What was done
Working with "four well-preserved continuous sediment sequences from the southern flank of the Skagerrak [approximately 58.2-58.6°N, 7.6-8.2°E]," which he describes as "a current-controlled sedimentary basin between the North and Baltic Seas," the author carried out "granulometric and stable oxygen isotope analyses ... in order to reconstruct climate fluctuations and to evaluate climate impact during the upper Holocene."

What was learned
Hass' data and analyses led him to conclude that the "Modern Climate Optimum was reached between 1940 and 1950, when temperatures exceeded the present day mean by 0.5°C." Prior to that was the Little Ice Age, which he places at about AD 1350-1900, while before that the Medieval Warm Period (AD 800/1000-1350) held sway, the climate of which, in his words, "was characterized by warm summers, mild winters and little storm activity." Preceding this interval was what we would call the Dark Ages Cold Period, which Hass does not name but places between AD 400 and 700, while preceding that cold spell was the Roman Warm Period, running from approximately 400 BC to AD 400. And preceding these climatic epochs was another pair of cold and warm periods.

What it means
As so many other studies have revealed, both before and after this one from 1996, Hass' work adds to the mountain of evidence supporting the reality of a repetitive worldwide cycling of climate between Medieval Warm Period- and Little Ice Age-like conditions. In addition, he notes that "at the onset of the Modern Climate Optimum ... conditions change again to a level comparable to the Medieval Warm Period," even though there was not nearly as much CO2 in the air back then as there was at the time of his study.