Chapter 4 Climate Impacts and Responses

Impacts of Climate Change on Australia > The marine environment

Evidence of a global rise in sea level over the past 100 years of between 10 and 20 cm comes from measurements around the world, corrected for land movements. The rise is primarily a result of increasing water temperatures and consequent expansion, with some contribution from melting land ice. Additional evidence of sea level rise in Australia has come to light following the discovery of 160 year old records of observations taken at Port Arthur, Tasmania. The observations, compared with data from a modern tide gauge, indicate an average sea level rise of about 1mm a year, consistent with other Australian observations and the lower end of estimates from the IPCC. The project, by researchers at the University of Tasmania and the UK’s Southampton Oceanography Centre, follow the investigation of a benchmark cut into a vertical rock face at the Isle of the Dead, Port Arthur, long before any effect of global warming was apparent.

Another consequence of rising water temperatures is coral bleaching. Coral reefs around the world are becoming stressed by a number of factors: bleaching due to warmer oceans, occasional reductions in salinity due to extreme river outflows, increased cloudiness of water, chemical pollutants, local fishing practices and damage from tropical cyclones. The frequency of occurrence of mass coral bleaching (when reef-building corals lose their symbiotic algae and associated pigments) have increased globally since the late 1970s. During 1997-98 coral bleaching was reported on many of the world’s coral reefs and also affected coral reefs in parts of Australia’s Great Barrier Reef (GBR) and northwest shelf. Mass coral bleaching was again observed on the GBR in early 2002. Daily monitoring of water temperatures from the Australian Institute of Marine Science’s (AIMS) Automatic Weather Station network during the 2001-02 summer season, combined with previously determined coral bleaching thresholds, led to early warning of conditions conducive to coral bleaching to the Great Barrier Reef Marine Park Authority. This enabled a coral bleaching strategy to be quickly implemented and this bleaching event was observed and documented in great detail. Water temperatures were again regularly monitored during the summer of 2002-2003 and regular updates of bleaching potential posted online ( www.gbrmpa.gov.au/corp_site/info_services/science/bleaching/conditions_ report.html). Fortunately, water temperatures did not reach coral bleaching thresholds.

Coral bleaching as severe as the event that occurred in 1998 may become common by 2020 due to projected global warming (CSIRO, 2002). The effect of higher carbon dioxide levels results in more acidic ocean waters and may lead to reduced coral growth rates. Natural adaptation will probably be too slow to avert a decline in the quality of the coral reefs. AIMS has several research activities assessing the threat to coral reefs posed by global warming. These include: determining the thermal tolerance levels of corals; determining the capacity of corals to adapt or acclimatise to new thermal regimes; determining the historical frequency of coral bleaching events; and determining whether there are spatial variations in the risk of coral bleaching along the GBR. These projects involve field and laboratory experiments of thermal thresholds, analysis of bleaching signals in massive coral skeletons, as well as application of in situ measurements of water temperature, weather conditions observed on reefs and satellite sea temperature observations.

AIMS researchers studied the relationship between patterns of coral bleaching impacts and patchiness in heat stress in GBR waters during the 2001-02 summer. A moderately strong relationship was found with impacts generally highest in the hottest areas. There were, however, some hot areas with low bleaching impacts, apparently due to lesser sensitivities among some coral community types and favourable acclimatisation regimes and local oceanography. In a collaborative study with CSIRO and US National Oceanic and Atmospheric Administration - National Environmental Satellite, Data and Information Service (NOAA-NESDIS), AIMS investigated the implications of high and low rates of global warming and high and low rates of autonomous adaptation in coral communities projected to 2050. All combinations saw some setbacks in reef ecology compared to a future without bleaching impacts. For high climate change and no adaptation, the modelled index of coral state fell below the 1990 baseline before 2050. To maintain the index above the 1990 baseline required a low rate of climate change combined with high rates of autonomous adaptation. The latter scenario was, nevertheless, setback relative to the no bleaching scenario.

AIMS researchers are also investigating whether the Great Barrier Reef will survive global warming. A rise of 1°C, predicted to occur within 50 years, could cause mass bleaching of coral reefs when combined with seasonal fluctuations. AIMS researchers heated to various temperatures tanks containing different corals. They found some corals bleached or even died, while others coped under the same conditions. They conclude global warming will not destroy the Great Barrier Reef, but there may be a reduction in species of coral, leaving only those species with a special protein that protects them from prolonged temperature rises. However, CSIRO research has shown an increase of 2°C is likely to change the tropical near-shore marine life from coral to algal dominated communities.