Volume 8, Number 7: 16 February 2005
In an important new study, Moberg et al. (2005) present a new temperature history of the Northern Hemisphere that spans the past two millennia and improves significantly upon the highly controversial reconstruction of Mann and Jones (2003), which evolved from the earlier and even more controversial studies of Mann et al. (1998, 1999). The new temperature history, which represents a major move in the right direction, was produced from two different sources of paleoclimatic data: tree-rings, which capture very high frequency climate information, and lake and ocean sediments, which, in the words of Moberg et al., "provide climate information at multicentenial timescales that may not be captured by tree-ring data."
The new temperature history clearly reveals the existence of one full cycle of the roughly 1500-year climatic oscillation that reverberates throughout the Holocene and across prior glacial and interglacial periods alike (see Climate Oscillations (Millennial Variability) in our Subject Index). Its creators note, for example, that "high temperatures - similar to those observed in the twentieth century before 1990 - occurred around AD 1000 to 1100, and minimum temperatures that are about 0.7°C below the average of 1961-90 occurred around AD 1600," while the 20th century has seen a return to a new period of relative warmth.
Where does the new low-frequency variability that is missing from the temperature reconstructions of Mann et al. originate? It comes from a set of eleven non-tree-ring proxy climate records that cover most of the past two millennia, nine of which data sets had already been calibrated to local/regional temperatures by their developers. Speaking to the logic and straightforwardness of their new analysis (which we applaud), Moberg et al. write that simple averages of temperature proxy series, such as the ones they used, "can yield adequate estimates of Northern Hemisphere century-scale mean-temperature anomalies," citing the work of von Storch et al. (2004) as authority for this statement; and when this procedure (simple averaging) is all that is done, as noted in Moberg et al.'s Figure 2a, the Medieval Warm Period (MWP) is observed to peak just prior to 900 AD and is strongly expressed between about 600 to 1100 AD, which is very possibly the most correct temperature reconstruction of all.
What does this result tell us about modern temperatures? Where the temperature history discussed above ends, it is at approximately the level at which the MWP begins, which makes the final 20th-century temperature of that record cooler than all of the temperatures of the entire 500-year time span of the MWP. Also, at the point where Moberg et al.'s full reconstruction (which includes tree-ring results) ends, it is cooler than the MWP temperatures they find "around AD 1000 to 1100." In fact, it is only when the directly-measured instrumental temperatures of the latter part of the 20th century are added to the new temperature history that the Swedish and Russian scientists observe an extremely recent ("post-1990") modern warming that "appears to be unprecedented" over the prior two millennia; and in carefully stating that this appears to be the case, Mobeg et al. speak appropriately, for one cannot make a definitive comparative judgment on the matter when the two types of data involved are significantly different from each other. That is to say, one cannot compare real apples with reconstructed oranges, especially when the apples may have been contaminated by an environmental factor (the growing urban heat island effect) that likely had little to no influence on the oranges.
So where does all of this leave us? It leaves us with more and stronger evidence that the Northern Hemisphere some 900 to 1400 years ago, when there was 100 ppm less CO2 in the atmosphere than there is currently, was as warm as, or warmer than, it has been since that time. This observation is extremely important, for it means that the opposite trend of whatever change(s) in climate forcing factor(s) brought the earth down into the depths of the Little Ice Age may well have been what has restored the lost warmth of the Medieval Warm Period and established the Modern Warm Period.
And - mark our words - ever more such evidence will be discovered as scientists continue to address this important topic. Why are we so sure? Because we have been reporting the results of such studies almost weekly for months on end.
Sherwood, Keith and Craig Idso
References
Mann, M.E., Bradley, R.S. and Hughes, M.K. 1998. Global-scale temperature patterns and climate forcing over the past six centuries. Nature 392: 779-787.
Mann, M.E., Bradley, R.S. and Hughes, M.K. 1999. Northern Hemisphere temperatures during the past millennium: Inferences, uncertainties, and limitations. Geophysical Research Letters 26: 759-762.
Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 10.1029/2003GL017814.
Moberg, A., Sonechkin, D.M., Holmgren, K., Datsenko, N.M. and Karlen, W. 2005. Highly variable Northern Hemisphere temperatures reconstructed from low- and high-resolution proxy data. Nature 433: 613-617.
von Storch, H., Zorita, E., Jones, J.M., Dimitriev, Y., González-Rouco, F. and Tett, S.F.B. 2004. Reconstructing past climate from noisy proxy data. Science 306: 679-682.