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New Paper: China temperatures warmer during the 1700s, linked to solar, volcanic, and AMO/PDO forcing

By Kenneth Richard

A new 368-year tree ring temperature reconstruction has established that regional (China) summer temperatures were warmer than they are now (2012) during the mid-1600s and early 1700s, and that the temperature variations can be linked to variations in solar activity, volcanic forcing (cooling), and natural oceanic-atmospheric oscillations (AMO/PDO).

The authors are intent on pointing out that it is “noteworthy that 20th century warming was not very obvious in our reconstruction.” This “noteworthy” finding is mentioned four different times in the paper.

The lack of a conspicuous 20th century warming — and the warmer periods during the 1600s and 1700s — are clearly shown in the summer temperature graph below:

Annual and 11-year smoothed regional July–August mean maximum temperature reconstruction (blue and red lines, respectively); superimposed is the 11-year smoothed instrumental record (pink line).

Zhu and co-authors also do not mention CO2 as a “possible forcing mechanism” in their extensive discussion of the causes of temperature variations for the last 368 years, probably because there is little to no correlation between the decadal-scale temperature variations (warming and cooling periods) and the precipitous rise in atmospheric CO2 during the 20th and 21st centuries.

Below is a summary of the key points from the paper Zhu et al., 2016:

A 368-year maximum temperature reconstruction

During the period 1875–1955, late summer temperature fluctuated less strongly than before or thereafter. In general, the average length of cold periods was shorter than that of warm periods. The cold period of 1869–1877 was the longest and coldest cool period had a mean of 17.63°C. The longest warm period extended from 1655 to 1668, and the warmest period in AD 1719–1730 had a mean of 20.37°C. However, we should point out that the rapid warming during the 20th century was not especially obvious in our reconstructed RLST.

[S]even cold periods and three warm periods were identified during the past 368 years (Fig. 4d). All the cold periods were during the Maunder (1708–1711) or Dalton (1818– 1821, 1824–1828, 1832–1836, and 1839–1842) solar minima periods, except for the cold periods of 1765–1769 and 1869–1877 (Eddy, 1976; Shindell et al., 1999), which indicated that RLST [mean maximum temperature] variations in the NWSP [northwestern Sichuan Plateau, China ] might be driven by solar activity (Fig. 7b). On the other hand, volcanic eruptions in the corresponding periods might also be a cooling factor (Fig. 7b). A longer cold period (e.g., 1820s–1840s) was interrupted by transient warming, thus forming a plurality of discontinuous short cold periods. Warm periods of 1719–1730 and 1858–1859 both had more sunspots (Eddy, 1976; Shindell et al., 1999) and lower volcanic forcing (Fig. 7b). The cold (1765–1769 or 1869–1877) and warm (1655–1668) periods were highly consistent with other studies (Fig. 7).

Possible Forcing Mechanism

Accompanied by significant peaks at 60.2 and 73 years, the continuously periodicities around 49–114 years in our regional temperature reconstruction might tentatively be related to PDO, Atlantic Multidecadal Oscillation (AMO; En- field et al., 2001) as well as solar activity (Eddy, 1976; Shindell et al., 1999; Peristykh and Damon, 2003; Raspopov et al., 2004; Braun et al., 2005). The AMO was an important driver of multidecadal variations in summer climate not only in North America and western Europe (Kerr, 2000; Sutton and Hodson, 2005) but also in the East Asia (Feng and Hu, 2008; Liang et al., 2008; Wang et al., 2011, 2015; Zhu et al., 2011). The 60.2-year peak associated with AMO demonstrated that multidecadal variations in late summer temperature in the NWSP NWSP [northwestern Sichuan Plateau, China ] might be controlled by AMO.

[S]ignificant multidecadal- and centennial-scale cycles of our temperature reconstruction might include the signs of solar activity, such as the Gleissberg cycles (Peristykh and Damon, 2003), Suess cycles (Braun et al., 2005), Bruckner cycles (Raspopov et al., 2004), and Schwabe cycles (Braun et al., 2005). The Maunder (ca. AD 1645–1715) and Dalton (ca. AD 1790–1840) solar minima periods were generally associated with temperature depressions (Eddy, 1976), and the Damon (ca. AD 1890– 1920) solar maximum period occurred in a relatively warm period, which further confirmed that late summer temperature variation in the NWSP [northwestern Sichuan Plateau, China ] might be driven by solar activity (Fig. 7b).

Conclusion

Overall, the RLST [mean maximum temperature] variability in the NWSP [northwestern Sichuan Plateau, China ] might be associated with global land–sea atmospheric circulation (e.g., ENSO, PDO, or AMO) as well as solar and volcanic forcing.