Background
The authors write that "numerous studies have evaluated the dynamics of Arctic tundra vegetation throughout the past few decades, using remotely sensed proxies of vegetation, such as the normalized difference vegetation index (NDVI)," but they say that "while extremely useful, these coarse-scale satellite-derived measurements give us minimal information with regard to how these changes are being expressed on the ground, in terms of tundra structure and function."

What was done
In an effort to better understand what has happened on the ground in this regard, Epstein et al. indicate that they used "a strong regression model between NDVI and aboveground tundra phytomass, developed from extensive field-harvested measurements of vegetation biomass, to estimate the biomass dynamics of the circumpolar Arctic tundra over the period of continuous satellite records (1982-2010)."

What was learned
The six U.S. scientists found that the three southernmost subzones of Arctic tundra (C, D and E, going from north to south) "exhibited extensive increases in aboveground phytomass (20.9%, 25.6% and 20.6%, respectively), whereas the two northernmost subzones (A and B) showed substantially smaller increases (2.1% and 6.4%, respectively)." And because of the fact that subzones C, D and E comprise 87.5% of the tundra landmass and accounted for 95.5% of the initial tundra biomass in 1982, they determined that the total aboveground phytomass of the entire Arctic tundra increased by 19.8% over the past three decades.

What it means
The unstoppable warming- and CO2-induced greening of the earth continues. And in the case of the Arctic tundra, Epstein et al. conclude that the greening there "has major implications for nearly all aspects of tundra ecosystems, including hydrology, active layer depths, permafrost regimes, wildlife and human use of Arctic landscapes," as life proliferates across the circumpolar region.