In response to the high-variability recosntruction of Esper et al.  posted up here , Mann and Hughes [Science 296, 848 (2002)] replied:
The basis used by Esper et al. for comparison with previous studies of NH temperature trends over the past millennium is flawed [imagine Mann calling someone else's work "flawed"] …
Some of these differences may have a geographic basis. Esper et al. estimate extratropical temperature changes using an entirely extratropical tree ring data set. In contrast the Mann et al reconstruction estimates temperature trends over the full NH using both extratropical and tropical proxy data and targeting the full NH temperature. Half of the surface area of the NH temperature rcord estimated by Mann et al. lies at latitiudes below 30N, whereas the Esper et al. estimate is based entirely on latitudes above 30N. Tropical surface temperatures are typically less variable than extratropical continental surface temperatures at almost all time scales.
This explanation sounds plausible on the surface. However, if you look at the actual proxies used in the MBH99 reconstruction (which are not the same as the 112 proxies of the post-1820 period), you see that MBH99 does not use ANY proxies from 0-30N in their MWP reconstruction.