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position analysis:
climate change, sea-level rise and extreme events: impacts and adaptATion issues
position analysis
www.acecrc.org.au
Position Analysis: Climate change, sea-level rise and extreme events: impacts and adaptation issues. PA01–0809011 ISSN: 1835-7911 © Antarctic Climate & Ecosystems Cooperative Research Centre 2008. This work is copyright. It may be reproduced in whole or in part for study or training purposes subject to the inclusion of an acknowledgement of the source, but not for commercial sale or use. Reproduction for purposes other than those listed above requires the written permission of the Antarctic Climate & Ecosystems Cooperative Research Centre. Requests and enquiries concerning reproduction rights should be addressed to: The Manager Communications Antarctic Climate & Ecosystems Cooperative Research Centre Private Bag 80 Hobart Tasmania 7001 Tel: +61 3 6226 7888 Fax: +61 3 6226 2440 Email:
[email protected] www.acecrc.org.au
Established and supported under the Australian Government’s Cooperative Research Centre Program.
Photo cover: ACE CRC Photo this page: ACE CRC
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climate change, sea-level rise and extreme events: impacts and adaptATion issues
climate change, sea-level rise & extreme events: impacts and adaptATion issues
1. introduction This position analysis outlines recent developments in the science of sea-level rise and its influence on the effects of extreme events such as high tides and storm surges. It also identifies some of the key issues around the potential impacts of sea-level rise and extreme events and the consequences they will have for Australia and its neighbours.
The aims of this position analysis are to: 1.
inform Australian governments and community about recent results in the science of sea-level rise and associated events that are a consequence of climate change;
2.
outline some of the likely impacts of these changes on Australia and neighbouring countries in the South Pacific and Indian Ocean regions; and
3.
identify issues that will need to be considered in developing policies to respond to sea-level change and its impacts.
This position analysis draws heavily on papers by Dr John Church et al. (2001; 2006a) and Dr John Hunter (2007) of the ACE CRC, and the Third and Fourth Assessment Reports (TAR and AR4) published in 2001 and 2007, respectively, by the Intergovernmental Panel on Climate Change (IPCC). The term ‘extreme sea-level events’ in this document refers to events during which the sea level exceeds some specific fixed level.
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2. the science of sea-level rise Climate has changed throughout Earth’s history, varying through four cold (glacial) cycles over the past 400,000 years (IPCC 2007, Ch 6). These cycles are believed to have been driven by, among other things, small changes of the Earth’s orbit that caused subtle changes in the amount and timing of incoming solar radiation. This small effect was amplified by a number of processes, such as changes in the reflectivity of the ground when covered by ice or snow, or removal of carbon dioxide (a greenhouse gas) from the atmosphere into the ocean or by marine plants.
Photo: Simon Marsland
Accompanying this climate change have been large changes in sea-level as the Earth’s climate oscillated between the glacial periods and warmer (inter-glacial) periods over thousands of years. This sea-level change was driven predominantly by melting and freezing of ice on land. Cooling during the glacial cycles led to the build-up of ice on land, primarily in the Northern Hemisphere. This locked up water as land-based ice rather than water in the ocean, resulting in sea-level falls of more than 100 m. When the atmosphere warmed again, much of the land ice melted and flowed into the ocean, resulting in a rise in sea level (Figure 1). There would also have been some warming and cooling of the ocean, resulting in expansion of a warmer ocean (termed thermal expansion) and contraction of a cooler ocean.
Figure 1. p History of global sea level over the past glacial–interglacial cycle relative to modern-day levels (indicated by the horizontal line at 0 on the graph). The thickness of the green line represents the uncertainty. The red rectangle (top right) indicates the period shown in Figure 2. After Lambeck and Chappell, 2001; Lambeck et al., 2002; Lambeck, 2002; Lambeck et al., 2004.
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climate change, sea-level rise and extreme events: impacts and adaptATion issues
climate change, sea-level rise & extreme events: impacts and adaptATion issues
2. the science of sea-level rise Following the peak of the last ice age 21,000 years ago (the ‘glacial maximum’) sea level rose by as much as 4 m per century (Church et al., 2001) as the climate warmed and land-based ice melted and drained into the ocean. Around 2,000 years ago, sea-level rise had almost stopped (Figure 2) and from 1,000 years ago to the late 19th century, sea-level was relatively stable, varying by less than about 0.2 m over the whole period.
But we are now living in a world in which the climate is being substantially modified by human activity, and it is happening at a pace that the Earth has not experienced for at least 5,000 years. Increases in greenhouse gases in the atmosphere during and since the 19th century, caused primarily by humaninduced emissions, have contributed substantially to warming of the atmosphere.
Figure 2. p Global sea level over the past 6,000 years relative to modern-day levels (indicated by the horizontal line at 0 on the graph). The thickness of the green line represents the uncertainty. The plotted record ends at about 1900. After Lambeck and Chappell, 2001; Lambeck et al., 2002; Lambeck, 2002; Lambeck et al., 2004.
Global atmospheric temperature increased by 0.76°C from 1850–1899 to 2001–2005 (IPCC, 2007) resulting in both warming of the oceans and melting of ice on land. The sea level rose significantly during the 20th century, at a rate that has not been experienced for the past 5,000 years (Figure 3 on page 6). Church and White (2006) used a combination of tide-gauge records and satellite altimeter data to estimate sea levels from 1870 to 2004 and showed a global average rise of 0.17 m during the 20th century. Figure 3 also shows that the sea-level rise for Australia (the light blue line) was only slightly less than the global average.
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2. the science of sea-level rise The analysis also suggests that sea level has risen faster over the past decade than at other times in the last century, a conclusion supported by other analyses using only tide-gauge data (Holgate and Woodworth, 2004; Jevrejeva et al., 2006). Unlike the sea-level changes associated with the glacial-interglacial cycles, recent sea-level rise has increasingly been driven by a warming ocean and the corresponding thermal expansion.
Thermal expansion and the melting of glaciers and ice caps (excluding the Greenland and Antarctic ice sheets) contribute similar amounts to current sea-level rise (IPCC 2007). Domingues et al. (2008) have recently shown that ocean thermal expansion contributed 50% more to total sea level rise for the second half of the 20th century (1961–2003) than estimated by the IPCC (2007). Extremes of high sea-level events depend on changes in long-term average sea level as well as fluctuations around that
average caused, for example, by tides, atmospheric pressure and storm surges. Any changes in these periodic events will also have effects on the extremes of sea level experienced at the coast. Even without any increase in weather-related events, a rise in the average sea level will mean that extreme sea levels that currently cause occasional inundation or wash-over at a given location will do so with increasing frequency in the future, causing increasingly severe impacts on coastal communities, infrastructure and habitats.
Figure 3. q Sea-level reconstruction of Church and White (2006). Historical sea-level reconstruction is shown by the thin black line, with grey shading indicating ±95% confidence limits. Magenta dots and arrows indicate the estimate of sea-level rise from 1842–2001 from Port Arthur, Tasmania (Hunter et al.2003). Dashed grey lines indicate the upper and lower limits of sea-level projections (1990–2100) from the IPCC Third Assessment Report (Church et al., 2001). The darker green shading indicates the range in projected sea-level rise under different greenhouse gas emissions scenarios averaged across several global climate models, whilst the broader pale green shading shows the range of projected sea-level rise across all models and emissions scenarios, excluding allowance for changes in land-based ice. The red line indicates the sea level measured by satellite altimeters, which shows that observed sea level has been tracking the upper limit of projections since 1990. The light blue line shows the estimate of relative sea level around Australia (Church et al., 2006a). The dark blue curve is a quadratic fitted to the sea-level data from 1870 to 2001 Figure courtesy Neil White, CSIRO.
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climate change, sea-level rise and extreme events: impacts and adaptATion issues
climate change, sea-level rise & extreme events: impacts and adaptATion issues
2. the science of sea-level rise Studies of extreme sea levels from many locations around the world have indicated that sea-level rise is generally the dominant cause of more frequent extreme sea-level events (IPCC 2007; Woodworth and Blackman 2004). The long tide-gauge records from Fremantle and Fort Denison (Sydney) show that the return period1 of flooding that occurred on a yearly to decadal basis pre-1950, decreased by a factor of about three from the pre-1950 period to the post-1950 period (Figure 4, Church et al., 2006a). This was mainly caused by sea-level rise. The relatively small rise in sea level that has occurred during the 20th century has therefore already caused a significant change in the frequency of extreme sealevel events around Australia’s coast.
Sea level will continue to rise during the 21st century. Mitigation of greenhouse gas emissions might attenuate sea-level rise in the long-term but we are already locked into significant sea-level rise during this century. Global sea levels are likely to rise by around 0.03 m in the next decade and by 0.2–0.8 m by the end of this century (IPCC, 2007).
Sea-level rise is a significant long-term climate change issue that has important implications now and into the future for us all, internationally, nationally and locally.
Even a moderate rise in sea level of 0.2–0.5 m will have serious implications for coastal zone management, including for communities, infrastructure, planning schemes, aquaculture, and coastal ecosystems – in Australia, the Indian Ocean and South Pacific regions – and globally.
Figure 4. q Change of return period for extreme high levels from pre-1950 to post-1950 conditions. Plots for Fort Denison (left) and Fremantle (right) show the return periods (on a logarithmic scale on the horizontal axis) for sea-level events of specific levels (vertical axis) prior to 1950 (red lines) and since 1950 (blue lines). The shift of the curves to the left (from red to blue) indicates that events of given levels (examples indicated by horizontal dashed grey lines) have happened approximately 3 times more often on average since 1950 (vertical dashed green lines) than before 1950 (vertical dashed orange lines). Source: Church et al., 2006a.
1 The return period (or average recurrence interval) is the average period between extreme events of a given height.
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3. contributions to sea-level rise There is still much uncertainty
While the total of the contributions statistically agrees with sea-level rise observed by satellites for 1993–2003, three caveats are important:
surrounding what contributes to the sea-level rise we are seeing today. In particular, the
1.
The uncertainties are quite large, about 25% of the total.
2.
There are now better estimates of the ocean thermal expansion (Domingues et al., 2008) than cited in Table 1. Future estimates of thermal expansion are likely to further improve the agreement between the total of the sea-level rise contributions and the satellite observations.
3.
Land-based (terrestrial) storage2 of water is possibly a significant influence on sea-level rise but has been omitted from the contributions to sea-level rise listed in Table 1. This land storage was estimated to contribute -0.35 ± 0.75 mm/year to mean sea levels during 1910–1990 by the IPCC Third Assessment Report. It was omitted from the Fourth Assessment Report because it is “poorly known” and because “...indirect evidence from considering other contributions to the sea-level budget…suggests that the land contribution either is small (
