We constantly hear that the warmest years on record have all occurred in the most recent decades, and of course, we are led to believe this must be a result of the ongoing buildup of greenhouse gases. In most places, we have approximately 100 years of reliable temperature records, and we wonder if the warmth of the most recent decades is unusual, part of some cyclical behavior of the climate system, or a warm-up on the heels of a cold period at the beginning of the record. A recent article in Geophysical Research Letters has an intriguing title suggesting a 2,000 year temperature record now exists for China – we definitely wanted to see these results of this one.
The article was authored by six scientists with the Chinese Academy of Sciences in Beijing, the State University of New York at Albany, and Germany’s Justus-Liebig University in Giessen; the research was funded by the Chinese Academy of Sciences, National Natural Science Foundation of China, and the United States Department of Energy. In their abstract, Ge et al. tell us “The analysis also indicates that the warming during the 10–14th centuries in some regions might be comparable in magnitude to the warming of the last few decades of the 20th century.” From the outset, we knew we would welcome the results from any long-term reconstruction of regional temperatures.
The authors begin noting that “The knowledge of past climate can improve our understanding of natural climate variability and also help address the question of whether modern climate change is unprecedented in a long-term context.” We agree! Ge et al. explain that “Over the recent past, regional proxy temperature series with lengths of 500–2000 years from China have been reconstructed using tree rings with 1–3 year temporal resolution, annually resolved stalagmites, decadally resolved ice-core information, historical documents with temporal resolution of 10–30 years, and lake sediments resolving decadal to century time scales.” However, the authors caution “these published proxy-based reconstructions are subject to uncertainties mainly due to dating, proxy interpretation to climatic parameters, spatial representation, calibration of proxy data during the reconstruction procedure, and available sample numbers.”
Ge et al. used a series of multivariate statistical techniques to combine information from the various proxy methods, and the results included the reconstruction of regional temperatures and an estimate of uncertainty for any given year. They also analyzed temperature records from throughout China over the 1961 to 2007 period and established five major climate divisions in the country (Figure 1).
Figure 1. Types, lengths, and locations of proxy temperature series and observation used in the Ge et al. study. The five climate regions were based on a “factor analysis” of the 1961–2007 instrumental measurements. Grey shading indicates elevation (from Ge et al., 2010).
The bottom line for this one can be found in our Figure 2 that shows the centennially-smoothed temperature reconstruction for the five regions of China. With respect to the Northeast, Ge et al. comment “During the last 500 years, apparent climate fluctuations were experienced, including two cold phases from the 1470s to the 1710s and the 1790s to the 1860s, two warm phases from the 1720s to the 1780s, and after the 1870s. The temperature variations prior to the 1500s show two anomalous warm peaks, around 300 and between approximately 1100 and 1200, that exceed the warm level of the last decades of the 20th century.” The plot for the Northeast shows warming in the 20th century, but it appears largely to be somewhat of a recovery from an unusually cold period from 1800 to 1870. Furthermore, the plot shows that the recent warming is less than warming that has occurred in the past.
Figure 2. Five regionally coherent temperature reconstructions with 100-year resolution; the dashed line is the part with fewer series used; and the solid line is the mean value. The shaded areas are the two coldest periods, during the 1620s–1710s and 1800s–1860s (from Ge et al., 2010).
The Central East region also has a 2,000 year reconstruction and Ge et al. state “The 500-year regional coherent temperature series shows temperature amplitude between the coldest and warmest decade of 1.8°C. Three extended warm periods were prevalent in 1470s–1610s, 1700s–1780s, and after 1900s. It is evident that the late 20th century warming stands out during the past 500 years. Considering the past 2000 years, the winter half-year temperature series indicate that the three warm peaks (690s–710s, 1080s–1100s and 1230s–1250s), have comparable high temperatures to the last decades of the 20th century.” No kidding – the plot for the Central East region shows that the warmth of the late 20th century was exceeded several times in the past.
Commenting on the Tibet reconstruction, Ge et al. state “The warming period of twenty decadal time steps between the 600s and 800s is comparable to the late 20th century.” In the Northwest, they note “Comparable warm conditions in the late of 20th century are also found around the decade 1100s.” Unfortunately, no long-term reconstruction was possible for the Southeast region.
In summarizing their work, Ge et al. report :
From Figure 3 [our Figure 2 –eds.] , the warming level in the last decades of the 20th century is
unprecedented compared with the recent 500 years. However, comparing with the temperature variation over the past 2000 years, the warming during the last decades of the 20th century is only apparent in the TB region, where no other comparable warming peak occurred. For the regions of NE and CE, the warming peaks during 900s–1300s are higher than that of the late 20th century, though connected with relatively large uncertainties.
We get the message – the recent warming in at least several regions in China has likely been exceeded in the past millennium or two, the rate of recent warming was not unusual, and the observed warming of the 20th century comes after an exceptionally cold period in the 1800s.
Declaring that anthropogenic greenhouse gas emissions have pushed modern temperature beyond their historical counterparts disregards the lessons of 2,000 years of Chinese temperatures.
Ge, Q.-S., J.Y. Zheng, Z.-X. Hao, X.-M. Shao, W.-C. Wang, and J. Luterbacher. 2010. Temperature variation through 2000 years in China: An uncertainty analysis of reconstruction and regional difference. Geophysical Research Letters, 37, L03703, doi:10.1029/2009GL041281.