Background
The authors write that "quantitative high-resolution global, hemispherical and regional climate reconstructions covering the last millennium are fundamental in placing modern climate warming into a long-term context," in order to "assess the sensitivity of the climate system to natural and anthropogenic forcings, and thus to reduce uncertainty about the magnitude and impact of future global climate change." And they note, in this regard, that for the entire Southern Hemisphere, "Mann and Jones (2003) considered only five data sets suitable for their work on surface temperature reconstructions for the past two millennia." What is more, they say that "only two of these data series are from South America," one of which is a tree-ring record "with unknown preservation of the low-frequency component of climate variability" and the other of which is a δ¹⁸O ice core record that they describe as "arguably putative at best" with respect to its temperature signal.

What was done
To begin to fill this vast data void, von Gunten et al. developed a continuous high-resolution (1-3 years sampling interval, 5-year filtered reconstruction) austral summer (December to February) temperature reconstruction based on chloropigments derived from algae and phototrophic bacteria found in sediment cores retrieved from Central Chile's Laguna Aculeo (33°50'S, 70°54'W) in 2005 that extended back in time to AD 850, which they describe as "the first quantitative temperature reconstruction for Central Chile for the last millennium."

What was learned
The Swiss, German and UK scientists report that their data "provide quantitative evidence for the presence of a Medieval Climate Anomaly (in this case, warm summers between AD 1150 and 1350; ΔT = +0.27 to +0.37°C with respect to (wrt) twentieth century) and a very cool period synchronous to the 'Little Ice Age' starting with a sharp drop between AD 1350 and AD 1400 (-0.3°C/10 years, decadal trend) followed by constantly cool (ΔT = -0.70 to -0.90°C wrt twentieth century) summers until AD 1750." Looking at the graph of their data, the peak warmth of the Medieval Climate Anomaly appears to be about 0.7°C warmer than the last decade or so of the 20th century, but only about 0.25°C warmer than the peak warmth of the 20th century, which occurred in the late 1940s for both their reconstructed temperatures and their instrumental data, which are essentially identical over most of the 1900s. In addition, they state that the "structure of variability" that is present in their data "is consistent in great detail with annually resolved tree-ring based warm-season temperature and river discharge reconstructions from northern Patagonia for the past 400 years, with qualitative climate reconstructions from Andean glacier fluctuations, and with hydrological changes in Patagonian lake sediment records."

What it means
The meticulous work of the five researchers clearly demonstrates the existence of both the Medieval Warm Period (MWP) and Little Ice Age in the Southern Hemisphere, as well as the fact that the MWP was warmer (and for a lot longer) than the Current Warm Period has been to date. Hence, there is nothing unusual, unnatural or unprecedented about the planet's current level of warmth -- or the rate at which it was achieved -- which further suggests there is no need to invoke current higher concentrations of atmospheric CO2 as the cause of these nondescript features of our current climate.

Reference
Mann, M.E. and Jones, P.D. 2003. Global surface temperatures over the past two millennia. Geophysical Research Letters 30: 1-4.