Getting into hot water global warming and rising sea levels
This 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.
In 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:
- Non-polar glaciers
If non-polar glaciers such as those in New Zealand and Norway melted, they would release water that may enter the ocean and contribute to a sea-level rise. Glaciers are rather sensitive to climate change and they could melt rapidly.
- Greenland ice sheet
In Greenland, ice increase from snowfall is balanced by ice loss from melting and the discharge of glaciers. Projections indicate that increased melting from higher temperatures would exceed any increases in precipitation. This change in the ice balance would add water to the ocean.
- Antarctic ice sheet
Nearly all of Antarctica is covered by an ice sheet that is, on average, 2.5 kilometres thick. If all the land-ice covering Antarctica were to melt that's around 30 million cubic kilometres of ice the seas would rise by over 60 metres! However, in the Antarctic it is so cold that even with increases of a few degrees, temperatures would remain below the melting point of ice. In fact, warmer temperatures could lead to more snow, which would increase the amount of ice in Antarctica.
- Thermal expansion
While thermal expansion is a less obvious process than melting ice (mainly because you can't see it happening) the IPCC projects that thermal expansion will be the main component of expected sea-level rises over the 21st century.
Uncertainty in estimates
It is difficult for scientists to be more precise with sea-level projections because there are a number of uncertainties:
- Greenhouse gas concentrations
While scientists agree that the levels of greenhouse gases are rising, future increases depend on many factors, including population growth, energy use and the development of new technologies.
- Climate sensitivity
Climate sensitivity is the amount of atmospheric warming that results from a doubling of atmospheric carbon dioxide concentrations. It depends on the presence of greenhouse gases, and on feedback processes from clouds, water vapour and ice. This is a significant source of uncertainty in projections of long-term climate change.
- Ocean heat exchange
Heat moves between the atmosphere and the ocean's surface. The temperature at the surface at any one time is influenced by what is going on in the ocean. Quite small changes in the transport of heat or salt can have large effects on surface temperature, and ultimately on climate. Ocean models have developed rapidly over the last two decades but accurately representing the most important ocean features remains a challenge.
There is uncertainty about the response of the ice sheets in Greenland and Antarctica to hundreds of years of warmer temperatures. Scientists are concerned that there could be a rapid disintegration of the West Antarctic ice sheet, causing a rapid rise in sea level.
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.
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.
Floods already cause more damage in Australia than any other natural disaster, in terms of cost to the community. CSIRO researchers believe that damage costs associated with coastal flooding would more than double in southern Queensland and northern New South Wales if sea levels were to rise by 40 centimetres.
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.
Future planning should take global warming and consequent sea-level rises into consideration. For example, building protective sea walls and restricting coastal development in areas at risk are planning measures that could minimise damage from rising sea levels over the next century.
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.
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Page updated May 2008.