The reason anyone flies to a field station 200 miles north of the Arctic Circle to examine plants through a hand lens [16] is that the list of things keeping those plants short is short enough to write down. Anne Bjorkman of the University of Gothenburg gives two items: in winter, whatever pokes above the shallow insulating snow gets damaged by cold and wind, or eaten [6], and roots stop where the permafrost starts [7]. That is about as close to a single-variable system as field ecology gets, which is why the place has long been a usable window on what makes plants grow at all [19].
The variable moved. This part of Alaska is 2.7C (4.8F) warmer than it was 40 years ago, mostly from burning coal, oil and gas [5], which works out to roughly 0.68C per decade [1], in a region warming four times faster than the rest of the planet and doing it hardest in winter [4] - the season that does the cutting [6]. The response arrives as two separate things that look identical in a greenness index: plants already present are growing taller [8], and taller species that previously could not survive the cold are moving in [9].
Neither response leaves the driver alone. Sarah Elmendorf of the University of Colorado describes canopies dark enough and tall enough to stand above the snow, absorbing solar energy that white snow would have reflected [10]; Duncan Menge of Columbia calls darker tundra one of the main reasons for Arctic amplification [12]. The same shrubs catch more snow, the deeper snow keeps the ground warmer, and winter microbes release more methane [13]. Against that, Elmendorf offers one partial credit: the added greenery takes up more carbon dioxide [14]. Two amplifying mechanisms named by these researchers, one offset, and no sign attached to the sum [2].
So the long-term work Menge describes, establishing what controls how much plants grow and how green the tundra gets [15], is chasing a subject that is being edited by its own answer. The Toolik question of whether harsh conditions push plants to compete or cooperate [16] presumes a cast of species, and immigration is changing the cast [9]. Downstream, Chris Neill and Linda Deegan of the Woodwell Climate Research Center report willow stands spreading far enough to shade streams and alter the ecology of fish, insects and birds [17], a thermal effect delivered by vegetation rather than by the water. Their multiyear study asks whether an ecosystem can adapt and even evolve genetically under that stress; Deegan says that if it is, it is grounds for hope for the most fragile Arctic systems [18].
Elmendorf calls shrubification a major fingerprint of change that researchers can see [11]. It is also the thing doing the erasing. The cold was the control condition, and the cold is what is being withdrawn [8][9].
