Effects of sea-level rise |
AS MENTIONED in the introduction, small islands and low-lying coastal areas have
received much attention in the literature, including the IPCC assessments in
the recent years. Deltas fall into the same category of areas very vulnerable
to sea-level rise. But unlike deltas and other coastal areas, small islands
have no hinterland to move to in the case of loss of land. In addition, their
land resources are very limited.
According to Nicholls and Leatherman (1995), a 1m sea-level rise would affect
6 million people in Egypt, with 12% to 15% of agricultural land lost, 13
million in Bangladesh, with 16% of national rice production lost, and 72
million in China and "tens of thousands" of hectares of agricultural land.
More than direct land loss due to seas rising, indirect factors are generally
listed as the main difficulties associated with sea-level rise. These include
erosion patterns and damage to coastal infrastructure, salinization of wells,
sub-optimal functioning of the sewerage systems of coastal cities with resulting
health impacts (WHO 1996, chapter 7), loss of littoral ecosystems and loss of
Deltas pose a set of more specific problems, as they are the areas where sea
and land most closely interact: their average elevation is usually very low,
to the extent that tidal effects can be felt for several tens of km, and in
some cases hundreds, inland. Note that the deltas in closed seas, such as
the Nile and Danube, do not normally suffer the same difficulties. This
land/sea interaction results in very complex agricultural systems, where
irrigation and rain-fed agriculture may be practised in alternate seasons,
with attention to irrigation water quality (salinity) and to the washing
out of salts by rains before planting crops.
The Mekong delta provides a clear example of this complexity (see, for example,
Jelgersma et al., 1993). Siltation is usually very active during certain
seasons of the year: deposition in the Mekong Delta continues to extend the
Ca Mau peninsula to the south and west at a rate of 150 m/annum in some
places (Fedra et al, 1991). The delta, therefore, is as much the result of
conditions up-stream as the result of local or coastal interactions. Sea-level
rise, therefore, cannot be examined without some sound assumptions about
climate and rainfall changes in the river catchment as a whole.
Contrary to many other "normal" coasts, deltas are difficult and expensive
to protect, due to the very dissected nature of the coastline. A measure
of the difficulty is provided by the "multiplier"  used by CZMS (1990)
to evaluate protection costs. Multipliers for deltas usually vary from 6
to 8, but may reach higher values for the Magdalena (10.4, Venezuela),
Orinoco (11.3), Parana (26.7), Mekong (9.8) or Ganges-Brahmaputra-Mahanadi
(7.5) CZMS (1990) stresses that the multiplier may be affected by large
errors (100%), but the parameter definitely gives a measure of the level
of intricacy of land and sea in deltas.
Although deltas tend to be densely populated, it is not really relevant
to assess exactly how many people live in them . Due to their very
high productivity (generally fertile soils, water availability, multiple
cropping, especially in tropical areas) they produce significantly more
food than the local consumption. In Vietnam, for instance, 50% of national
rice production comes from the Mekong delta in the south, while 20% is
produced in the Red River delta near Hanoi. Although much of this production
is for export, a disaster in the deltas would have profound effects on the
whole country. This indicates a fragile situation in which any major
disturbance would result in economic and possibly political shock waves
well beyond the delta proper.
Figure 4. Difference
between "coastal" and "continental" rainfall in the three conventional
continents defined in Table
indicate rainier conditions along the coasts, defined a 0.5 square
(up to 60 km from the coast at the equator), while land areas are
all land found at the same latitude. Based on the IIASA data set
of Leemans and Cramer, 1990. The graph shows 5 degree moving-averages.
While it is possible, at any location, to define a "climate complex", i.e.
a set of inter-related climatic variables (Sombroek and Gommes, 1996),
it should be noted in the current context that coastal climate complexes
tend to be rather different from the climate complexes at some distance
from the sea (Figure 4 above). Since delta agriculture typically uses a
combination of irrigated and rain-fed crops, a change in the relative
behaviour of the local delta climate and the climate in the basin may
lead to unprecedented difficulties. It might impose, for instance, a
shift to more salt tolerant crops.
It is unknown how global climate change would alter these patterns, but
is it obvious that zonal scenarios are unlikely to apply on the coast.
Similarly, the sea- and land breeze patterns are very likely to be altered.
In coastal areas, and particularly deltas, we thus have to take into
account: projected sea-level rise, modified ocean circulation patterns
as they affect the building and erosion of the coast, climate change
in the catchment basin and change of the coastal climate, and changes
in the frequency of extreme events. This also constitutes a major difference
with small islands where remote land-based changes are likely to be of
However, major disasters or changes in deltas and small islands could
have repercussions over large areas. Given the populations potentially
involved, this is more likely to seriously affect the major deltas, such
as the Ganges-Brahmaputra, Mekong and Nile. In both the cases of deltas
and small islands, a likely scenario could be outmigration when disasters
due to sea-level rise reach levels or frequencies considered unacceptable.
It is at such thresholds that maximum damage and loss of life could be expected.
It is also noteworthy that deltas, although not specifically fragile, are
the result of a long evolution and that they are, in the words of Riebsame
et al. (1995), tuned to the current climate. Any departure from the current
equilibrium, even one that would a priori be positive (e.g. better rainfall/
evaporation ratio), results in a temporary disturbance of the ecology and
It may be useful to more closely examine the evolution of coastal lowlands
during the period from about 10,000 to 6,000 years ago: the sea level rose
faster than ever since, by some 125 m in 4000 years. In comparison, the sea
level on the coast of the Netherlands rose by 4 m over the last 4000 years.
Speculating on the impacts of that extremely rapid and prolonged sea level
rise on human society, we might note that near its beginning, some 9000
years ago, arable agriculture began. Just after the end, some 6,000 to 5,500
years ago, there were suddenly a series of well planned cities, with well
ordered agricultural areas surrounding them, in various coastal lowlands or
lower river valleys, e.g. in the Euphrates-Tigris, the Nile, the Indus.
Where are the precursors of these cultures, so sophisticated in their spatial
planning? Did they originate near the coast much earlier, at lower elevations,
with those societies that survived and prospered being those that could best
plan their successive evacuations to higher ground, re-establish their physical
infrastructure and maintain their social organization? A re-reading of the
several "Flood" records in ancient literature worldwide might provide new
insights. Also, subaqueous investigation of the shallow parts of the relevant
undersea deltas - for example, by seismic means or possibly ground-penetrating
radar - might yield new information on that intriguing period. We might learn
something of use to present society, which is facing similarly great changes,
albeit of a different kind.
Figure 5. Recent
population density changes according to major agro-climatic regions
graph prepared using data from Tobler et al. 1995 and 1997. The 9
regions covered are Warm humid tropics, Warm seasonally dry tropics,
Cool tropics, Arid regions, Subtropics (summer rains), Subtropics
(winter rains), Temperate zone (oceanic), Temperate zone (continental)
and Boreal zone.
Finally, one observes marked differences in population growth according
to agroclimatic zones. Population densities grow fastest in the seasonally
dry tropics, while in temperate countries little change takes place. This
will lead to the need to produce more food in the former zones, which have
quite sever ecological and agricultural limits (Gommes, 1992).
5. The "multiplier" expresses how many times the coast is more dissected than
would be estimated from the length measured as straight lines "as the crow flies".
6. The main deltas are the Ganges-Bahmaputra delta in Bangladesh, the Nile
in Egypt, and the Mahanadi and Ganges in West Bengal. There are about nine
major deltas in the Americas (two flowing into the glacial Ocean and seven
in sub-tropical and tropical areas), three in Europe, one in glacial Asia,
10 in tropical Asia, four in Australasia and five in Africa (Jelgersma et al