Level 1 Studies
Studies that allow a quantitative comparison to be made between the temperatures of the Medieval Warm Period (MWP) and the Current Warm Period (CWP). These reports are very important, especially those that reveal the MWP to have been warmer than the CWP and that were published after the papers of Mann et al. (1998, 1999) appeared, because the authors of such Level 1 reports likely knew that their findings were not in harmony with the contemporary position of the Intergovernmental Panel on Climate Change, which strongly endorsed the Mann et al. papers that claimed the warmth of the latter part of the 20th century was unprecedented over the entire past millennium. Authors of more recent Level 1 papers have additionally had to contend with the contrary force of the paper of Mann and Jones (2003), which claimed that the warmth of the latter part of the 20th century was unprecedented over the past two millennia. Hence, it can be appreciated that the authors of many Level 1 papers were really "sticking their necks out" when reporting something considered by much of the scientific community to be incorrect, which would tend to give special credence to the sincerity of their belief in the validity of their data.
Level 2 Studies
Studies that allow a qualitative comparison to be made between the temperatures of the Medieval and Current Warm Periods. They enable one to determine if peak MWP temperatures were warmer than, equivalent to, or cooler than, peak CWP temperatures, but they do not enable one to determine how much warmer or cooler they may have been. Many of these studies, i.e., those that indicate the MWP was warmer than the CWP, were also published by scientists who likely knew their findings were not in harmony with the position of the scientific establishment, nor, for that matter, with the views of the political establishments of most of the world's nations, which again bears testimony to the strength of their faith in their findings.
Level 3 Studies
Studies that simply indicate the Medieval Warm Period did indeed occur in the studied regions. They may seem rather innocuous, but many of them may also be considered to be "politically incorrect," in that they contradict the climate-alarmist claim that the MWP, if it occurred at all, was only a regional phenomenon experienced by lands significantly influenced by the North Atlantic Ocean. In this level we include certain studies that are based on data related to parameters other than temperature, such as precipitation. These studies, however, are only used to help define the timeframe of the MWP; they are not employed to infer anything about either its quantitative or qualitative thermal strength.
Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787.
Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.
Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 10.1029/2003GL017814.