Coasts and sea level rise

Global sea levels are rising and increasing the risk to coastal communities from inundation and erosion.

The principal components contributing to global average sea level rise are the melting of land-based snow and ice reserves and the thermal expansion of the ocean water mass.

While global average sea level rise is relatively consistent, there are significant regional variations throughout the ocean basins of the world, which are attributable to:

  • variations in the distribution of thermal expansion
  • local and regional meteorological effects
  • regional responses to modes of climate variability (for example, the El Nino-Southern Oscillation).

There is also considerable short and long-term variability in sea levels at any particular location, with this variability often extending over multiple decades.

There are two related measures of sea rise:

  • absolute sea level rise, which is the increase in the ocean water level
  • relative sea level rise, which is the increase in sea level recorded relative to land and is affected by land movement at the tide gauge site.

NSW Government policy

Guidance on the NSW framework for managing existing and future coastal management issues, including those related to climate change and sea level rise, is outlined here.

Recorded historical sea level rise

Recorded sea levels are influenced by factors such as tides, waves, storm surges, seasonal temperature effects, and longer-term effects due to large-scale phenomena like the El Nino-Southern Oscillation. Therefore, short-term sea level records reflect short-term trends that may be different from long-term trends.

A 2011 analysis of global tide gauge records and satellite altimetry has found that:

  • Global average sea levels increased by 210 mm from 1880 to 2009.
  • While there was considerable variability in the rate of rise during the 20th century, there has been a statistically significant acceleration since 1900.
  • Between 1993 and 2009, the estimated rate of rise was 3.2 ± 0.4 mm per year from the satellite altimetry data and 2.8. ± 0.8 mm per year from tidal records.

Further information on observed historical mean sea levels is available from:

Projected future sea level rise

The increase in global mean sea level over the last century is thought to be associated with global warming as a result of increasing greenhouse gas concentrations in the atmosphere. The IPCC concludes it is extremely likely that human influence has been the dominant cause of the observed warming since the mid-20th century and that future changes to climate are likely to depend on future greenhouse gas emissions.

In its fifth assessment report (2013), the IPCC has developed a range of future sea level rise projections associated with different greenhouse gas emission scenarios (representative concentration pathways (RCPs)). These indicate the following:

Scenario

Likely global mean sea level rise range by 2100

(relative to 1986-2005)

Significantly Reduced Emissions (RCP 2.6)

0.24–0.61 m

Highest Emissions (RCP 8.5)

0.54–1.06 m

They also suggest the possibility of up to several tens of centimetres above these values if marine-based sectors of the Antarctic ice sheet collapse.

Beyond 2100, the IPCC concludes that it is virtually certain that global mean sea level will continue to rise for many centuries owing to thermal expansion of the oceans.

Data provided by the IPCC also indicate that sea level rise along the east coast of Australia might be 0% to 10% above the global average by 2100 (relative to 1986–2005), with higher rates offshore.

Further information on sea level rise projections is available from:

Coastal impacts from sea level rise

Of all the impacts from climate change, the projected rise in mean sea level is a concern for management of the coastal zone in the longer term.

The potential implications of sea level rise over time include:

  • Higher projected storm surge and inundation levels.
  • Landward recession of sandy shorelines. Depending on the rate and scale of sea level rise, the environmental, social and economic consequences of recession within low lying inter-tidal areas, in particular, may be significant in the medium-long term.
  • Salt water intrusion and landward advance of tidal limits within estuaries. This may have significant implications in the medium-long term for freshwater and saltwater ecosystems and development margins, particularly building structures and foundation systems within close proximity to the shoreline.
  • Existing coastal gravity drainage, stormwater infrastructure and sewerage systems may become compromised over time as mean sea level rises.
  • Sea level rise will influence the entrance opening regimes for intermittently closed and open lakes and lagoons (ICOLLs) and alter catchment flood behaviour over time.
  • The level of protection provided by existing seawalls and other hard engineering structures will decrease over time due to the increasing threat from larger storm surges and inundation at higher projected water levels.

Further information on the coastal impacts of sea level rise is available from:

Historical and future impacts of East coast 'lows'

East coast 'lows' (ECLs) are amongst the most dangerous meteorological events affecting Australia, and certainly the most significant for coastal eastern Australia. They frequently have severe consequences in terms of flash flooding, wind damage, storm surge, and heavy and damaging seas. They are also responsible for heavy rainfall events that contribute significantly to total rainfall and runoff along the Australian East Coast.

The coarse resolution of the global climate models means they are currently unable to adequately model ECLs due to their relatively small spatial extent and short life span. Consequently, it is difficult to project how climate change will impact on this weather phenomenon.

Research has been undertaken to better understand the historical and future impacts of East coast lows and other major weather patterns on the eastern seaboard.

A snapshot of future sea levels: photographing the king tide

More than 250 people joined in photographing our foreshores in January 2009 when NSW experienced a king tide. See these images and learn more about tides, sea levels and climate change.

We had an overwhelmingly enthusiastic response to its request for people to assist in the photographic survey of the impact of the king tide which occurred on the morning of 12 January 2009. We had received over two thousand images of the event within a week of the king tide. These photographs provide a statewide snapshot of areas currently vulnerable to tidal inundation will assist planning for future implications of sea level rise.

The king tide was predicted to peak in Sydney at 2.05 metres at Fort Denison, but on the day the peak recorded was only 1.96 metres. Although some 9 cm short of the predicted king tide, the water level reached was still a very high spring tide level. It is important to understand that local meteorological and weather effects can have a very significant impact on water levels with variances of 20 cm not uncommon compared to the tide predictions. The fair weather and the presence of a high-pressure system across most of coastal NSW at the time were the primary reasons for the tide level not reaching the predicted maximum.

We would like to thank all those who volunteered their time and photographed the king tide.

Photographing King Tides

A Snapshot of Future Sea Levels: Photographing the King Tide (2009) (PDF 2.62 MB)

For more information please contact Principal Coastal Specialist on (02) 4320 4209 or by email info@environment.nsw.gov.au.

For additional technical information Fort Denison Sea Level Rise Vulnerability Study (PDF 1.26 MB)