Preparing for Sea-Level Rise
Pyper, Wendy, Ecos
The past 12 months have seen a deluge of newspaper articles, scientific papers and reports on climate change and global sea-level rise. One of the most anticipated was the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report--Summary for Policymakers, released in February, whose projections, with respect to sea-level rise, have been variously supported and criticised.
Key factors in the disagreement over the projections are scientific uncertainty about the future response of polar ice sheets to global warming and their subsequent contribution to sea level, and the way this uncertainty was reported by the IPCC.
'When you've got an unknown and you don't know how to quantify it, you're going to get disagreement on it" says oceanographer and IPCC reviewer, Dr John Hunter, of the Antarctic Climate and Ecosystems Cooperative Research Centre (ACE CRC). 'Some people will come up with a big number; others will come up with a smaller one.'
What is agreed is that global sea levels have risen about 20 cm since 1870, (2) due mainly to the thermal expansion of the ocean (3) and melting of non-polar glaciers, (4) and with a small contribution from the melting of the Greenland and Antarctic ice sheets, at least over the last decade. In 2001, the IPCC Third Assessment Report projected that these processes would see the global average sea level rise another 9-88 cm between 1990 and 2100.
Early this year, world renowned sea-level rise expert, Dr John Church, of the CSIRO and ACE CRC, along with international colleagues, compiled the most recent observed climate trends for carbon dioxide concentration, mean air temperature and sea level, and compared these to the 2001 IPCC model projections. (5) They found that sea-level rise is occurring faster than models projected and closely follows the upper trajectory towards an 88 cm rise by 2100.
'Sea-level data from satellites and tide gauges show that between 1993 and 2003 the rate of sea-level rise was about 3.1 mm/ year,' Church says.
'This is more than 50 per cent larger than the average rate (1.7 mm/year) over the 20th century. However, it is still unclear whether this increase reflects a further acceleration in sea-level rise or a natural fluctuation on a multi-decadal timescale.'
The IPCC Fourth Assessment Report projects a global sea-level rise of between 18 and 59 cm to 2095, relative to 1990 sea levels. At first these numbers seem significantly less than the range projected in the Third Assessment Report. But they come with an important caveat: a further 10-20 cm rise in sea levels could occur due to increased loss of ice from Greenland and, to a lesser extent, Antarctica, and 'larger values cannot be excluded' from these sources.
'If these 10 to 20 cm are added to the projections, the upper limit result is close to the upper limit in the Third Assessment Report of 88 cm by 2100,' Church says.
The '10-20 cm or more' caveat stems from the present inability to comprehensively model the ice sheets' responses to warming, and as a result the IPCC projections 'do not include the full effects of changes in ice sheet flow' because a basis in published literature is lacking.
'The IPCC is working from published data and has made its assessment based on the recent published material on accelerated ice sheet melt,' says Professor Nick Harvey, a lead author of the second volume of the IPCC's Fourth Assessment Report.
'Some scientists are warning of the potential for a greater sea-level rise, but the IPCC felt that there was not sufficient evidence at the time of its report to quantify the impact on projections.'
Professor Will Steffen, former Executive Director of the International Geosphere-Biosphere Program and an advisor to the Australian Greenhouse Office, says the uncertainty in the projections needs to be carefully interpreted.
'The Fourth Assessment Report has better estimates for the contribution of thermal expansion and glacier loss to sea level, which provides a narrower projection range, but a larger potential contribution of the polar ice sheets is not included,' he says. …