Reference
Keigwin, L.D. 1996. The Little Ice Age and Medieval Warm Period in the Sargasso Sea. Science 274: 1504-1508.
Background
Lloyd Keigwin -- Senior Scientist in Geology and Geophysics at the Woods Hole Oceanographic Institution, Woods Hole, Massachusetts, USA -- introduces his classic paleoclimatic study of the northern Sargasso Sea by stating that "it is important to document natural climate variability in order to understand the effects of anthropogenic forcing," that is to say, in order to correctly determine if the warming of the past several decades is unusual, unnatural or unprecedented in terms of what has occurred in the past.
What was done
Working with two subcores of a sediment box core retrieved from 33°41.6'N, 57°36.7'W on the undulating plateau of the northeast Bermuda Rise, Keigwin measured the oxygen isotope ratios (δ18O) of the white variety of the planktonic foraminifera Globigerinoides ruber, which he says "lives year-round in the upper 25 meters of the northern Sargasso Sea and has a relatively constant annual mass flux and shell flux" to the sediments. Calibrating these data against temperature and salinity data obtained at Ocean Station "S" (32°N, 62°30'W) over the prior 42 years, he first determined that "temperature accounts for about two-thirds of the isotopic signal, whereas salinity accounts for one-third." And based on these results, he calculated sea surface temperatures (SSTs) of the prior three millennia, after which he "stacked the temperature proxy data from the two subcores by averaging results in 50-year bins," obtaining the results depicted in the figure below.
Figure 1. Fifty-year averages of mean annual sea surface temperature calculated from the δ18O data of the two Bermuda Rise sediment subcores, together with the mean annual SSTs measured at Ocean Station "S" over the period 1954-1996. Adapted from Keigwin (1996).
What was learned
Keigwin states that the northern Sargasso Sea SST "was ~1°C cooler than today ~400 years ago (the Little Ice Age) and 1700 years ago [the Dark Ages Cold Period], and ~1°C warmer than today 1000 years ago (the Medieval Warm Period)."
What it means
In terms of his reason for conducting the study, which was to document natural climate variability in order to understand the effects of anthropogenic forcing, Keigwin states that "over the course of three millennia, the range of SST variability in the Sargasso Sea is on the order of twice that measured over recent decades," and, therefore, he concludes that "at least some of the warming since the Little Ice Age appears to be part of a natural oscillation." In addition, he says that "because the changes described here for surface waters over the Bermuda Rise are probably typical of a large part of the western Sargasso Sea, they most likely reflect climate change on the basin or hemispheric scale." Thus, one can have a high degree of confidence that the Medieval Warm Period in this particular part of the world -- and maybe a much wider region as well -- was significantly warmer than what it is there today.