Background
The authors write that "the Arctic is greening (Bhatt et al., 2010; Epstein et al., 2013) and sequestering increasing amounts of atmospheric CO2 (Epstein et al., 2013)," while "at the same time, permafrost thawing is releasing ancient soil C (Schuur et al., 2013) to the atmosphere." And they note, in this regard, that "the timing and balance of these two processes determine the sign and strength of the Arctic's C cycle feedbacks to climate change," citing Schaefer et al. (2011).

What was done
Lupascu et al. quantified net ecosystem exchange of CO2, gross primary productivity and ecosystem respiration in a High Arctic semi-desert in a set of long-term (about 10 years) summer warming and/or wetting treatments in northwest Greenland, while also measuring sources of ecosystem respiration and below-ground CO2 based on their radiocarbon (¹⁴C) contents, which approach allowed them to detect changes in land-atmosphere C exchange and within-soil C dynamics.

What was learned
The six scientists report that warming (by 4°C) decreased the summer CO2 sink strength of the semi-deserts they studied by up to 55%; but they indicate that warming combined with wetting (equivalent to an extra 50% of summer precipitation) increased the CO2 sink strength by a full order of magnitude, totally dwarfing and countering the warming-alone effect, due in large part to the fact that wetting relocated recently assimilated plant C deeper into the soil and thereby decreased old C loss compared to that experienced in the warming-only treatment.

What it means
Due to the fact, as Lupascu et al. put it, that "an overall wetting of the Arctic is expected as a consequence of increased moisture in the atmosphere due to reduced sea ice (Higgins and Cassano, 2009) and added transport into the Arctic (Zhang et al., 2013)," it would appear that the warming expected by many climate scientists to occur in response to rising atmospheric CO2 concentrations should produce a significant negative feedback throughout the high Arctic that would tend to reduce the degree of warming that has historically been projected to occur there.

References
Bhatt, U.S., Walker, D.A., Raynolds, M.K., Comiso, J.C., Epstein, H.E., Jia, G., Gens, R., Pinzon, J.E., Tucker, C.J., Tweedie, C.E. and Webber, P.J. 2010. Circumpolar Arctic tundra vegetative change is linked to sea ice decline. Earth Interactions 14: 1-20.

Epstein, H.E., Meyers-Smith, I and Walker, D.A. 2013. Recent dynamics of arctic and sub-arctic vegetation. Environmental Research Letters 8: 10.1088/1748-9326/8/1/015040.

Higgins, M.E. and Cassano, J.J. 2009. Impacts of reduced sea ice on winter Arctic atmospheric circulation, precipitation, and temperature. Journal of Geophysical Research 114: 10.1029/2009JD011884.

Schaefer, K., Zhang, T.J., Bruhwiler, L. and Barrett, A.P. 2011. Amount and timing of permafrost carbon releases in response to climate warming. Tellus 63: 165-180.

Schuur, E.A.G., Vogel, J.G., Crummer, K.G., Lee, H., Sickman, J.O. and Osterkamp, T.E. 2009. The effect of permafrost thaw on old carbon release and net carbon exchange from tundra. Nature 459: 556-559.

Zhang, X., Juanxiong, H., Zhang, J., Polyakov, I., Gerdes, R., Inoue, J. and Wu, P. 2013. Enhanced poleward moisture transport and amplified northern high-latitude wetting trend. Nature Climate Change 3: 47-51.