Read here and here. Using non-tree ring proxies from 18 different locations across the globe, the scientists were able to confirm the actual global existence of both the Medieval Warming Period and the Little Ice Age. Proxies used included: borehole temperature measurements, pollen remains, Mg/Ca ratios, oxygen
isotope data from deep cores or from stalagmites, diatoms deposited on
lake bottoms, and reconstructed sea surface temperatures. (click image to enlarge)
Why not tree rings?
“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.....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.”