The "Little" Medieval Warm Period in Southeast Tibet

Late 20th-century temperatures have been described by the IPCC as unprecedented over the past one to two millennia, whereas other researchers have contended that the Medieval Warm Period of a thousand or so years ago was equally as warm as, or even warmer than, it has been recently. In doing so, however, both sides of the debate have neglected to focus attention on a much shorter period of time in the early 1400s that a number of studies have also identified as being anomalously warm, present study included.

Yang et al. (2010) developed a tree ring-width history spanning the time interval AD 1377-1998 from Tibetan juniper (Cupressus gigantea) trees growing at a site (29°22'N, 94°16'E) just north of the deep gorge of the Yarlung Tsangbo River of southeast Tibet, from which they developed a linear regression model between ring-width and mean January-June temperature that accounts for 35% of the variance of this parameter over the period 1961-1998. According to the authors, the tree ring history revealed a number of relatively warmer and cooler intervals throughout its 622-year record, among the former of which were several that exceeded late 20th-century warmth. The two most striking of these short-term warm periods were those of 1443-1466 and 1482-1501; and as best as can be determined from the graphical representations of their data, annual temperatures during the second of these two warm periods exceeded those of the late 20th century by as much as 0.75°C, while 11-year smoothed temperatures of the first of the two warm periods exceed those of the late 20th century by as much as 0.3°C. Thus, evidence continues to accumulate that there were periods of warmth in the AD 1400s (when atmospheric CO2 was over 30% lower than that of today) that significantly exceeded late 20th-century warmth (when atmospheric CO₂ was much higher)..