Australia's marine climate shifting south

Science Alert 8 Aug 08;

Australian Institute of Marine Science

Since the 1950s, average sea surface temperatures in northeast and northwest tropical Australian waters have increased steadily, causing a 200km shift southwards of climate zones along the northeast coast and an expansion in the area that can be designated "the tropics".

According to senior AIMS scientist and climate change team leader Dr Janice Lough, who has published her findings in the journal Geophysical Research Letters, if current trends continue, annual sea surface temperatures in northern Australian tropical waters could be around half a degree warmer and those of more southern parts about two degrees warmer within the next 100 years, with dire consequences for our coral reefs, particularly those in the more southerly areas.

This work provides further evidence of a significant widening of the global tropical belt.

"These rapid changes in oceanic climate are already causing responses in Australia’s tropical marine ecosystems and, if present rates continue, these will only intensify," Dr Lough said.

Many components of Australia’s unique tropical ecosystems are sensitive and vulnerable to the changing climate, as are many of Australia’s marine flora and fauna. Although well protected, Australia’s tropical reefs have not been immune to already observed impacts of a changing climate, such as mass coral bleaching events linked to warmer waters.

Dr Lough has analysed temperature records going back to 1950, seeking answers to the following questions: has Australia’s tropical climate already changed? Are rates of warming similar along the northwest and northeast coasts? Are there latitudinal differences in the rate of warming?

Her study used instrumental sea surface temperature (SST) records to examine annual average, maximum and minimum sea surface temperatures. Each variable has a profound impact on coral growth and health. She has found that Australia’s tropical ocean climate has already changed and that the rates of change vary in different regions.

Dr Lough’s work is part of an attempt to gather hard data on regional variation in the impacts of climate change. It has long been known that climate change effects are not evenly distributed and will affect different areas in different ways.

She has found that annual sea surface temperatures down to around 30 degrees south (about level with Coffs Harbour on the east coast) have already warmed between 1950 and 2007. This warming has shifted average climatic zones by about 200km southwards on the east coast and about half that distance on the west coast.

"A possible indicator of greater thermal stress in the southern Great Barrier Reef is the evidence from three recent large-scale coral bleaching events," Dr Lough said. These were recorded in 1998 and 2002 over large areas of the Reef, and in a more limited range in 2006.

"The rapidity and magnitude of warming along Australia’s tropical coastal regions is of great concern for maintenance of the integrity of their diverse tropical ecosystems, especially coral reefs," she said.