Early 20th century Arctic warming in retrospect

The major early 20th century climatic fluctuation ~1920-1940 has been the subject of scientific inquiry from the time it was detected in the 1920s. This period was marked by especially warm Atlantic-Arctic air and sea-surface temperature anomalies and related ecosystem impacts. The papers of scientists who studied the event firsthand have faded into obscurity but their insights are relevant today. We review this event through a rediscovery of early research and new assessments of the instrumental record. Much of the interannual to decadal scale variability in surface air temperature anomaly patterns and related environmental impacts in the Arctic and elsewhere can be attributed to the superposition of leading modes of natural variability in the atmospheric circulation. Meridional circulation patterns were an important factor in the high latitudes of the North Atlantic during the early climatic fluctuation. Sea-surface temperature anomalies during this period were congruent with low-frequency variability in the climate system but were themselves most likely the result of anomalous forcing by the atmosphere. The consistency of multiple lines of evidence provides support for this hypothesis where high resolution ocean and atmospheric data are lacking. We find no evidence that this fluctuation was part of a temporal oscillation or cycle beyond what can be expected given a Markov (AR-1) process. Our findings indicate that the early climatic fluctuation is best interpreted as a large but essentially random climate excursion imposed on top of the steadily rising global temperatures associated with anthropogenic forcing.