climate change. Changes in unique physical systems can have serious consequences for unique biological and human systems.
Tropical glaciers are present on several mountains in Asia, Africa, and Latin America. These glaciers are valuable because, among other reasons, they are a major source of water for people living below them. For example, through a network of mountain streams, meltwater of the Himalayan glaciers contributes a sizeable portion of river flows to the Ganges, Brahmaputra, Indus, and other river systems in south Asia. Similarly, snow accumulates in winter in the high parts of the cordillera in Peru and melts during summer, becoming the main source of water for many rivers in Latin America. In addition, glaciers act as buffers that regulate runoff water supply from mountains to plains during dry and wet spells. Thus, tropical glaciers are instrumental in securing agricultural productivity and livelihoods and provide cultural inspiration for millions of people who live remote from their sources.
Because of the narrow range of ambient temperatures in the tropics, tropical glaciers are more sensitive to climate change than glaciers elsewhere (see Section 4.3.11). Indeed, records spanning several decades show accelerated retreat of several Himalayan and other tropical glaciers (see Section 220.127.116.11).
In the transient phase of melting, increasing discharge will generate floods in the mountains and immediate vicinity, increased siltation of rivers, and larger sediment load in dams and reservoirs. Riparian mountain ecosystems will be impacted during their dry seasonsin the transient phase by a significant increment of downstream flow, as well as following the transient phaseby significant reduction of this flow. These changes will have tangible economic and cultural implications (see Section 18.104.22.168). This example of a tropical unique entity provides an "early warning" for nontropical glaciers and their potential impacts.
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