What was done
Working with distributions of dinoflagellate cyst assemblages obtained from sediment cores retrieved from Malangen Fjord in northern Norway (69°27.67'N, 18°23.64'E), the authors sought to determine the thermal and hydrologic characteristics of the region over the prior 1500 years.

What was learned
Rorvik et al. discovered four major climatic/hydrologic periods that they describe as follows: "zone 1 from c. AD 500 to 790, representing the Dark Ages Cold Period, represents the coldest time interval during the last 1500 years ... zone 2, from c. AD 790 to 1500, including the Medieval optimum, reflects strong advection of warm saline water ... zone 3, from c. AD 1500 to 1940, representing the Little Ice Age, reflects cool and low saline surface water conditions ... zone 4, from c. AD 1940 to the present (AD 1999)" is described by them as "the Modern Climate Optimum."

What it means
Just as has been discovered numerous times and throughout the entire world -- see Climate Oscillations (Millennial Variability) in our Subject Index -- northern Norway has experienced alternating multi-century periods of relative cold and warmth over prior millennia. And this pervasive natural phenomenon appears to be totally sufficient to explain 20th-century warming (there has been no more so far this century) without any need to invoke the help of the greenhouse effect of CO2 or any of the atmosphere's other greenhouse gases.