This paper commences with a discussion of the sea level rise policy of the NSW
Government. The link between sea level rise and coastal erosion is discussed,
and the first half of the paper concludes with a review of amendments to
coastal protection legislation introduced in May 2010 by the NSW Government.
The second half of the paper reviews the science behind measuring and
projecting global sea levels. It is readily apparent that scientists are
grappling with an extraordinarily difficult science, with many unknowns and
sparse and often conflicting data.
Global mean sea level is not fixed. It changes in response to such things as
ocean temperature and the amount of water trapped as ice on land. The
Intergovernmental Panel on Climate Change (IPCC) provides reports on the
current state of knowledge of climate change. The most recent report –
Assessment Report 4 (IPCC AR4) in 2007 projected sea level rise to be between
18 cm and 59 cm higher in the decade 2090-2099 than compared to
1980–1990. Some scientists are now arguing that this is a conservative
estimate, and that a rise of up to 110 cm is not out of the question.
Consequences of sea level rise include inundation of land and the erosion of
coastlines.
[1]
The NSW Sea Level Policy
In October 2009 the NSW Government released a Sea Level Rise Policy. Whilst
there are several elements to this policy, the central tenet is that of
promoting adaptive risk-based management. The Policy states that planning and
investment decisions should consider the sea level rise projections over
timeframes that are consistent with the intended timeframes of the decision.
The NSW Government has adopted sea level rise planning benchmarks to support
this adaptive risk-based approach. The primary purpose of the benchmarks is to
provide guidance supporting consistent considerations of sea level rise
impacts, within applicable decision-making frameworks
The NSW sea level rise planning benchmarks are an increase above 1990 mean sea
levels of 40 cm by 2050 and 90 cm by 2100. The policy states that the
benchmarks were established by considering the most credible national and
international projections of sea level rise and by taking into consideration
the uncertainty associated with sea level rise projections.
The sea level rise planning benchmarks can be used for purposes such as:
Incorporating the projected impacts of sea level rise on predicted flood
risks and coastal hazards;
Designing and upgrading of public and private assets in low-lying coastal areas
where appropriate, taking into account the design life of the asset and the
projected sea level rise over this period;
Assessing the influence of sea level rise on new development;
Considering the impact of sea level rise on coastal and estuarine habitats
(such as salt marshes) and identifying valuable habitats at most risk from sea
level rise; and
Assessing the impact of changed salinity levels in estuaries, including
implications for access to fresh water. [2]
Sea Level Rise and Coastal Erosion
It is considered well established that sea level rise generally leads to
erosion and causes the shoreline to retreat landwards. Australia’s
coastline has been remarkably stable over the past 200 years or so, given that
sea levels have risen by 17 centimetres over that time. Generally in Australia,
beaches appear not to be receding on a large scale, except in some localised
places where natural recession is occurring.
Generally, the most common experience of acute coastal erosion in Australia has
been associated with transient erosion due to storm events involving large
waves and abnormally high water levels, especially when storm surges coincide
with spring tides.
Coastal scientists have determined that for the most part, property and
infrastructure losses associated with extreme storms reflect the inappropriate
construction of property within dynamic coastal environments that were
mistakenly thought to be stable. Almost all the chronic coastal erosion
hotspots around Australia lie in regions where sand is transported along the
shore by natural processes. Erosion occurs when this sand transport is
interrupted, and in most cases this disturbance is caused by coastal
engineering works.
Scientists have not been able to detect an Australian coastal erosion response
to sea level rise to date. One of the main reasons for this is the wide
availability of sand in the coastal shore face – that area seaward of the
foreshore and to a distance beyond where the waves break. Evidence for sand
supply to beaches from the lower shore face is common around the Australian
coast. For example, off the NSW coast sand is estimated to accumulate onto
beaches from the shore face at the rate of 6m
3 per metre per year.
Off the Victorian coast the estimate is 10m
3 per metre per year.
Results from geological investigations in south-east Australia and sediment
transport modelling from the Netherlands coast indicates that sand supply from
the lower shore face fully compensates for the effects of significant relative
sea level rise.
Coastal erosion, especially that of private property, is the cause of
significant community concern. Understandably, owners of waterfront properties
are generally reluctant to stand back and watch their land, and any development
on it, be eroded away. For their part, local and state government have
difficult decisions to make concerning the extent, if any, to which they should
permit or prevent land owners from protecting their property.
[3]
The Coastal Protection and Other Legislation Amendment Bill 2010
With the introduction of the
Coastal Protection and Other Legislation
Amendment Bill 2010, the NSW Government stated that its approach is
to provide the tools needed to achieve an appropriate balance between the two
competing goals of protecting private property and preventing the transference
of erosion onto other sites.
To address coastal erosion and projected sea level rise, the Bill makes
amendments to the
Coastal Protection Act 1979 and other legislation. The
Bill has two main components:
It allows landowners to place emergency coastal protection works such as
sandbags on beaches and sand dunes to mitigate erosion in specified
circumstances without obtaining development consent or other specified
permission;
It allows local councils to make and levy an annual charge for the provision of
coastal protection services (such as the building of a rock wall) on rateable
land that benefits from such services. [4]
The Science of Sea Level Rise
Over decadal and longer time scales, global mean sea level change results from
two major processes that alter the volume of water in the global ocean: thermal
expansion; and the exchange of water between oceans and other reservoirs of
water such as the ice caps, glaciers and ice sheets. The measurement of global
sea level is reliant on two techniques: tide gauges and satellite altimetry.
On the available evidence, the IPCC AR4 concluded that global sea level had
risen during the 20
th century at 1.7 ±0.5mm per year, and from 1961
to 2003 at 1.8 ±0.5mm per year. Since 1992, global mean sea level can be
computed by satellites. Using this method, the IPCC AR4 reported that sea level
over the period 1993 to 2003 showed a rise of 3.1 ±0.7mm per year.
Global sea level rise is subject to considerable decadal variability. The rate
of sea level change was found to be larger in the early part of last century
(2.03 ±0.35mm per year 1904-1953) in comparison to the latter part (1.45
±0.34mm per year 1954-2003). The highest decadal rate of rise occurred in the
decade centred on 1980 (5.31 mm per year), whilst the lowest rate of rise
occurred in the decade centred on 1964 (-1.49 mm per year). Whilst satellite
measurements showed global mean sea level rising at a rate of 3.1 ±0.7mm in the
decade to 2003, for the period 2003 to 2008 it has since slowed to about 2.5mm
per year.
Much of the uncertainty about the rate of sea level rise in the future centres
on the behaviour of the large polar ice sheets. In recent years, the velocities
of outlet glaciers in coastal regions of Greenland and Antarctica have
accelerated, showing that a large fraction of ice-mass loss occurs through
dynamic processes rather than surface melting. The dynamic response of the ice
sheets to present-day climate may thus play a much larger role than previously
assumed.
[5.2] – [5.4]
Projections of Future Sea Level
Climate models are used to project sea levels into the future. The IPCC has
projected sea level to rise 18 cm by 2090-2099 at the lower end of the scale,
and at the upper end 59 cm. On a per year basis, the rate of sea level rise by
2090-2099 is calculated to be 1.5mm at the lower end, and 9.7mm per year at the
other extreme. In all scenarios, the IPCC stated that average rate of rise
during the 21
st century is very likely to exceed the 1961 to 2003
average rate of 1.8 ±0.5mm per year.
The IPCC noted that thermal expansion of the ocean is the largest component of
the projected sea level rise, contributing 70 to 75% of the central estimate in
these projections for all scenarios. The dynamical response of ice sheets was
not factored into the IPCC projections due to the fact that they cannot be
modelled quantitatively with confidence.
Since the publication of the IPCC AR4 report in 2007, scientists have projected
sea level rise from 75 to 190cm, approximately three times as much as the IPCC
AR4 projections. There is still considerable uncertainty surrounding estimates
of future sea level rise. Steffen observes that nearly all of these
uncertainties operate in one direction – towards higher rather than lower
estimates.
Sea level rise will impact on extreme sea level events. With a mid-range
sea-level rise of 0.5 metres in the 21st century, sea level events that now
happen every 10 years would happen about every 10 days in 2100.
[6]
Susceptibility of the Australian Coast to Sea Level Rise
Australia wide, a study found that a sea level rise of 1.1 metres combined with
a 1 in 100 year storm surge would result in the flooding of between 157,000 and
247,600 properties. Nearly 39,000 buildings are located within 110 metres of
‘soft’ shorelines and at risk from accelerated erosion due to
sea-level rise and changing climate conditions. For NSW the analysis suggested
that between 40,800 and 62,400 residential buildings may be at risk of
inundation.
Local government areas in NSW that have the greatest level of risk are Lake
Macquarie, Wyong, Gosford, Wollongong, Shoalhaven and Rockdale, which
collectively represent over 50 per cent of residential buildings at risk in New
South Wales. The inundation analysis also indicated that the local government
areas of Great Lakes, Rockdale and Shellharbour have a high proportion of
existing residences at risk within their boundaries, with a substantial
18–20 per cent of existing buildings potentially affected by 2100.
Erosion due to higher sea levels is also a key risk for coastal areas. In New
South Wales there are approximately 3,600 residential buildings located within
110 metres of ‘soft’ erodible shorelines, of which approximately
700 are located within 55 metres of ‘soft’ coast. Of the coastal
local government areas, Sutherland and Port Stephens have the highest number of
potentially affected properties.
[7]