What was done
Many researchers have examined historical proxy temperature changes over the past millennia and beyond in an attempt to quantify the magnitude, frequency and causes of natural climate variability.  However, temperature is not always the best measure of climate, and it is certainly not the only measure.  Few studies, for example, have examined the millennial range and rate of change of hydrologic and atmospheric circulation; yet changes in these parameters are important because they are involved in more than half of the earth's poleward transfer of heat (Peixoto and Oort, 1992).

In the present study, Maasch et al. attempt to remedy this deficiency by examining changes in eight well-dated high-resolution non-temperature records over the past two millennia: (1) K⁺ concentrations from the GISP2 ice core in Greenland, (2) Na⁺ concentrations from the Siple Dome ice core in Antarctica, (3) percent Ti from an ocean sediment core in the Cariaco basin, (4) Fe intensity from a marine core near the coast of mid-latitude Chile, (5) oxygen isotope fractions from Punta Laguna near the Yucatan, (6) carbon isotope data from a speleothem in Makapansgat, South Africa, (7) percent of shallow water diatoms from Lake Victoria, and (8) lake levels from Lake Naivasha in equatorial Africa.  The eight data sets were then compared with a history of atmospheric Δ¹⁴C, a proxy for solar variability that was obtained from tree rings, to ascertain what, if any, solar influence operated on these parameters.

What was learned
Comparison of the Δ¹⁴C solar proxy data with the eight climate-related data sets revealed that over the past 2000 years there has been, in the authors' words, a "strong association between solar variability and globally distributed climate change."  This "remarkable coherence" among the data sets was particularly noticeable in the Medieval Warm Period to Little Ice Age transition, as well as throughout the Little Ice Age.

What it means
As we continue to report here at CO2 Science, and contrary to the strident claims of climate alarmists, the results of this study suggest that the Medieval Warm Period and Little Ice Age were indeed global phenomena that were likely the products of natural climate variability driven by changes in solar activity.  As for the Current Warm Period, we believe it to be much less due to increases in atmospheric CO2 concentration than to the natural - and likely solar-induced - recovery of the planet from the coldest period of the current interglacial, i.e., the Little Ice Age.

Reference
Peixoto, J.P. and Oort, A.H.  1992.  Physics of Climate.  American Institute of Physics, New York.