Ran, L., Jiang, H., Knudsen, K.L. and Eiriksson, J. 2011. Diatom-based reconstruction of palaeoceanographic changes on the North Icelandic shelf during the last millennium. Palaeogeography, Palaeoclimatology, Palaeoecology 302: 109-119.
The authors write that "the growing concern about global warming has led to renewed interest in past temperature changes, particularly the question of whether the present climate change exceeds the natural variability of the past."
What was done
Working with two ocean sediment cores -- one obtained in 1999 at 66°33.10'N, 17°41.99'W and one obtained in 2006 at 66°33.18'N, 17°42.04'W -- Ran et al. reconstructed summer sea surface temperature (SST) on the North Icelandic shelf for the period AD 940-2006, based on their high-resolution and precisely dated diatom records, along with the help of "a modern diatom-environmental dataset from around Iceland [that] was established as a basis for quantitative reconstruction of palaeoceanographic conditions on the North Icelandic shelf (Jiang et al., 2001, 2002)."
What was learned
In the words of the four researchers, their diatom-based SST record indicates that the sea surface on the North Icelandic shelf "was not as warm during the last century as during the Medieval Warm Period (MWP)." More specifically, they write that "warm and stable conditions with relatively strong influence of the Irminger Current on the North Icelandic shelf are indicated during the interval AD 940-1300, corresponding in time to the MWP," and that "a considerable cooling at ~AD 1300 indicates the transition to the Little Ice Age (LIA) with increased influence of Polar and Arctic water masses deriving from the East Greenland and East Icelandic currents." Then came "an extended cooling period between AD 1300 and 1910," followed by "a two-step warming during the last 100 years" that was "interrupted by three cool events around AD 1920, in the AD 1960s and in the late AD 1990s." And they end by stating that "the data suggest that solar radiation may be one of the important forcing mechanisms behind the palaeoceanographic changes."
What it means
Once again we have a situation where the warmth of the more distant past clearly exceeded that of the recent past, with the peak temperature of the MWP exceeding that of the Current Warm Period by about 0.6°C, as best we can determine from the graphical representation of Ran et al.'s data, which thus indicates there is nothing unusual, unnatural or unprecedented about the earth's current level of warmth in this particular part of the planet, as in the many other such parts of the planet documented in our Medieval Warm Period Project.
Jiang, H., Seidenkrantz, M.-S., Knudsen, K.L. and Eiriksson, J. 2001. Diatom surface sediment assemblages around Iceland and their relationships to oceanic environmental variables. Marine Micropaleontology 41: 73-96.
Jiang, H., Seidenkrantz, M.-S., Knudsen, K.L. and Eiriksson, J. 2002. Late-Holocene summer sea-surface temperatures based on a diatom record from the North Icelandic shelf. The Holocene 12: 137-147.Reviewed 11 May 2011