Robert ‘Bob’ Watson is a physical chemist specialising in atmospheric science issues and a leading authority on the science of climate change due to human activity. His research on halogen and hydroxyl free-radical reactions significantly informed models of how chlorofluorocarbons and other manmade chemicals deplete the Earth’s ozone layer. Bob’s career spans research and advisory roles, including key roles with NASA, as a science policy adviser to US President Bill Clinton and at the World Bank. For the UK government, Bob was Chief Scientific Adviser to the Department for Environment, Food and Rural Affairs. He was Chair of the Intergovernmental Panel on Climate Change from 1997 to 2002.