The Review 2009

32 The Review 2009 • Research Scientists trying to predict global warming and other aspects of climate change draw on comparisons with data from the past, but research by Dr Liu Zhonghui, Assistant Professor of Earth Sciences, and his colleagues suggests this information may be incomplete. Dr Liu led a study that used ocean sediments to look back 34 million years ago when rapid cooling led to the formation of massive ice sheets in Antarctica but probably not in the Arctic. They found that at the start of the cooling transition, temperatures in the higher latitudes were likely at least 10 degrees Celsius warmer than today, much warmer than current climate models would predict. The scientists also separated the impacts of atmospheric temperature and the cooling effect of ice volume to show that all of the glaciation could have occurred in Antarctica during the transition, even though temperatures were dropping worldwide. “This was a time of dramatic changes. Although the change was towards cooling and not warming, the fact that current climate models cannot simulate warmer high latitudes in the past suggests that our understanding of climate change is incomplete,” Dr Liu said. The findings were published in 2009 in Science , along with a second climate change study that Dr Liu participated in concerning sea surface temperatures during the early Pliocene period (about four million years ago). That study found that although carbon dioxide (CO 2 ) concentrations in the atmosphere during the Pliocene were similar to today, the Earth was much warmer and temperature differences were flatter because of less cooling in higher latitudes. There were also larger tropical warm pool regions and the atmospheric (Hadley) circulation was greatly weakened. To account for this discrepancy, Dr Liu and his colleagues tinkered with the widely-used atmosphere-ocean coupled general circulation model, a type on which current climate change projections are based. When they applied only the atmosphere climate model and used prescribed ocean conditions likely to have prevailed at the time, they found anomalies in atmospheric behaviour that had not been observed in coupled climate models. “As temperatures rose, water vapour in the atmosphere increased greatly. Water vapour is another greenhouse gas, it’s stronger than CO 2 . So changes in the water vapour content and cloud distribution in the atmosphere can account for some of the warming we observed during the Pliocene,” Dr Liu said. “The important implication is that we don’t know everything yet. CO 2 is probably not the only thing that can cause climate change and there may be more complicated factors involved.” Climate Change Knowledge is Still Incomplete “The fact that current climate models cannot simulate warmer high latitudes in the past suggests that our understanding of climate change is incomplete” 33 The Review 2009 • Research Dr Liu Zhonghui