MWP and LIA in IPCC reports
From Wikipedia, the free encyclopedia
This page discusses the description of the Medieval Warm Period and Little Ice Age in various reports of the Intergovernmental Panel on Climate Change.
In assessing the significance of recent global warming and in particular in considering the attribution of recent climate change, the question of how much temperatures have varied over the last millennia or more arises. In this context, the best known fluctuations are the Medieval Warm Period and Little Ice Age. It is sometimes claimed that the MWP or LIA have been airbrushed out of the 2001 IPCC Third Assessment Report whilst being prominently featured in earlier reports [1]. That is not true.
Contents |
[edit] 1990 report
With the growth of interest in global warming in the 1980's came renewed interest in the past temperature record, and the question of whether past times had been warmer or colder than "present". However, at this 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, 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 [19th century] 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).
[edit] 1992 supplement
The 1992 report (appendix C) used only two graphs or pre-instrumental temperatures, from Wang and Wang (1991). They show air temperature based on documentary evidence in East and North China from 1350-1950. Fluctuations are of the order of 0.5-0.75 °C and indicate, variably, colder-than-present temperatures before the 20th century. The graph stops in 1350 and does not show a MWP. The only text reference to the MWP is qualified by in this region in boldface.
[edit] 1995 report (SAR)
By 1995, research in the subject had advanced and hemispherical reconstructions of temperature were available, though only for the summer season (because tree rings are often most strongly influenced by summer temperatures). The 1995 IPCC report used a northern hemisphere summer temperature reconstruction (fig 3.20) from 1400 to 1979 by Bradley and Jones (1993). This too shows no MWP (it only goes back to 1400) and colder temperatures otherwise before the 20th century, of the order of 0.5 °C colder. Fig 3.21 shows 8 ice core records from 1200 to present, which display a mixed pattern. The MWP and LIA are introduced, in the text, as two periods which have received special attention... These have been interpreted, at times, as periods of global warmth and coolness, respectively. Recent studies have re-evaluated the interval commonly known as the MWP... the available evidence is limited (geographically) and is equivocal.
[edit] 2001 report (TAR)
The 2001 report used northern hemisphere warm-season and annual reconstructions from 1000 AD to present by Mann et al (1999), Jones et al (1999) and Briffa (2000) [2].
The IPCC TAR says of the MWP that the posited Medieval Warm Period appears to have been less distinct, more moderate in amplitude, and somewhat different in timing at the hemispheric scale than is typically inferred for the conventionally-defined European epoch. The Northern Hemisphere mean temperature estimates of Jones et al. (1998), Mann et al. (1999), and Crowley and Lowery (2000) show temperatures from the 11th to 14th centuries to be about 0.2°C warmer than those from the 15th to 19th centuries, but rather below mid-20th century temperatures [3].
The TAR discusses Was there a “Little Ice Age” and a “Medieval Warm Period”? and says Thus current evidence does not support globally synchronous periods of anomalous cold or warmth over this timeframe, and the conventional terms of “Little Ice Age” and “Medieval Warm Period” appear to have limited utility in describing trends in hemispheric or global mean temperature changes in past centuries. (ibid)
