Background
The authors write that "Northern Hemisphere climate is believed to have fluctuated from being generally mild on average in the early millennium (the classic Medieval Warm Period) to being cool and variable during the subsequent Little Ice Age, followed by recent warming."

What was done
To see to what extent western Canada had followed this basic pattern over the past thousand years, Edwards et al. employed a coupled isotope response-surface model "to resolve multi-dimensional patterns of climate variability using carbon- and water-isotope time series developed from tree-ring cellulose," based on "16 subfossil snags and living-tree sequences of Picea engelmannii (Engelmann spruce) from upper alpine treeline sites near Athabasca Glacier and subfossil material from the forefield of Robson Glacier plus living and snag material of Pinus albicaulis (whitebark pine) adjacent to Bennington Glacier, spanning AD 951-1990."

What was learned
The four researchers report that "high inferred winter temperatures ~AD 1100-1250 stand out in particular, corresponding with the Medieval Climate Anomaly," adding that the "climate shifted broadly in western Canada from warm in winter and atmospherically moist during the growth season during medieval times to being cool in winter and atmospherically dry during the growth season in the subsequent Little Ice Age." Nevertheless, they note that "independent proxy hydrologic evidence suggests that snowmelt sustained relatively abundant streamflow at this time in rivers draining the eastern Rockies," while during the Medieval Warm Period there was "evidence for reduced discharge in rivers draining the eastern Rockies and extensive hydrological drought in neighboring western USA." Finally, they write that "declining streamflow in rivers draining the eastern Rockies over the past century (Rood et al., 2005) may indicate that conditions are in the process of returning to a similar state [our italics]," which suggests that the Current Warm Period has not yet achieved the more extreme climatic status of the Medieval Warm Period.

What it means
The results of this study from western Canada clearly delineate the classic cycling of climate that brought the earth the Medieval Warm Period and subsequent Little Ice Age, as well as the 20th-century transition to the Current Warm Period, all independent of the air's CO2 content. In fact, Edwards et al.'s data clearly indicate that both the minimum temperature of winter and the yearly average of the winter minimum and summer maximum temperature were greater during the Medieval Warm Period than they were during the late 20th century, between which times the air's CO2 concentration rose by approximately 100 ppm and still could not force a temperature increase equal to that of a thousand years ago.

Reference
Rood, S.B., Samuelson, G.M., Weber, J.K. and Wyrot, K.A. 2005. Twentieth-century decline in streamflows from the hydrographic apex of North America. Journal of Hydrology 306: 215-233.