Coral samples support global warming theory
	Long-lived corals give an excellent record of global climate change
By Julian Lee Warming of the Pacific Ocean 6,000 years ago may have produced drier conditions in Northeast Australia - similar to those produced today by the El Niño weather pattern, researchers at the ANU's Research School of Earth Sciences (RSES)have revealed. Analysing geochemical data obtained from ancient Great Barrier Reef corals thousands of years old, Dr Michael Gagan and colleagues in RSES and James Cook University in North Queensland found that, 6,000 years ago, tropical ocean surface water was warmer and saltier than today. "It appears that changes in the climate of the last several millenia may have been more dramatic than we ever believed," Dr Gagan said. The findings, reported in Science last week, agree with research on land (using pollen samples and tropical ice cores) which indicate a warmer past climate - but the higher ocean salinity implies that the climate was drier. Parts of North America and Europe were warmer and strong monsoonal rains produced wetter climates in northern Africa and central Asia. There were even permanent lakes and hippopotamus in the Sahara Desert. Such a long period of warm dependable weather may have been instrumental in the rise of ancient civilisations including the Egyptians, Mayas, Indians and Chinese. However, Dr Gagan said the nature of this ancient "climatic optimum"was less clear for the southern hemisphere. "The unexpected twist in our results is that the warming of the ocean in the southwestern Pacific may have helped to produce a drier climate in northeast Australia," Dr Gagan said. The question is whether any future warming of the atmosphere and ocean, due to the post-industrial rise in carbon dioxide, will result in a similar drying of northern Australia. "It's difficult to say because carbon dioxide concentrations in the atmosphere 6,ooo years ago were much lower than today," Dr Gagan said. "Future global warming will be driven by changes in the atmosphere and it is likely that changes in ocean circulation were driving the Pacific warming 6,000 years ago. "We know that the warming of the tropical Pacific, driven by El Niño also warms the global atmosphere. The dry conditions we're seeing in northeast Australia may be the result of a semi-permanent El Niño." Long-lived reef corals are excellent monitors of environmental change and Dr Gagan and colleagues are measuring coral samples for both oxygen isotope ratios and strontium-calcium ratios to create a wealth of new data on past climate. By correlating oxygen measurements with strontium-calcium ratios- which measure temperature only- the water evaporation and precipitation component in oxygen measurements can be isolated. "We can learn a lot about the oceans and the atmosphere simultaneously with this method." Doubts about the reliability of climate records extracted from corals were mostly quashed when results from several corals growing in highly variable conditions concurred. "We tried to go to environments that are sub-optimal for coral growth to test a range of potentially confounding factors. None of the test sites are a diver's paradise." "The results certainly indicate that Great Barrier Reef waters were warmer 6,000 years ago - even warmer than the 1990s," Dr Gagan said.