"In this study, eighteen 2000-year-long series were obtained that were not based on tree ring data.
...
The mean series shows the Medieval Warm Period (MWP) and
Little Ice Age (LIA) quite clearly, with the MWP being approximately 0.3°C warmer than 20th century values at these eighteen sites.
...
Climate histories are commonly reconstructed from a variety of sources, including ice cores, tree rings, and sediment. There are reasons to believe that tree ring data may not capture long-term climate changes (100+ years) because tree size, root/shoot ratio, genetic adaptation to climate, and forest density can all shift in response to prolonged climate changes, among other reasons (Broecker, 2001; Falcon-Lang, 2005; Loehle, 2004; Moberg et al., 2005). Most seriously, typical reconstructions assume that tree ring width responds linearly to temperature, but trees can respond in an inverse parabolic manner to temperature, with ring width rising with temperature to some optimal level, and then decreasing with further temperature increases (D’Arrigo et al., 2004; Kelly et al., 1994). This response is most likely due to water limitation at higher temperatures, because higher temperatures increase evaporation rates.
...
If climate is reconstructed from tree ring data, therefore, the response will primarily reflect noise, with the average response being flat, which will make it look like past climates have been stable."