Coherent Detection of the Medieval Warm Period in Multiple Data Sets

Many are the studies that have attempted to reconstruct the mean temperature history of the earth over the past millennium; and in the two companion analyses of Dergachev and Raspopov (2010a,b), the degree of harmony among the reconstructions of those earlier studies -- as well as the degrees of goodness of their individual correlations with various indices of solar activity -- are evaluated. Following this protocol, the authors initially show that climate reconstructions that rely heavily on tree-ring data do not agree very well with each other; and that when they are compared to ice core data, they appear to lose much of the low-frequency signal that is preserved in that other medium.

The Russian researchers next note that a detailed 750-year temperature reconstruction from an ice core in Siberia agrees well with measures of solar modulation based on sunspot number and carbon-14 and Be-10 estimates, and that the agreement is remarkable at multi-decadal time scales. Then, they examine borehole thermometry data, noting that although such data lose annual- and decadal-scale detail, the temperature history thereby derived agrees well over recent decades with local instrumental data. In addition, they find that multiple boreholes from around the world agree with each other on the scale of the last millennium, showing thereby that borehole-derived temperatures are a valid and consistent representation of reality.

Finally, Dergachev and Raspopov compare the solar indices of the past millennium with the borehole temperature reconstructions, demonstrating that the borehole data and solar indices agree on the long-term temperature pattern of the past thousand years. That is to say, the two parameters imply the existence of a solar-induced Medieval Warm Period (MWP) around AD 1000 to 1300 and a Little Ice Age (LIA) in the 1600s to 1700s. Thus, their study pretty much proves the existence of a global MWP, while demonstrating the link between the MWP-LIA oscillation and solar activity. And it indicates that the MWP was roughly as warm as -- or possibly even warmer than -- it has been to date during the Current Warm Period.