1. Department of Geography and Environment, London School of Economics and Political Science, Houghton Street, London WC2A 2AE, England
2. Faculdade de Ciências Económicas e Empresariais, Universidade Católica Portuguesa, Palma de Cima, 1695-095 Lisboa, Portugal
3. T H Huxley School of Environment, Earth Sciences and Engineering, Imperial College of Science, Technology and Medicine, 48 Prince's Gardens, London SW7 2PE, England