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Three Thousand Years of Climate Change in Central Iceland
Reference
Larsen, D.J., Miller, G.H., Geirsdottir, A. and Thordarson, T. 2011. A 3000-year varved record of glacier activity and climate change from the proglacial lake Hvitarvatn, Iceland. Quaternary Science Reviews 30: 2715-2731.

Background
The authors note that there have been many attempts to produce temperature reconstructions that place 20th-century warmth within the context of the last few millennia, because an important goal of paleoclimate research is to understand the causes and scale of past climate variability in order to better understand the likely causes of more recent climate change; and this worthy goal was the rationale for the study of Larsen et al. as well.

What was done
Noting that the varve thickness of annually-laminated sediments laid down by Hvitarvatn, a proglacial lake in the central highlands of Iceland, is controlled by the rate of glacial erosion and efficiency of subglacial discharge from the adjacent Langjokull ice cap, the four researchers say that a suite of environmental proxies contained within those sediments were used to reconstruct regional climate variability and glacial activity over the past 3000 years, which proxies included varve thickness, varve thickness variance, ice-rafted debris, total organic carbon (mass flux and bulk concentration), and the C:N ratio of sedimentary organic matter.

What was learned
Larson et al. report that "all proxy data reflect a shift toward increased glacial erosion and landscape destabilization from ca 550 AD to ca 900 AD and from ca 1250 AD to ca 1950 AD, separated by an interval of relatively mild conditions," and they say that "the timing of these intervals coincides with the well-documented periods of climate change commonly known as the Dark Ages Cold Period [DACP], the Medieval Warm Period [MWP], and the Little Ice Age [LIA]." In speaking of the MWP, they additionally note that "varve thickness decreases after 950 AD and remains consistently low through Medieval time with slightly thinner annual laminations than for any other multi-centennial period in the past 3000 years," which suggests that the MWP was the warmest period of the past three millennia. And they say that "the LIA was the most severe multi-centennial cold interval of the late Holocene" and "likely since regional deglaciation 10,000 years ago."

What it means
The research team's findings clearly suggest that (1) there is nothing unusual, unnatural or unprecedented about the warmth of the post-1950 Current Warm Period or CWP, and that (2) it is not surprising that at the conclusion of what was likely the coldest period of the entire Holocene (the LIA), there would be a significant warming of the globe, all of which further suggests that (3) there is no compelling reason to believe that 20th-century warming (which essentially ceased about 15 years ago) is a man-made phenomenon produced by the burning of coal, gas and oil. Quite to the contrary, the CWP is much more likely to be merely the most recent phase of the natural millennial-scale oscillation of earth's climate that has been shown to be operative throughout glacial and interglacial periods alike. See our Subject Index for many examples archived under the heading of Climate Oscillations (Millennial Variability).

Reviewed 9 November 2011