Arctic browning: extreme events and trends reversing arctic greening.

NOAA’s recent assessment of Arctic greenness has reported a remarkable finding: the Arctic is browning (Epstein et al., 2015). Whilst a clear greening trend has been apparent for most of the satellite record’s 33 year history (indicating an increase in biomass and productivity), there is now an overall decline in greenness from 2011 to 2014. If this is a new direction of travel for arctic vegetation, rather than just a temporary departure from long-term greening, this has major implications for not only our understanding of the future of arctic vegetation, but also arctic carbon, nutrient and water cycling, surface energy balance and permafrost degradation, and therefore feedback to climate, all of which are strongly influenced by vegetation composition, productivity and biomass. The urgency in understanding what is happening here is clear. Most models predict arctic greening; to what extent are they wrong, and why? Arctic greening has rightly received much attention. Satellite and observational data have consistently confirmed an increase in vegetation cover and productivity in many regions (Xu et al., 2013) caused most notably by the expansion of large stature deciduous shrubs (Myers-Smith et al., 2015). Likewise, field simulation experiments provide strong evidence that greening is driven by warming (Elmendorf et al., 2012). However, the magnitude of the recent browning is large and cannot be ignored. For both the Eurasian Arctic and the Arctic as a whole, Epstein et al. report the 2014 maxNDVI (greenness) to be below the 33-year average. To find lower values than 2014, you have to go back to 1996 for the whole Arctic and to 1993 for the Eurasian Arctic. However, while browning is the overall trend, there is considerable regional variation and the Arctic is not browning everywhere. These findings raise important questions that represent priority challenges, including (1) what is driving the browning? (2) is browning the new trajectory or only a temporary reversal of greening? and (3) what arctic regions and vegeta-