Medieval Warm Period and the World’s Oceans

By | October 29, 2013


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Keigwin (1996) introduced 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.” And, therefore, working with two subcores of a sediment box core retrieved from 33°41.6’N, 57°36.7’W of the undulating plateau of the northeast Bermuda Rise, he measured the oxygen isotope ratios (?18O) of the white variety of the planktonic foraminifera Globigerinoides ruber, which 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 below.