It's not quite official yet, but 2009 was probably one of the 11 hottest years in Hong Kong history. The average temperature for the year was provisionally 23.5 degrees Celsius - the same as in 1991 and 2006, which are tied as the ninth-hottest years in the Observatory's records, which began in 1885. But the Observatory has yet to confirm the average temperature for December, and the annual figure is calculated based on the 12 monthly averages. Among the hottest years on record are 2001, 2002, 2003, 2006, 2007 and 2009 - making the past 10 years the hottest decade in the city's history. The Observatory, in a press release last month, attributed this to global warming and dense urban development. The World Meteorological Organisation forecast last month that 2009 would likely rank among the 10 hottest years globally. The organisation will update its assessment and publish final figures in March. The past decade has also seen hot weather occurring more frequently than the previous one. Between 2000 and 2009, 134 'very hot days' of 33 degrees or higher were recorded, a 50 per cent increase from the 89 days between 1990 and 1999. The number of 'hot nights' rose from 183 to 201 during the same period. Hot nights, when the daily minimum temperature fails to drop below 28 degrees, are measured by the Observatory. The steadily warming weather is also affecting plants, which are changing their growth cycle, according to Leon Lau Man-chung, an arborist who conducts tree surveys in various areas of Hong Kong. For example, Lau said, the rough-leaved holly, a native shrub that sheds its leaves in winter and sprouts in spring, was found to be sprouting new leaves near Shek Pik Reservoir, in Lantau, in mid-December. The Taiwan cherry, a foreign species that usually blooms in February, has been found flowering in Tai Po's Kadoorie Farm as early as December for the past few years. 'It's getting more and more common to see plants sprout or flower prematurely in winter, and to see plants that are supposed to shed their leaves, but don't - because the weather is not so cold any more,' Lau said. 'The phenomenon is also visible in the countryside, where the weather is less affected by the urban heat.'