The Naturally Oscillating Climate of Northern Norway
Rorvik, K.-L., Grosfjeld, K. and Hald, M. 2009. A late Holocene climate history from the Malangen fjord, North Norway, based on dinoflagellate cysts. Norwegian Journal of Geology 89: 135-147.
In analyzing the data, the authors report discovering 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."
Just as has been discovered numerous times and throughout the entire world, 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.