Larsen, D.J., Miller, G.H., Geirsdottir, A. and Olafsdottir, S. 2012. Non-linear Holocene climate evolution in the North Atlantic: a high-resolution, multi-proxy record of glacier activity and environmental change from Hvitarvatn, central Iceland. Quaternary Science Reviews 39: 14-25.
Noting that "Iceland is well situated to monitor North Atlantic Holocene climate variability," and that "terrestrial sites there offer the potential for well-dated, high-resolution, continuous records of environmental change and/or glacier activity," the authors go on to state that "laminated sediments from the proglacial lake Hvitarvatn provide a continuous record of environmental change and the development of the adjacent Langjokull ice cap for the past 10.2 thousand years."
What was done
Working with replicate sediment cores collected from multiple locations within the lake basin, and utilizing "multiple proxies, including sedimentation rate, bulk density, ice-rafted debris, sediment organic matter, biogenic silica, and diatom abundance," Larsen et al. were able to develop an annual- to multi-decadal-resolution history of what they call "dynamic Holocene terrestrial climate."
What was learned
Following the Holocene thermal maximum, the four researchers note that neoglaciation is manifest in their data as a non-linear transition toward cooler summers, as well as landscape destabilization and the inception and expansion of the Langjokull ice cap, which began about 5500 years ago. Thereafter, they say that "the climate of the past two millennia is characterized by two significant periods of landscape instability and glacial activity beginning in the 6th and 13th centuries AD, that correspond to the DACP [Dark Ages Cold Period] and LIA [Little Ice Age]," which were "separated by an interval of relative warmth from ca 950 to 1150 AD" that is known as the Medieval Warm Period (MWP). Perhaps most important of all, however, is their finding that "the LIA contained the most extensive glacial advance of the neoglacial interval," combined with their conclusion that "the LIA was the coldest period of the last 8 thousand years."
What it means
The first important point to be gleaned from the work of Larsen et al. - within the context of current climate concerns - is that the Holocene has experienced alternating multi-century periods of relative cold and warmth, the most recent and repeatable of which sequences is composed of the pre-DACP period (which is typically referred to as the Roman Warm Period), which was followed by the DACP, which was followed by the MWP, which was followed by the LIA, which was followed by the Current Warm Period or CWP, because this set of observations suggests that it was only natural for the CWP to have begun when it did. The second important point is their finding that the LIA was the coldest period of the last 8 thousand years, which suggests that starting from such an unprecedented low temperature, it is only natural that the warming that produced the CWP should have been one of the most significant warmings of the current interglacial. And these two observations suggest that there is absolutely no need to invoke anthropogenic CO2 emissions as the cause of 20th-century warming. In fact, it would have been an aberration of natural history if the warming of the past century or so had not occurred as it has, in terms of both the rate of temperature rise and the level of warmth so far achieved.