What was done

A new tree-ring width data set was derived from 14 white spruce chronologies obtained from the Seward Peninsula, Alaska, covering the years 1358-2001. The Seward data were then combined with additional tree-ring width chronologies from northwest Alaska to produce two versions of a much longer data series that extended all the way back to 978 AD. The first chronology was created using traditional methods of standardization (STD), which do not perform well in capturing multi-decadal or longer climate cycles, while the second chronology utilized the regional curve standardization (RCS) method, which better preserves low-frequency variations at multi-decadal time scales and longer.

What was learned

Analysis of the Seward Peninsula chronology revealed a general warming at the start of the record in 1358 that continued until the mid-1500s, whereupon cold conditions set in and prevailed until the end of the 17th century. Near 1700 there was another pronounced increase in temperature to the highest levels in the record after which the temperature fell to the coldest point in the record around 1780. Modest warming returned thereafter and temperatures remained relatively average for most of the 20th century. There was a slight warming in the 1950s, but the authors note that it was "not unusual relative to other above-average periods in these records."

With respect to the STD and RCS chronologies, each of them revealed, in the words of D'Arrigo et al., "several intervals of persistent above-average growth that broadly coincide with the timing of the late Medieval Warm Period." However, the warming is much more pronounced in the RCS chronology, where the greatest warmth occurred in the early to middle 1200s, with lesser peaks in the early to middle 1100s and early 1400s. In addition, the severe cold of the Little Ice Age is more pronounced and of longer duration in the RCS chronology, where it occurred between about 1500 and 1700.

What it means

The three chronologies of this study provide further evidence for natural climate fluctuations on centennial-to-millennial time scales, capturing the temperature oscillations that produced the Medieval Warm Period (11-13th centuries), Little Medieval Warm Period (1350-1450), and Little Ice Age (1500-1700). What the records fail to do is provide evidence for what climate alarmists call unprecedented warmth in the last decade of the 20th century. Quite to the contrary, in fact, the northwest Alaska temperatures of the last four decades have actually hovered around the long-term average.