Read here. There is an abundance of both empirical and anecdotal evidence that the Arctic and sub-Arctic regions were warmer during the early 20th century than the more recent recorded temperatures. (For an indication of the anecdotal evidence about the 1920s and 1930s go here and scroll down to those decades.)
Scientists and Arctic experts now believe that the early 20th century saw exceptional polar and Northern Hemisphere warming that is yet to be adequately explained since atmospheric CO2 levels were so low at the time.
Now a new study by Halfar et al. adds to growing empirical evidence that indeed the North Atlantic area may have been warmer during the 1920s and 1930s.
"The authors write that "mid- and high-latitude crustose coralline algae are an emerging extra-tropical marine climate archive," as was demonstrated during a field calibration study (Halfar et al., 2008), since "they are amongst the longest-lived shallow marine organisms," and since "they show constant growth over their lifespan...Using a regional network of specimens of the coralline alga Clathromorphum compactum spanning portions of the Labrador Current inshore branch from the Gulf of St. Lawrence to both latitudinal extremes of the eastern Newfoundland shelf,...The new temperature reconstruction revealed "the well-documented regime shift and warming in the northwestern Atlantic during the 1990s." But in addition, the eight researchers report that "large positive changes in algal growth anomalies were also present in the 1920s and 1930s, indicating that the impact of a concurrent large-scale regime shift throughout the North Atlantic was more strongly felt in the subarctic Northwestern Atlantic than previously thought." And they specifically state that this regime shift "may have even exceeded the 1990s event with respect to the magnitude of the warming," as "has recently been suggested for the central and eastern North Atlantic," citing the study of Drinkwater (2006)." [Halfar, J., Hetzinger, S., Adey, W., Zack, T., Gamboa, G., Kunz, B., Williams, B. and Jacob, D.E. 2011: Palaeogeography, Palaeoclimatology, Palaeoecology]