Getting into hot water – global warming and rising sea levelsThis topic is sponsored by the Australian Greenhouse Office. The 20th century saw the greatest increase in temperature of any century during the last thousand years, and the last decade was the warmest since records began. As the temperature rises, so does the sea level – with profound consequences for us all. Key textIn its 2001 assessment of global warming, the Intergovernmental Panel on Climate Change (IPCC) projected that global mean sea level is expected to rise between 9 and 88 centimetres by 2100, with a 'best estimate' of 50 centimetres.Higher temperatures lead to higher sea levels A warmer world will have a higher sea level because as the land and lower atmosphere of the world warm, heat is transferred into the oceans. When materials are heated they expand (thermal expansion). So the heat that is transferred causes sea water to expand, which then results in a rise in sea level. In addition, water from land-based ice such as glaciers and ice sheets may enter the ocean, thus adding to the rise. A point to remember is that no extra water is added to the oceans when ice floating in the ocean melts. As floating ice melts, it only replaces the volume of water that it originally displaced. Melting or expansion? Which contributes the most to sea-level rise – melting ice or the thermal expansion of water? The answer depends on the time-scale you're interested in. Warmer temperatures could lead to the following scenarios:
Uncertainty in estimates It is difficult for scientists to be more precise with sea-level projections because there are a number of uncertainties:
As well as these global uncertainties, the rate and magnitude of sea-level change will vary from place to place in response to changes in ocean currents and vertical movements of the land itself. In some areas, sea level may actually fall. What is the impact of 50 centimetres? On average, it is expected that by 2100 sea levels will have risen in most places by around half a metre. Reduced to a raw number like this it doesn't sound like too much. What impact does 50 centimetres have on anything? Maybe you'll just have to build your sandcastles a little higher up the beach. The reality promises to be a little grimmer. In many places, 50 centimetres would see entire beaches being washed away, together with a significant chunk of the coastline. For people living on low-lying islands such as Tuvalu, Kiribati or the Maldives, where the highest point is only 2-3 metres above current sea levels, an extra 50 centimetres could see significant portions of their islands being washed away by erosion or covered by water. Even if they remain above the sea, many island nations will have their supplies of drinking water reduced because sea water will invade their freshwater aquifers. While these islands have sizeable populations, they're insignificant compared to the tens of millions of people living in the low-level coastal areas of southern Asia. These include the coastlines of Pakistan, India, Sri Lanka, Bangladesh and Burma. Australian coasts Each centimetre of sea-level rise will lead to increasing impacts on low-lying coastal land. Modelling predicts the inundation would cause sandy beaches on the Australian coastline to recede by the order of 100 times the vertical sea-level rise. For example, if the sea level rises by a metre, the coastal beaches could retreat by about 100 metres unless some preventative action is taken. Given that about 85 per cent of Australia's population lives within an hour's drive of the coast, this is particularly relevant.
Low-lying coastal ecosystems, such as the freshwater wetlands that make up about 90 per cent of the coastal zone of Kakadu in the Northern Territory, are also vulnerable. Hundreds of species of birds, reptiles and amphibians depend on these freshwater areas. Intrusion of salt water is already a major management issue in Kakadu. If sea levels around Australia rise by about 50 centimetres, these freshwater wetlands will become saltier. A 1-metre rise in sea level would transform lowland Kakadu almost totally into mangrove forest.
A changing world Even if greenhouse gas emissions could be stabilised by the end of the 21st century, sea-level rise from ocean thermal expansion may only have reached half its eventual level by the year 2500. To minimise the impacts of climate change, we need to start changing our habits as soon as possible – Australia is one of the largest per capita greenhouse gas emitters in the world. The longer we delay, the less effective our actions will be. Related Nova topics: Enhanced greenhouse effect – a hot international topic The Southern Ocean and global climate Carbon currency – the credits and debits of carbon emissions trading Coral bleaching – will global warming kill the reefs? Warmer and sicker? Global warming and human health Impact of global warming on biodiversity
Activities
Further readingAustralasian Science April 2008, page 10 Rising sea levels already threatening coral atolls.
November/December 2006, pages 19-22 Sea change threatened by climate change (by John Church) Warns that sea level rise will have serious consequences during the 21st century and will impact on millions of people.
March 2006, pages 10-11 Sea rises have accelerated Satellite and tide gauge records indicate that sea levels rose during the 20th century.
November/December 2005, page 9 Acidic oceans threaten species Explains how increasing atmospheric carbon dioxide is causing oceans to become more acidic.
May 2005, pages 31-33 Ancient mangroves reveal rapid sea rise (by Kerie Hull) Describes evidence of rapid sea level rise, indicating a sudden climate shift.
Cosmos June/July 2008, pages 70-77 The big melt (by Tom Clynes) Describes the effect of global warming on ice sheets.
Ecos No. 137, 2007, pages 14-17 Preparing for sea-level rise (by Wendy Pyper) Looks at the preparation and mitigation planning for sea level rises in Australia.
No. 133, 2006, page 4 Climate change planning call for ‘sea change’ regions (by Craig Macaulay) Looks at the need to manage climate change, focussing on the rise of sea levels.
No. 119, 2004, page 4 Sea levels are rising while storms intensify Suggests that higher seas and changes to cyclone intensity are likely due to global warming.
No. 99, 1999, pages 18-19 Glimpses of a warmer world (by Graeme O'Neill) Explains that climate change may be different in the southern hemisphere because of effects of the Southern Ocean.
New Scientist 4 June 2008, pages 42-45 Climate scientists go with the floe (by Ben Crystall) Describes an expedition to observe climate changes in the Arctic.
28 July 2007, pages 30-34 Huge sea level rises are coming – unless we act now (by James Hansen) NASA physicist discusses the potential impact of greenhouse gas emissions on sea levels.
15 April 2006, pages 42-46 Climate change: The great Atlantic shutdown (by Stephen Battersby) Looks at the possible effects of slowing of the thermohaline currents in the Atlantic Ocean due to melting ice.
18 February 2006, pages 8-9 Disappearing deltas could spell disaster (by Jeff Hecht) Looks at the numbers of people in danger of displacement due to rising sea levels.
2 October 2004, pages 8-9 Melting ice, global warning (by Bob Holmes) Looks at three studies that are tracking the melting of glaciers in Antarctica.
14 August 2004, page 14 Rome's ancient fisheries confirm sea-level fears (by Jeff Hecht) Ancient coastal fish pens have provided the most accurate record so far of changes in sea level over the past 2000 years.
15 September 2003, pages 6-7 Global warming: The new battle (by Jenny Hogan) Suggests that climate change is unstoppable and the priority is to prepare for its consequences.
20 July 2002, page 12 Islanders 'can't go home' (by Fred Pearce) Explains that people removed from Chagos Archipelago may not be able to return because these coral islands are threatened by rising sea levels.
1 June 2002, page 6 The icehouse effect (by Fred Pearce) Explains why there is more ice around Antarctica even though temperatures have increased.
20 April 2002, pages 38-41 Deep secrets (by Kate Ravilious) Describes how a record of sea level changes over the past two ice ages was elucidated from a stalagmite in an Italian cave.
RTD Info Mysteries of the ocean Describes changes in the ocean due to global warming. Includes What is happening in the North Atlantic?, The unique relationship between the sea and CO2 and The strange world of oceanic methane.
Science 17 February 2006, pages 963-964 The Greenland ice sheet and global sea-level rise (by Julian Dowdeswell) Suggests that current estimates of sea-level rise are too low, due to the accelerated rate of melting of Greeenland ice sheets.
Scientific American 13 June 2008 Are Arctic sea ice melts causing sea levels to rise? Describes the effect of melting ice sheets on sea levels.
24 March 2005 Climate model predicts greater melting, submerged cities (by David Biello) Describes research predicting the extent of sea rises due to global warming.
March 2004, pages 40-49 Defusing the global warming time bomb (by James Hansen) Presents geologic evidence that human activities are shifting global climate but also suggests that practical actions could slow the process.
December 2002, pages 66-71 On thin ice? (by Robert A. Bindschadler and Charles R. Bentley) Discusses the shrinking of West Antarctica's ice sheet.
Useful sitesGlobal warming and rising oceans (Actionbioscience.org)
A good overview article. Also provides annotated links to relevant sites.
Global sea-level rise (CSIRO Marine Research, Australia) Looks at causes of sea-level rise, past changes in Australian sea levels and future predictions.
Australian Antarctic Division
Australian Broadcasting Corporation
Sea levels, oceans and coastal areas (Climate change information kit, United Nations Framework Convention on Climate Change)
Understanding 20th century sea-level rise and projections for the future by Dr John Church (Australian Academy of Technological Sciences and Engineering)
This paper discusses instrumental observations of sea-level change, the contributions to sea-level rise from
different sources (eg, thermal expansion of the ocean, ice sheets) and estimations of sea-level rise to 2100 and beyond. Uses some technical language.
Changes in sea level (Global Resource Information Database, Norway) This chapter on sea-level change is from the IPCC's third assessment report, Climate Change 2001: The Scientific Basis. The report is a comprehensive and up-to-date scientific assessment of past, present and future climate change. The chapter includes an executive summary and six sections (eg, 'Factors contributing to sea level change' and 'Can 20th century sea level changes be explained?') that can be accessed individually.
Maps and graphics (United Nations Environment Progamme GRID Arendal)
Provides a wide range of graphics showing the impact of climate change, including rising sea level.
Sea level rise (OzEstuaries, Australia)
Provides estimates of global sea level rise and a table of observed sea level changes around Australia.
Sea level rise (Firetree.net)
Shows map of Australia with possible sea levels due to global warming. Controls for sea level rise and zooming in on a region are available.
Glossaryaquifer. A layer of rock or sand that contains water. For more information see Aquifers (Tasmanian Department of Primary Industries and Water, Australia). feedback. The process whereby the output of a system affects the input. Positive feedback reinforces or increases something; negative feedback acts to keep a process within certain limits. Positive feedback can work in systems by amplifying a very small effect, changing the previous equilibrium. greenhouse gas. A gas that is transparent to incoming solar radiation and absorbs some of the longer wavelength infrared radiation (heat) that the Earth radiates back. The result is that some of the heat given off by the planet accumulates, making the surface and the lower atmosphere warmer. For more information see The greenhouse effect (CSIRO Atmospheric Research, Australia). ice sheet. A very large body of land-based ice. Ice sheets are found in Greenland and Antarctica and are also known as continental glaciers. Intergovernmental Panel on Climate Change (IPCC). An organisation established in 1988 by the World Meteorological Organization and the United Nations Environment Programme to provide the most widely accepted information available about climate change. The IPCC does not conduct new research or monitor climate-related data, its mandate is to assess existing data and to produce balanced and objective reports. For more information see Intergovernmental Panel on Climate Change (Switzerland).
External sites are not endorsed by the Australian Academy of Science. Posted March 2004. The Australian Foundation for Science is also a supporter of Nova.
This topic is sponsored by the Australian Greenhouse Office.
|