"We would like to thank Susan Solomon for the critical role she played in the organisation and progressing of the article. We also thank her for the very many useful comments and suggestions. We also wish to thank Olivier Boucher, Chris Folland and Peter Stott for their constructive comments. But of course the ultimate acknowledgement must be made to the hundreds who authored, edited and reviewed the AR4 itself. This article would not exist without their colossal efforts."