Background
The authors write that "the recent Fifth Assessment Report of the Intergovernmental Panel on Climate Change anticipates the continuing retreat of sea ice and warming accompanying future anthropogenic emissions of greenhouses gases and aerosols." However, they say that "natural variability, such as that associated with the Atlantic Multi-decadal Oscillation (Chylek et al., 2009), has been suggested to be an important driver of climate variations in the Arctic region and responsible for a portion of the recent warming trend." And they further state that "recent results also indicate that sea surface temperature (SST) changes outside the Arctic have played a role in forcing the recent tropospheric warming in the Arctic (Screen et al., 2012.)."

What was done
Ding et al., as they describe it, used "observational analyses and modelling to explore the relative contributions of anthropogenic forcing and natural variability to recent warming trends in the Arctic," based on "post-1979 observations only, because the analyses of geopotential height and other variables over the Northern Hemisphere polar region are more reliable during the modern satellite era," citing Bromwich et al. (2007) in this regard.

What was learned
First of all, the seven scientists determined that "the most prominent annual mean surface and tropospheric warming in the Arctic since 1979 has occurred in northeastern Canada and Greenland." And they also showed that "the recent warming in this region is strongly associated with a negative trend in the North Atlantic Oscillation, which is a response to anomalous Rossby wave-train activity originating in the tropical Pacific." In addition, they say that "experiments from the Coupled Model Intercomparison Project Phase 5 [CMIP5] models with prescribed anthropogenic forcing show no similar circulation changes related to the North Atlantic Oscillation or associated tropospheric warming," while noting that "the most salient circulation feature in the observations is the pronounced localized rise in upper-tropospheric geopotential height and surface and tropospheric temperature over northeastern Canada and Greenland," which features they again note "are not reproduced in the ensemble average of the historical simulations using the CMIP5 models."

What it means
In the words of Ding et al., these several findings suggest that (1) "a substantial portion of recent warming in the northeastern Canada and Greenland sector of the Arctic arises from unforced natural variability," that (2) this natural variability "is intrinsic to the coupled atmosphere-ocean system," that (3) "these features are not reproduced in the ensemble average of the historical simulations using the CMIP5 models," which further implies that (4) the output of the most up-to-date CMIP5 models is still a far, far cry from reality.

References
Bromwich, D.H, Fogt, R.L., Hodges, K.I. and Walsh, J.E. 2007. A tropospheric assessment of the ERA-40, NCEP, and JRA-25 global reanalyses in the polar regions. Journal of Geophysical Research 112: 10.1029/2006JD007859.

Chylek, P., Folland, C.K., Lesins, G., Dubey, M.K. and Wang, M.Y. 2009. Arctic air temperature change amplification and the Atlantic multidecadal oscillation. Geophysical Research Letters 36: 10.1029/2009GL038777.

Screen, J.A., Deser, C. and Simmonds, I. 2012. Local and remote controls on observed Arctic warming. Geophysical Research Letters 39: 10.1029/2012GL051598.