Attribution of climate variations and trends to human influences and natural variability

Past attribution studies of climate change have assumed a null hypothesis of no role of human activities. The challenge, then, is to prove that there is an anthropogenic component. I argue that because global warming is "unequivocal" and 'very likely' caused by human activities, the reverse should now be the case. The task, then, could be to prove there is no anthropogenic component to a particular observed change in climate, although a more useful task is to determine what it is. In Bayesian statistics, this change might be thought of as adding a 'prior'. The benefit of doubt and uncertainties about observations and models are then switched. Moreover, the science community is much too conservative on this issue and too many authors make what are called 'Type II errors' whereby they erroneously accept the null hypothesis. Global warming is contributing to a changing incidence of extreme weather because the environment in which all storms form has changed from human activities. (C) 2011 John Wiley & Sons, Ltd.