1. ODINAFRICA/GLOSS Training Workshop on Sea-Level Measurement
and Interpretation. Oostende, Belgium, 13-24 November 2006
Tanzania Fisheries Research Institute
P.O. Box 9750
Dar es Salaam
TANZANIA
EMAIL: SHIGALLA@YAHOO.CO.UK
20 NOVEMBER 2006
SHIGALLA MAHONGO
IMPACTS OF SEA LEVEL CHANGE
2. The Greenhouse Effect
Infrared (IR) active gases, principally water vapor (H2O),
carbon dioxide (CO2) and ozone (O3), naturally present in
the Earth’s atmosphere, absorb thermal IR radiation
emitted by the Earth’s surface and atmosphere.
The atmosphere is warmed by this mechanism and, in
turn, emits IR radiation, with a significant portion of this
energy acting to warm the surface and the lower
atmosphere.
As a consequence the average surface air temperature of
the Earth is about 30° C higher than it would be without
atmospheric absorption and re-radiation of IR energy.
This phenomenon is popularly known as the greenhouse
effect, and the IR active gases responsible for the effect
are likewise referred to as greenhouse gases.
3. Incoming Solar
Radiation 343 W/m2
Reflected Solar
Radiation 103 W/m2
Long-wave Radiation
240 W/m
2
CO2 CH4,
N2O, O3,
Water
vapour,
aerosols,
clouds
Earth’s ground temperature, approx 13o
C with greenhouse
effect, approx - 20o
C without it. Doubling CO2 increases
temperature by between 1.5o
C and 4o
C.
The Greenhouse Effect
“Blanket” of Greenhouse Gases
5. The Greenhouse Effect
The rapid
increase in
concentrations
of greenhouse
gases since
the industrial
period began
has given rise
to concern
over potential
resultant
climate
changes
6. Greenhouse Gases and Global Climate Change
The principal greenhouse gas concentrations that
have increased over the industrial period are carbon
dioxide (CO2), methane (CH4), nitrous oxide (N2O),
and chlorofluorocarbons (CFCs).
The observed increase of CO2 in the atmosphere
from about 280 ppm in the pre-industrial era to
about 364 ppm in 1997 has come largely from fossil
fuel combustion and cement production.
Of the several anthropogenic greenhouse gases,
CO2 is the most important agent of potential future
climate warming because of its large current
greenhouse forcing, its substantial projected future
forcing, and its long persistence in the atmosphere.
8. -3 -2.5 -1.5 -1 -.5 -.1 .1 .5 1 1.5 2.5 3.4
2005 Temperature Changes
Compared to 1951-1980
9. Ozone Layer Depletion and Climate Change
The ozone layer absorbs harmful ultraviolet-B radiation from
the sun. Over the past 30 years ozone levels over parts of
Antarctica have dropped by almost 40% during some months
and a 'hole' in ozone concentrations is clearly visible in
satellite observations.
Ozone is been damaged mainly by:
1. Chlorofluorocarbons (CFCs) that are used in refrigerators,
aerosols, and as cleaners in many industries.
2. Halons that are used in fire extinguishers.
3. Aircraft emissions of nitrogen oxides and
water vapour.
As Ozone is considered to be a greenhouse gas, a depleted
ozone layer may partially dampen the greenhouse effect. This
may therefore lead to increased global warming.
Conversely, efforts to tackle ozone depletion may result in
increased global warming!
10. Some Impacts of Climate Change
The hydropower-dependent energy sector in Tanzania has been
seriously affected by drought. The country is turning to coal and
natural gas as new sources of energy.
Some cities in Europe and USA experienced power shortages
during summer of 2006 due to effects of increased temperatures
(The infrastructures failed to cope with the record heat -
unsuitable wires, pipes, etc not designed for higher
temperatures).
About 82% of the icecap on mount Kilimanjaro in 1912 is now
gone. If recession continues at the present rate, the majority of
the mountain glaciers could vanish in the next 15 years.
The area covered by glaciers on the Rwenzori Mountains halved
between 1987 and 2003, expected to disappear in the next 20
years.
12. RELATIVE SEA LEVEL CHANGE
Sea level varies as a result of processes operating on a great range of
time-scales, from seconds to millions of years, so that current sea level
change is also related to past climate change.
The local change in sea level at any coastal location as measured by a
tide gauge depends on the sum of global, regional and local factors and is
termed relative sea-level change.
It is so called because it can come about either by movement of the land
on which the tide gauge is situated or by the change in the height of the
adjacent sea surface.
Relative sea levels are also measured by dating buried coastal vegetation
(salt marshes, mangroves, etc.).
Most of the tide gauges are located in mid-latitude northern hemisphere,
few in middle of oceans, and contaminated by earth movements.
The main source for the uncertainties in using tide gauge records still
remain: poor historical distribution of tide gauges, lack of data from Africa
and Antarctica, the GIA corrections used, and localized tectonic activity.
13. CLIMATE CHANGE AND SEA LEVEL RISE
Sea-level rise due to global warming occurs primarily
because water expands as it warms up. The melting ice
caps and mountain glaciers also add water to the
oceans, thus rising the sea level.
The contribution from large ice masses in Greenland
and Antarctica is expected to be small over the coming
decades. But it may become larger in future centuries.
Sea-level rise can be offset up by irrigation, the
storage of water in reservoirs, and other land
management practices that reduce run-off of water
into the oceans. Changes in land-levels due to coastal
subsidence or geological movements can also affect
local sea-levels.
14. Average Rate of Sea Level Rise and the Estimated
Contributions from Different Processes: 1910 - 1990
Factor Min Mid value Max
Ocean thermal expansion 0.3 0.5 0.7
Glaciers and ice caps 0.2 0.3 0.4
Greenland – 20th
Century effects 0.0 0.05 0.1
Antarctica – 20th
Century effects -0.2 -0.1 0.0
Ice sheets – Adjustment since LGM 0.0 0.25 0.5
Permafrost 0.00 0.025 0.05
Sediment deposition 0.00 0.025 0.05
Terrestrial storage -1.1 -0.35 0.4
Total -0.8 0.7 2.2
Estimated from tide gauge records 1.0 1.5 2.0
15. CLIMATE CHANGE AND SEA LEVEL RISE
About 20,000 years ago during the LGM, large ice sheets
melted causing a rise in sea level of about 100m, most of
the melting occurred about 6,000 years ago.
Over the past 1,000 years and prior to the 20th century,
the average global sea level rise was of the order of 0.2
mm/yr.
The rate of sea level rise climbed to about 1-2 mm/yr
during the 20th century, with a central value of 1.5 mm/yr
(IPCC TAR). The most recent estimate during the 20th
century is 1.4 -2.0 mm/yr, with a central value of 1.7 ±
0.3 mm/yr (Church & White, 2006).
This significant rate of rise in sea level is attributed to
global warming caused by industrialization during the
second half of the 19th century.
16. CLIMATE CHANGE AND SEA LEVEL RISE
There is no evidence for any acceleration of sea level rise
in data from the 20th century data alone. Mediterranean
records show decelerations and even decreases in sea
level in the latter part of the 20th century.
Most records show evidence of a gradual rise in global
mean sea level over the last century. However, signals
caused by land movements (e.g. uplift or submergence)
can mask this signal due to actual changes in sea level.
The IPCC has estimated that, if the emission of
greenhouse gases continues at the current rate, the level
of the sea surface will rise by an additional 8-20 cm by
2030, 21-71 cm by 2070 and 31-110 cm by 2100.
18. THE PROSPECT OF SATELLITE
ALTIMETRY IN SEA LEVEL STUDIES
Satellite altimetry provides near-global coverage of the
world’s oceans and thus the promise of determining the
global-averaged sea level rise, its regional variations, and
changes in the rate of rise more accurately and quickly
than is possible from the sparse array of in situ gauges.
TOPEX/Poseidon satellite altimeter mission with its (near)
global coverage from 66°N to 66°S was launched in August
1992. Estimates of the rates of rise from the short T/P
record are 2.5 ± 1.3 mm/ yr over the 6-yr period 1993–98
(Church et al, 2004).
Using a combination of tide gauge records and satellite
altimetry, Jevrejeva et al. (2006) have estimated this rate
to be 2.4 mm/yr over the same period.
19. THE PROSPECT OF SATELLITE ALTIMETRY
IN MEAN SEA LEVEL STUDIES
Whether this larger estimate is a result of an increase in
the rate of rise, systematic errors in the satellite and/or
in situ records, the shortness of the satellite record, or a
reflection of the large error bars is not clear.
Analysis of TOPEX/Poseidon satellite altimeter data has
demonstrated that meaningful estimates of global averaged
mean sea level change can be made over much shorter periods
than possible with tide gauges because the global satellite data
account for horizontal displacements of ocean mass.
However, achieving the required sub-millimeter
accuracy is demanding and requires satellite orbit
information, geophysical and environmental corrections
and altimeter range measurements of the highest
accuracy. It also requires continuous satellite operations
over many years and careful control of biases.
20. Mean Sea Level Variations at Selected
Locations [Data: www.pol.ac.uk/psmsl]
21. Mean Sea Level Variations at Selected
Locations [Data: www.pol.ac.uk/psmsl]
23. Mean Sea Level Variations in the Western
Indian Ocean [Data: www.pol.ac.uk/psmsl]
24. Mean Sea Level Variations in the Western
Indian Ocean [Data: www.pol.ac.uk/psmsl]
25. Mean Sea Level Variations in the Western
Indian Ocean [Data: www.pol.ac.uk/psmsl]
26. Mean Sea Level Variations in the Western
Indian Ocean [Data: www.pol.ac.uk/psmsl]
27. Some Observations on Sea
Level Trends in East Africa
Tide gauge records in the region are not long enough to give
any conclusive evidence on sea level rise. Fairly long records of
at least 50 years are needed because of the influence of natural
variability in the climate system (Douglas, 1992).
Some of the stations, including the oldest ones such as
Mombasa (1932) and Port Louis (1942) have significant record
gaps, making it difficult to examine the trends with certainty.
From the available data, sea level has been rising in some
stations whereas in others it has been observed to fall (e.g.
Zanzibar at - 3.9 mm/yr), probably due to decadal variability.
The decadal variability can be explained by the Aberdeen record
(1862-1965). There is a consistent fall in sea level during the
first 23 yrs, then a consistent rise during the remaining 77yrs.
The East African region is of special interest in the aspect of sea
level rise. Whilst there is worldwide trend of rise in sea level,
both tide gauge and satellite altimetry (e.g. Church et al, 2006)
indicate a falling trend in sea level in some parts of the region.
28. Some Observations on Sea
Level Trends in East Africa
The falling trend in sea level, though a rare occasion, is
not unique to the East African region. The sea level at
Helsinki has been declining at an average rate of 2mm/yr
over the past century (1879 and 2001).
There are records from a number of sea level stations
which are not in the PSMSL database but can be used to
observe the trend in sea level (e.g. Port Victoria, Saint
Paul, Dar es Salaam, Lamu, Dzaoudzi, Reunion, etc.).
Comparison cannot be made on sea level trends at
different periods. E.g., the sea level at Port Louis (1942-
1947) showed a rising trend (+2.8mm/yr), that of nearby
Port Louis II (1986-2003) indicates a falling trend of
1mm/yr.
Some stations have longer records than those in the
PSMSL database. However, use of such data requires
reducing the data to a common datum. Recommended
data sets of the PSMSL are the monthly and annual
“Revised Local Reference” (RLR) means.
29. Some Observations on Sea
Level Trends in East Africa
SLPR2 software can be used to convert data from the
UHSLC into generic format to observe the rate of sea level
rise or acceleration.
For long records, small, non-consecutive data gaps cannot
alter observations of the sea level trends (e.g. Brest).
Station records should be examined before rushing into
conclusions. E.g. at Takoradi, “data from 1966 onwards
looks irregular and unreliable”,. The entire records of
Durban and Rodriguez are also “flagged for attention”. At
Maputo, a benchmark was destroyed 1999 and replaced
by another one nearby. No direct relationship between the
two as they did not exist at the same time.
While the region is embarking on expansion and upgrade
of the existing sea level network, there is an urgent need
to build capacity in satellite altimetry so that current
trends in sea level can be monitored by both methods.
30. PHYSICAL IMPACTS OF SEA LEVEL RISE
PRIMARY IMPACTS
Inundation and displacement of wetlands and lowlands
Increased vulnerability to coastal storm damage and
flooding
Shoreline erosion
Saltwater intrusion into estuaries and freshwater aquifers
SECONDARY IMPACTS
Altered tidal ranges in rivers and bays
Changes in sedimentation patterns
Decreased light penetration to benthic organisms
Increase in the heights of waves
31. Inundation and displacement of
wetlands and lowlands
This, the most obvious impact of sea
level rise, refers both to the
conversion of dryland to wetland and
the conversion of wetlands to open
water.
In salt marsh and mangrove habitats,
rapid sea-level rise would submerge
land, waterlog soils, and cause plant
death from salt stress.
32. Increased vulnerability to coastal
storm damage and flooding
Sea level rise would increase the impact of tropical
cyclones and other storms that drive storm surges.
The effects would be disastrous on small island
States and other low-lying developing countries,
such as the Maldives, where 90 per cent of the
population lives along the coast.
Flooding due to storm surges will increase under
conditions of higher sea level. As is true at present,
damage due to flooding will be most severe when
the surges strike during high tide.
33. Shoreline erosion
While acknowledging that erosion is also caused by
many other factors, Bruun (1962) showed that as
sea level rises, the upper part of the beach is
eroded and deposited just offshore in a fashion that
restores the shape of the beach profile with respect
to sea level.
A rise in sea level immediately results in shoreline
retreat due to inundation. However, a 1 m rise in
sea level implies that the offshore bottom must also
rise 1 m. The sand required to raise the bottom can
be supplied by beach nourishment. Otherwise waves
will erode the necessary sand from the upper part of
the beach.
34. Saltwater intrusion into estuaries
and freshwater aquifers
Sea level rise would generally enable saltwater to advance
inland in both aquifers and estuaries. In estuaries, the gradual
flow of freshwater toward the oceans is the only factor
preventing the estuary from having the same salinity as the
ocean.
A rise in sea level would increase salinity in open bays because
the increased the cross-sectional area would slow the average
speed at which freshwater flows to the ocean.
The impact of sea level rise on groundwater salinity could
make some areas uninhabitable even before they were actually
inundated, particularly those that rely on unconfined aquifers
just above sea level. Generally, these aquifers have a
freshwater "lens" floating on top of the heavier saltwater.
As sea level rises, the depth of the freshwater lens in the
coastal zone is greatly reduced, leading to salinization of water
supplies. In extreme cases exacerbated by over-pumping, the
aquifer may rapidly become unsuitable for drinking and even
for irrigation.
35. Altered tidal ranges in rivers and bays
Sea level rise could change tidal ranges by:
Removing barriers to tidal currents
Changing the resonance frequencies of tidal basins.
Greater tidal currents would tend to form larger ebb
tidal deltas, providing a sink for sand washing along
the shore and thereby causing additional erosion.
Some of the bathymetric changes that might amplify
tides would have the same impact on storm surges.
Finally, higher tidal ranges would further increase
the salinity in estuaries due to increased tidal
mixing.
36. Changes in sedimentation patterns
Under natural conditions, most of the sediment
washing down rivers is deposited in the estuary due
to settling and flocculation. Settling occurs
downstream from the head-of-tide because the
slowly moving water characterized by estuaries can
not carry as much sediment as a flowing river.
Flocculation is a process by which salty water induces
easily entrained fine-grained sediment to coalesce
into larger globs that settle out. A rise in sea level
would cause both of these processes to migrate
upstream, and thereby assist the ability of wetlands
in the upper parts of estuaries to keep pace with sea
level, while hindering their ability in the lower parts.
37. Decreased light penetration to benthic organisms
If sea levels were to rise at a pace faster than corals
could build their reefs upward, eventually light
conditions would be too low for the zooxanthellae to
continue photosynthesis.
On reefs near low-lying coastal areas, sea-level rise
would likely increase coastal erosion rates, thus
degrading water quality and decreasing light
penetration, thus reducing the depth to which reefs
can grow.
Losses of coral reefs would mean losses in the high
biodiversity of these systems as well as the fisheries
and recreational opportunities they provide.
38. Increase in the heights of waves
A rise in sea level would also increase the size of waves. In
shallow areas, the depth of the water itself limits the size
of waves, which could be the most important impact of sea
level rise along shallow tidal embayments with steep,
muddy shores.
The steep slopes imply that inundation would not be a
problem. However, with water depths one meter deeper,
waves could form large enough to significantly erode the
muddy shores.
Bigger waves could also increase the vulnerability of lands
protected by coral reefs. In many areas, these reefs
protect mangrove swamps or sandy islands from the direct
attach by ocean waves; but deeper water would reduce the
reef’s ability to act as a breakwater.
The extent to which this will happen would depend on the
ability of the corals to keep pace with sea level rise.
39. Impacts of Sea Level Change:
The East African Experience
The Eastern African coastal zone is very heavily populated today
because of its growing industrial infrastructure.
It is estimated that 13% of the 62 million people in East Africa reside
along the coast due to rapid development of coastal activities such as
fishing, sea ports for imports and exports, coastal tourism and
industries.
The East African coastal zone is presently experiencing some coastal
degradation due to erosion along some sandy and low-lying beaches.
In Dar es Salaam, accelerated marine erosion and flooding in the last
decade have uprooted settlements and resulted in the abandonment of
luxury beach hotels.
Coral reefs, especially near Malindi in Kenya, are being damaged due
to excessive siltation caused by coastal erosion.
In Seychelles, several parts of the country have already experienced
coastal flooding, coastal erosion and loss of infrastructure as a result
of increased intensity of wave action and probably sea-level-change.
40. Predicted Future Impacts:
The East African Perspective
The coastal countries and islands of East Africa are highly vulnerable to
sea-level rise, having many low-lying structural developments including
major ports and cities, extensive farmlands, settlements and tourist
facilities located along low-lying parts of the coast.
Sea level rise would cause inundation of the extensive mangroves of
Mozambique and Tanzania and these would retreat, thus increasing rates of
erosion of the shoreline.
In Mauritius, applying the Brunn rule for calculation of future coastal
retreat within 2.2 km of the west coast, the whole area from the actual
shoreline to the tarred road will disappear following a sea level rise of 1 m
(Beebeejaun, 2000).
In Tanzania, Mwaipopo (1997) employed a sea-level rise scenario of 1
mm/yr to determine that about 2,117 km2 of land would be inundated, and
another 9 km2 of land would be eroded.
In Kenya, the most vulnerable sites are the Watamu and Sabaki River
estuary. It is also projected that with a 0.3m increase in sea level, about
17% of Mombasa district will be submerged (Oyieke, 2000).
Seychelles will be severely affected by sea level rise by virtue of the
concentration of economic activities on the coast, uniqueness of the coastal
environment, as well as current direct impacts on coastal processes and
resources.
41. Predicted Future Impacts:
The East African Perspective
Although the region experiences calm conditions through
much of the year, sea-level rise and climatic variation may
reduce the buffer effect of coral and patch reefs along the
east coast, increasing the potential for erosion.
Growing coastal activities have attracted large populations to
the coast and this has exerted big strains on coastal
groundwater resources. For example, Dar es Salaam is
heavily populated with over 85% of the industries situated in
and around the city.
The depth to water-table in the coastal zone is often very
shallow and is subject to saline sea water contamination and
pollution. An increased global sea level rise is expected to
raise the water-table along the coast and result in increased
salinity of the groundwater.
Many of the island states are already experiencing this
phenomenon and the situation is expected to worsen with sea
level rise.
42. RESPONSE STRATEGIES
There are three response strategies to rising sea level and its
physical impacts: RETREAT, ADAPT or DEFEND. In practice, many
responses may be hybrid and combined elements of more than
one approach.
Retreat can involve chaotic abandonment of property and cultural
investments, or it can be an ordered, planned program that
minimizes losses from rising sea level and maximizes the cost-
effectiveness of the operation.
The operation also seeks to leave surrendered areas as aesthetic
looking as possible and to avoid abandoned structures that are an
operational hazard to other social and economic activities.
Adaptation/Accommodation – all natural system effects are
allowed to occur and human impacts are minimized by adjusting
human use of the coastal zone. For East African countries,
adaptation is the immediate priority to respond to sea-level rise.
Defence/Protection – natural system effects are controlled by soft
or hard engineering, reducing human impacts in the zone that
would be impacted without protection.
43. SETBACKS
The assessment of impacts of sea level rise over the
next century is hindered by lack of knowledge of the
detailed topography of the near shore.
New global elevation maps based on detailed surveys
at cm resolution will make it possible to accurately
determine the areas which will be inundated by storm
surges under conditions of rising sea level.
This will require a concerted effort by the satellite
altimetry community as well as local ground-based
geodetic surveyors in all coastal areas world-wide.
