from direct environmental stresses associated with climate change as well as from the indirect effects of climate change–induced environmental change on economic and political conditions. Climate change may constitute a direct environmental driver of either temporary or permanent migration via its effects on the availability of ecosystem services including, for example, the supply of freshwater, which may change under altered rainfall regimes; coastal flood protection, which may be lost as the result of sea level rise; and changes in the productivity of agricultural lands as a result of changes in temperature and precipitation regimes (Black et al., 2011b). Climate change may also affect the likelihood of droughts, coastal storms, and other types of hazardous climate events, which may temporarily or permanently displace susceptible households. Climate change may indirectly contribute to migration, whether temporary or permanent, via effects on economic, political, and social drivers. For example, climate change may influence agricultural and natural resource–related livelihood opportunities in a particular region, or it may contribute to political conflicts within a region over water or other resources. In all of these cases climatic shocks and stresses interact in complex ways with the other known drivers of migration so that the effects of climate events are not monotonic (i.e., more intense climate events do not necessarily lead to more migration) and they also depend on other causal variables (Warner, 2010; Black et al., 2011b).

When considering the migration decisions of individuals and households, it is important to keep in mind that climate changes may lead to alterations in a household’s characteristics, which, in turn, can influence the actions of individuals. For example, climate change may create conditions that result in a long-term decline in a household’s wealth and assets, which in turn can both increase the motivation to migrate and decrease the ability of members of the household to migrate in search of new economic opportunities. Policies intended to facilitate adaptation to climate change may provide incentives for a household either to relocate or to remain in a region. Another critical factor for migration decisions is the effectiveness of responses to climate events. As noted in the discussion of humanitarian crises, effective response is a key determinant of whether an extreme climate event becomes a humanitarian crisis. Extreme events have often resulted in temporary, internal population displacement but have rarely led to permanent migration (Perch-Nielsen et al., 2008; Lilleør and Van den Broeck, 2011). Effective immediate responses to extreme events and effective recovery efforts might therefore be expected to reduce population displacement or to shorten the duration of displacement.

Several types of security threats can be identified as possible results of climate change–related migration. Broad migration trends indicate that over the past 30 years large cities have been growing faster in low-elevation coastal zones than elsewhere and that these trends are likely to continue



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