ABC Science 14 Mar 12;
What causes sea level rise? Why is the average sea-level rise around Australia higher than the global mean? And why is the sea rising at different rates around Australia?
Looking out to sea it's hard to imagine that the seemingly flat expanse of ocean in front of you is rising. But according to satellite and tide-gauge data collected since 1993, global sea levels are rising on average about 3 millimetres per year, faster than the average for the 20th century as a whole.
The major factors contributing to global sea-level rise are expansion of the ocean as the water warms and melting of ice on land, leading to an increase in volume of the ocean, says Dr John Church from CSIRO Marine and Atmospheric Research.
"These processes will continue to contribute to rising sea levels during the 21st century and for centuries into the future," Church says, adding that "changes in ice sheets have caused changes in sea level of over 100 metres in the past."
Another smaller contribution is the change in volume of liquid (rather than frozen) water stored on land. For example, extensive flooding in Australia, Asia and South America during the 2010/11 La Niña event caused a small and temporary fall in sea level as water was transferred onto land. By the end of 2011 sea levels had begun to rise again as this water flowed back to the ocean.
Sea-level rise around Australia
Despite appearances, the ocean is not flat — it is a constantly shifting mass affected by factors such as ocean temperatures, currents and atmospheric winds. This means there are differences between the global-average sea-level rise and local rates of rise.
At an average of about 5 millimetres per year, sea levels are rising faster around Australia since 1993 than the global average. And the rate of sea-level rise varies around the coast of Australia.
In the north and north-west, sea levels are rising between 7 and 11 millimetres per year since 1993 — that's two to three times the global average. On the central east coast and areas to the south of Australia rates are mostly similar to the global average.
Regional variations in sea level rise are influenced by range of factors including interannual and decadal fluctuations in climate, says Church.
"That larger rate of rise in the north and north-west is probably dominated by climate variability — El Niño/La Niña cycles as well as a longer period decadal change like the Pacific Decadal Oscillation," says Church.
"This is a movement of water around the ocean, but with little change in overall volume, like sloshing water in a very big bathtub."
"If you go from a period of La Niña to El Niño or a period of El Niño to La Niña then you'll see a trend in sea level across the Pacific Ocean," says Church.
During La Niña periods, trade winds blow warm water westward across the Pacific, through the Indonesian Archipelego to the the Indian Ocean, where they are clearly seen in the Fremantle tide-gauge record, he says.
But during El Nino periods the trade winds relax and warm water flows eastward in the equatorial Pacific Ocean.
"For example, during 1997/98 sea levels mostly fell around Australia, and in the western equatorial Pacific and the eastern equatorial Indian oceans," he says.
While the El Niño/La Niña causes short term 'troughs' and 'spikes' in sea level, the Pacific Decadal Oscillation produces long-term patterns over several decades.
"The interannual variability produced by El Niño/La Niña is large, of the order of tens of centimetres in some spots, but superimposed on that is the rising trend which over time is likely to be larger than El Niño/La Niña," says Church.
Local anomalies
Within this rising trend are local anomalies, thought to be attributed to the changing "solid" Earth and shifting ocean currents.
"One example is Hillarys in Perth where groundwater extraction has led to subsidence and an apparent larger rate of sea-level rise compared to the land," says Church.
In contrast, coastal sea levels at Port Kembla in New South Wales aren't rising as fast as the ocean offshore. "We believe that is due to changes in the strength of the East Australian Current (EAC)," says Church.
The EAC is a major current that flows down the east coast of Australia, bringing warm water from the tropics.
"The EAC is extending south and that brings warm water and high sea levels to the offshore side of the EAC, but across the EAC there's also a slope in sea level so that rising sea level offshore is not felt as strongly at the coast."
Dr John Church from CSIRO Marine and Atmospheric Research was interviewed by Genelle Weule.
