Chinese and Swedish researchers examined a 2,485-year record of tree ring data from the eastern Tibetan Plateau. The researchers say:
The results showed that extreme climatic events on the Plateau, such as the Medieval Warm Period, Little Ice Age and 20th Century Warming appeared synchronously with those in other places worldwide. The largest amplitude and rate of temperature change occurred during the Eastern Jin Event (343–425 AD), and not in the late 20th century. There were significant cycles of 1324 years, 800 years, 199 years, 110 years, and 2–3 years in the 2485-year temperature series. The 1324-year, 800-year, 199-year, and 110-year cycles are associated with solar activity, which greatly affects the Earth surface temperature. The long-term trends (>1000 years) of temperature were controlled by the millennium-scale cycle, and amplitudes were dominated by multi-century cycles. Moreover, cold intervals corresponded to sunspot minimums.
The graph below shows their temperature reconstruction and their prediction for the next 120 years.
The researchers say that there are few records long enough to recognize millennium-scale variations. This particular record is well-correlated with seven other records in the northern hemisphere.
The researchers calculated the rate of temperature change for 10-year intervals. They found that the highest rate was 0.77°C/decade between 362-390 AD. The next highest rate was 0.35°C/decade between 881–908 AD. “In general, the calculations showed that the warming rate in the 20th century was not the highest in the past 2485 years.”
The global climatic system is greatly affected by the millennium-scale cycles. There has been much geological evidence of millennium-scale cycles, from North America to Europe and from the Middle East to East Asia.
There is evidence worldwide indicating that millennium-scale cycles are the dominant factors for climatic fluctuation during the Holocene, and the exact periodicity of the millennium-scale cycle was modified as 1374 ± 502 years by Bond et al. The inducement mechanism of these cycles may be associated with solar activity, and perhaps, the inherent solar cycle.
The researchers say that “two century-scale cycles (199 years and 110 years) have dominantly affected the amplitude of the temperature variations.” We experience temperature extremes through “constructive overlapping of multiple cycles” which, in the past 2,485 years appear to have produced 600-year cycles of general increases and decreases in temperature.
These tree-ring proxies agree with results of interpretations from ice cores, corals, speleothems, lacustrine deposits, and historical documents. The authors say there is still uncertainty in climate change but solar activity appears to have had a great effect on the Tibetan Plateau. The implication is that 20th Century warming is not unprecedented in either rate or magnitude. The authors say, “This moderate millennium-scale cycle [600 years] has lasted for such a long time that there is no reason for it to disappear in the last 2000 years.
The paper: Liu Y, Cai Q F, Song H M, et al., 2011, Amplitudes, rates, periodicities and causes of temperature variations in the past 2485 years and future trends over the central-eastern Tibetan Plateau. Chinese Sci Bull, 56: 2986 2994, doi: 10.1007/s11434-011-4713-7.
The full paper may be downloaded here.