1000 Years of Climate Change

The exceptional warmth of the 1990s – the warmest decade since instrumental records began – has sharpened concern over the nature of anthropogenic influences on climate. However, the causes of 20th century climate variability are difficult to resolve, because the period of instrumental records coincides with the time during which the atmosphere has increasingly been contaminated by greenhouse gases. What are the “natural” variations that underlie the observed warming of the 20th century? To the casual observer, it might seem that this question has a simple answer – 20th century warming was preceded by the “Little Ice Age ” and before that, a “Medieval Warm Period”. But to those who have looked into this question in depth, the answer is not so simple, as was apparent at a recent workshop (1). Many questions remain regarding the climatic changes that occurred over the last millennium. Hubert Lamb was the first to argue for the existence of a “Medieval Warm Epoch” from around A.D. 1000 to 1300, but his evidence was highly anecdotal and largely qualitative (2). Furthermore, his reconstruction of the mild winters and dry summers that led him to characterize the Epoch as he did was based on records from western Europe