Mit dem Kopf voran

I have four chief concerns with this chapter. First, there are numerous important references left out, and an over-emphasis on papers by the authors themselves, which do not accurately reflect the communities' view. In general, the certainty with which this chapter presents our understanding of abrupt climate change is overstated. There is confusion between hypothesis and evidence throughout the chapter, and a great deal of confusion on the difference between an abrupt "climate change" and possible, hypothetical cuases of such climate changes (e.g. Heinrich events). Second, the use of the terms "very likely", "likely", etc. are not in conformance with the rest of the IPCC document -- some things that are virtually certain are listed as "likely" and mere hypotheses, largely untested, are listed as "very likely". This carelessness does not add credibility to this chapter. Third, extensive reference is made to a very few recent papers that have not yet been thoroughly considered by the scientific community, and whose relevance to future climate is, in my judgement, greatly overstated. Finally, the choice of words to define -- or not define -- in the Glossary is strange. A definition (and a very poor one) of Heinrich events is given, but there is no definition for "Holocene", even though that term is used throughout the text. I would additionally note that overall, the chapter does a fine job at dealing with the "Hockey Stick" controversy, but a very poor job dealing with abrupt climate change and its possible relevance to the future. There are numerous glaring omissions of citations -- notably no mention is made of the work by Wunsch, Seager and Battisti, challenging the standard "Broecker-type" hypothesis for abrupt climate change. (WG I AR 4 Comments, Final Draft Comments Ch. 6, 6-42)