Background
The authors note that "the recent widespread warming of the earth's climate is the second of two marked climatic fluctuations to attract the attention of scientists and the public since the turn of the 20th century," and they say that the first of these -- "the major early 20th century climatic fluctuation (~1920-1940)" -- has been "the subject of scientific enquiry from the time it was detected in the 1920s." Furthermore, they write that "the early climatic fluctuation is particularly intriguing now because it shares some of the features of the present warming that has been felt so strongly in the Arctic [italics added]."

What was done
To learn more about the nature of both warmings, Wood and Overland reviewed what is known about the first warming through what they describe as "a rediscovery of early research and new assessments of the instrumental record," which allowed them to compare what they learned about the earlier warming with what is known about the most recent one.

What was learned
With respect to the first of the two warmings, the U.S. researchers say "there is evidence that the magnitude of the impacts on glaciers and tundra landscapes around the North Atlantic was larger during this period than at any other time in the historical period [italics added]." In addition, they report that "the ultimate cause of the early climatic fluctuation was not discovered by early authors and remains an open question," noting that "all of the leading possibilities recognized today were raised by the 1950s, including internal atmospheric variability, anthropogenic greenhouse gas (CO2) forcing, solar variability, volcanism, and regional dynamic feedbacks (e.g. Manley, 1961)," although they add that "greenhouse gas forcing is not now considered to have played a major role (Hegerl et al., 2007)." Thus, they suggest that "the early climatic fluctuation was a singular event resulting from intrinsic variability in the large-scale atmosphere/ocean/land system and that it was likely initiated by atmospheric forcing."

What it means
Wood and Overland conclude that the "early climatic fluctuation is best interpreted as a large but random climate excursion imposed on top of the steadily rising global mean temperature associated with anthropogenic forcing [italics added]." However, it could just as easily be concluded that the early warming is best interpreted as a large but random climate excursion imposed on top of the steadily rising global mean temperature associated with earth's natural recovery from the global chill of the Little Ice Age. And there is no reason not to conclude the same about the most recent Arctic warming; for in a major analysis of past rates of climate change in the Arctic, White et al. (2010) conclude that "thus far, human influence does not stand out relative to other, natural causes of climate change [italics added]."

References
Hegerl, G.C., Zwiers, F.W., Braconnot, P., Gillett, N.P., Luo, Y., Marengo Orsini, J.A., Nicholls, N., Penner, J.E. and Stott, P.A. 2007. Understanding and attributing climate change. In: Solomon, S., Qin, D., Manning, M. Chen, Z., Marquis, M., Averyt, K.B., Tignor, M. and Miller, H.L. (Eds.). Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. Cambridge University Press, New York, New York, USA.

Manley, G. 1961. Solar variations, climatic change and related geophysical problems. Nature 190: 967-968.

White, J.W.C., Alley,R.B., Brigham-Grette, J., Fitzpatrick, J.J., Jennings, A.E., Johnsen, S.J., Miller, G.H., Nerem, R.S. and Polyak, L. 2010. Past rates of climate change in the Arctic. Quaternary Science Reviews 29: 1716-1727.