Description of The Medieval Warm Period and Little Ice Age in IPCC Reports - 1990 Report (FAR)

1990 Report (FAR)

With the growth of interest in global warming in the 1980s came renewed interest in the past temperature record, and the question of whether past times had been warmer or colder than "present". However, at that time available records were few. The discussion in chapter 7 "Observed climate variations and change" of the last 1000 years (excluding the instrumental period) occupies less than a page.

A schematic (non-quantitative) curve was used to represent temperature variations over the last 1000 years in chapter 7. The vertical temperature scale was labelled as "Temperature change (°C)" but no numerical labels were given; it could be taken to imply that temperature variations of the MWP and LIA were each of the order of 0.5 °C from the temperature around 1900. The section specifically states recent climate changes were in a range of probably less than 2 °C. The 1990 report noted that it was not clear whether all the fluctuations indicated were truly global (p 202). The graph had no clear source (but can be traced to publications by Hubert Lamb representing the Central England Temperature; those publications have no explicit calibration against instrumental data, just Lamb’s qualitative judgement and interpretation of what he refers to as the ‘evidence’ ), and disappeared from the 1992 supplementary report.

Within the 1990 report, the LIA is taken to be global in extent but the MWP is not. Climate over the last 1000 years is mentioned very briefly in the SPM of the 1990 report. The MWP is not mentioned at all, and the LIA described by ...probably fluctuated by little more than 1°C. Some fluctuations lasted several centuries, including the LIA which ended in the and which appears to have been global in extent. The MWP is mentioned in the executive summary to chapter 7, as MWP around 1000 AD (which may not have been global).