Liability for climate change

摘要

As I write this article in January 2003, the flood waters of the River Thames are about 30 centimetres from my kitchen door and slowly rising. On the radio, a representative of the UK Met Office has just explained that although this is the kind of phenomenon that global warming might make more frequent, it is impossible to attribute this particular event (floods in southern England) to past emissions of greenhouse gases. What is less clear is whether the attribution of specific weather events to external drivers of climate change will always be impossible in principle, or whether it is simply impossible at present, given our current state of understanding of the climate system. The issue is important as it touches on a question that is far closer to many of our hearts than global sustainability or planetary survival ― who to sue when the house price falls? At the heart of the problem is the distinction between weather and climate. As Edward Lorenz put it, "climate is what you expect, weather is what you get". In the twenty-first century, climate is what you affect, weather is what gets you. Climate means 'possible weather", or what a statistician would call the 'expected weather' and its variability for a particular time of year, given all the properties of the ocean-atmosphere system, current levels of greenhouse gases, solar activity, and so on. The 'attribution problem' for externally driven changes in climate (as opposed to specific weather events) boils down to questions such as: "what would the climate have been like had we not increased greenhouse-gas levels?" This is a well posed question to which, if we define climate rigorously to encompass all the properties of the 'attrac-tor' of atmospheric and oceanic weather, there is only a single answer.