Background

The authors write that "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," which is perhaps the most burning scientific question of our day. In addition, they say that "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tree rings with 1-3 year temporal resolution, annually resolved stalagmites, decadally resolved ice-core information, historical documents with temporal resolution of 10-30 years, and lake sediments resolving decadal to century time scales," noting that "these proxies provide quantitative estimates of past climate through statistical calibration against instrumental temperature measurements."

What was done

Dividing the country into five sections, Ge et al. developed three composite temperature reconstructions that extended back in time a full two millennia (Northeast, Tibet, Central East), one that extended back approximately 950 years (Northwest), and one that only went back about 550 years (Southeast).

What was learned

With respect to the three sections of China that extended through the Medieval Warm Period and the one that extended into but not through it, the six scientists report that (1) in the Northeast, there was a warm period "between approximately 1100 and 1200 that exceeded the warm level of the last decades of the 20th century," (2) in Tibet, there was a "warming period of twenty decadal time steps between the 600s and 800s" that was "comparable to the late 20th century," (3) in the Central East, there were two warm peaks (1080s-1100s and 1230s-1250s) that had "comparable high temperatures to the last decades of the 20th century," although the graph of their data indicates that these two periods were actually warmer than the last decades of the 20th century, and (4) in the Northwest, "comparable warm conditions in the late 20th century are also found around the decade 1100s."

What it means

Since portions of two of the four sections of China for which temperature reconstructions extended far enough back in time to sample the Medieval Warm Period exhibited temperatures comparable to those of the late 20th century, and since portions of two other sections actually revealed parts of the Medieval Warm Period to have been warmer than the late 20th century, it is clear that there is nothing unusual, unnatural or unprecedented about the country's current level of warmth. Hence, by the reasoning set forth by Ge et al. in the introduction to their study, there is no compelling reason to attribute late 20th-century warmth in that sprawling country to 20th-century increases in the air's CO 2 concentration, nor to attribute it to concomitant increases in any other greenhouse gases.