The Earth system - comprising atmosphere, ocean, land, cryosphere and biosphere - is an immensely complex system, involving processes and interactions on a wide range of space- and time-scales. Thus begins the abstract of an enlightening essay on the many shortcomings of today's climate models (O'Neill and Steenman-Clark, 2002) in which the implications of this situation are discussed within the context of developing "reliable numerical models that can be used to predict how the Earth system will evolve and how it will respond to man-made perturbations." The challenge of this enterprise, as the authors describe it, is truly daunting. They...