World in Transition: Climate Change as a Security Risk - WBGU

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6 Conflict constellations
6.4.2
Causal linkages
6.4.2.1
From environmental change to increase in storm
and flood disasters
The likely environmental changes, particularly the
expected intensification of tropical cyclones, the
increase in extreme weather events, and rising sea
levels, have been discussed in detail in Chapter 5.
But climatic-meteorological mechanisms and rising
sea levels are not the only factors heightening the
risks of disaster: deforestation in the upper reaches
of rivers and land subsidence in major urban centres
also contribute to this development. Further considerations
are the growing concentration of cities and
settlements in coastal regions, the high proportion of
economically weak population groups in developing
and newly industrializing countries, and the particular
vulnerability of the urban and industrial infrastructure.
Deforestation of river catchments
No form of land cover does more to balance the discharge
behaviour of streams and rivers than forests.
They intercept and store the bulk of the precipitation,
and so attenuate the direct run-off from heavy
rainfall (FAO, 2005a). Localized studies show that
direct run-off in forests sometimes amounts to less
than one-thousandth of that on grassland (IPCC,
2001). Although forests cannot prevent floods completely,
the peak run-off in forested river catchments
is normally considerably lower than in unforested
areas. Overall this effect is less marked in large river
systems than in small ones. Nevertheless, in the case
of the Yangtze, one of the longest rivers in East Asia,
deforestation of the upper reaches was identified as a
major cause of recurrent catastrophic floods (UNEP,
2002a; MA, 2005a). Similarly, in other regions the
deforestation of catchment areas is acknowledged as
a significant flood risk factor. This applies to Haiti,
Honduras, Nicaragua and Bangladesh, to name but
a few countries which have repeatedly been stricken
by severe storm and flood disasters.
In many of these regions, deforestation continues
unabated. Whereas the forested areas of Europe
and North America have increased slightly on average
in recent years, the same does not hold true for
Africa, Latin America (including the Caribbean) and
the Asia-Pacific region, all of which have registered
decreases, sometimes on a major scale. Forested area
is shrinking by 0.7 per cent per year in Africa and
0.5 per cent per year in Latin America and the Caribbean
(UNEP, 2002b). In certain countries the fig-
ures are higher. In Honduras, almost 3 per cent of the
remaining forests are disappearing every year (calculations
based on data from FAO, 2005b).
Land subsidence
Many coastal cities have grown on the estuaries of
large rivers. These are traditionally locations of great
economic and political importance as transshipment
sites between marine and inland waterway transport
routes. Furthermore, river estuaries normally offer
a variety of benefits for agriculture (e.g. fertile soils
and irrigation potential). A specific problem of such
sites is that the geological substrate is composed of
recent, relatively uncompacted sediments. If an excessively
large volume of groundwater is withdrawn
from these sediment bodies, the substrate becomes
compacted, resulting in subsidence of the land surface
above (Nicholls, 1995; Galloway et al., 1999).
Heavy loading can also lead to settlement of the sediments
and hence to land subsidence. Both causes –
the abstraction of groundwater and heavy loading
– are consequences of a typically urban pattern of
production and consumption: due to the poor quality
of surface water, the bulk of urban water requirements
are often met by means of groundwater. Moreover,
urban building activity creates substantial land
loading. According to Nicholls (1995) and Klein et al.
(2002), at least eight to ten of the twenty-one largest
coastal cities have experienced marked subsidence in
the 20th century, including Tianjin, Shanghai, Osaka,
Tokyo, Bangkok, Manila, Jakarta and Los Angeles.
Subsidence has occurred at rates of up to 1m per decade
(Nicholls, 1995), with top rates of 30cm per year
recorded in Shanghai (Hu et al., 2004). But this phenomenon
is not confined to megacities alone: according
to Barends et al. (1995) more than 150 regions
worldwide are at risk. In China alone, Hu et al. (2004)
estimate the number of affected cities and districts
at 45.
In many cities, land subsidence is perceived as a
problem primarily because it results in damage to
buildings and infrastructure (Galloway et al., 1999).
But problems also arise where cities are built only
a few metres above sea level and any subsidence
implies a precarious rise in the relative sea level
(Coplin, 1999). The potential repercussions of such a
rise in relative sea level were seen when New Orleans
was hit by Hurricane Katrina. New Orleans has
subsided significantly in recent decades leaving some
districts at up to 3m below sea level (Burkett et al.,
2005). Many districts could only be kept permanently
dry by constructing dams and operating large pumping
stations. When Hurricane Katrina crossed the
Gulf of Mexico in August 2005, the dykes of neighbouring
Lake Pontchartrain were breached and the
city was flooded (Section 3.2.2).