Chapter 3 - Mann's Hockey Stick
You will probably have seen representations of this graph in the media over the
last few years.
Climatologist Michael Mann’s graph first appeared in the April 1998 edition of the journal Nature, causing quite a stir among the scientific masses. It very quickly became the signature image of the United Nations Intergovernmental Panel on Climate Change, the IPCC, in effect kick-starting the Global Warming
phenomenon in both the media and scientific communities. The graph, known as the ‘hockey stick’ because of its distinctive shape, did not sit well with a lot
of people. To the general public the graph is misleading. Having been presented as a graph of temperature over the past 1000 years, it is in fact a representation of temperature anomalies per year, over that period. An anomaly is a change from what is normal, and in Mann’s case he chose the period of 1961-1990 as his ‘normal’ temperature. Therefore first of all, it is not a straight representation of temperature.
The main reason for the dispute over Mann’s graph was not, however, that it was misleading. The problem was that it appeared to ignore two very distinctive climatic events during the 1000 year period, in the form of the Medieval Warm Period and the Little Ice Age. The exact time of the Little Ice Age is unknown, although it is thought to have occurred between the years of 1600 and 1850 AD, but there is no disputing that it happened, and that it lasted for a couple of hundred years. Annual festivals and shows were held on the River Thames in London, which froze over regularly during the winter months. In 1658 a Swedish army marched across the ice to Denmark and invaded Copenhagen, so we know that the Little Ice Age existed.
Also beyond dispute, the Medieval Warm Period began around 1000 AD and lasted for about 500 years. We read old Roman documents from that time describing wine grapes being grown in Britain, as far north as Newcastle.
Vine Street, London, famous at least for its presence on the Monopoly board, is testament to those warm temperatures. As the global temperature dropped
going into the Little Ice Age, grapes could not be grown in Britain until the 1960s. Since then, British grapes have been grown in small quantities, but nothing to the extent of the Medieval period. Greenland too, is a record of historical warmth, suggested even by the very name. Today, a barren wasteland of ice, just starting to show signs of life in terms of trees and shrubs, but back then the Vikings landed, built settlements, grew crops and generally survived. They would struggle to do the same today. Reports emerged last year of some very old trees in Greenland beginning to grow once again, having been dormant for over 600 years. Only now is the climate getting close to what it was then.
Canadians Ross McKitrick and Stephen McIntyre noticed the discrepancy in Michael Mann’s graph and in 2003 released a paper showing the errors in Mann’s calculations. Applying the same principles as Mann, but removing all major errors, they presented a corrected version of the graph, above, clearly showing that the period between 1000 and 1500 AD was equally warm, and at times warmer than the current global temperature.