HKU Bulletin November 2009 (Vol. 11 No .1)

17 November 2009 Dr Goodkin explains how variations in the NAO impact on the economies of Europe and North America. “In the 1980s,” she says, “we had the longest period of positive NAO with extreme values. This kind of event effects precipitation and temperature from the mid west of America all the way through Europe and into Russia. It causes severe droughts and heat waves – for instance the one in France and Europe in 2003, which was NAO related – it drastically changes fisheries in the north Atlantic, as well as changing wave heights and affecting shipping lanes. The economic impacts of changes in the NAO are quite large.” One question that troubles scientists attempting to predict the NAO is “whether human activity has shifted the way the NAO behaves.” “Our evidence is showing that there’s a distinctive difference in the way the NAO behaves in warm weather versus cold weather. There were indications in other records that this was the case, but the coral allowed us to explore the phenomena in detail.” What makes Dr Goodkin’s research unique is that it is the first time continuous NAO patterns have been documented in oceanic records. Previous studies have concentrated on terrestrial sources, such as tree rings, but says Dr Goodkin “corals are really the only paleo record where you can look at a seasonal time-scale. “With corals you can extract information at approximately monthly resolution and if you want to study climate features such as the NAO you need to be looking just at winter.” The research was conducted in Bermuda – the northernmost point where corals grow in the Atlantic – and resulted in a detailed 218- year-long NAO record. However, establishing a one-location record is insufficient to predict the future behaviour of the NAO. “It is a very dynamic system, but if we’re just looking at Bermuda and the Bermuda signal changes then that could just mean that the signal at Bermuda has changed. Not necessarily that the NAO changed,” explains Dr Goodkin. So she compared her results to larger geographical land records and confirmed that they show very similar behaviour. “That gives a stronger indication that it’s happening everywhere, but we need a lot more sites to improve prediction,” she adds. “The minute you can better understand the behaviour of the NAO the better you can prepare for the economic impacts of what will happen in the next five or ten years.” The impact of global warming is considered one of the greatest challenges facing mankind in the 21st century. Increasing greenhouse gas concentrations from the burning of fossil fuels and deforestation is blamed for escalating temperatures which are expected to continue soaring, causing rising sea levels and more extreme weather events. For nearly a century climatologists have been closely observing the behaviour of the North Atlantic Oscillation (NAO) – a measurement of the atmospheric pressure at sea-level between the polar low and subtropical high, that controls winter weather over large parts of North America, Europe and North Africa. The bigger the pressure difference, the stronger the storms, and the more northern the storms. Now, new research by Dr Nathalie Goodkin, Assistant Professor in the Department of Earth Sciences, and her co-workers at Woods Hole Oceanographic Institution, in the USA, has generated the first continuous ocean- based record of the NAO, and has discovered that climate change may be altering its long-term behaviour. The research, published in Nature Geoscience , found that the NAO has experienced larger changes from high to low values decade-to-decade during the latter part of the 20th century than it did in the early 1800s, pointing to the Industrial Revolution and human-related warming as culprits. Research READING the corals New research on Bermuda coral reveals a link between the North Atlantic Oscillation and climate warming.