Arctic browning: impacts of extreme events on vegetation and carbon balance in high latitude ecosystems

Climate change is happening faster in the Arctic than almost anywhere else in the world, and Arctic winters are warming especially rapidly. Among the consequences of this is an increase in the frequency of winter extreme events. These include climatic events, such as periods of extreme warmth, and biological events, such as outbreaks of defoliating insects. Such events are already having major impacts on Arctic landscapes, driving vegetation damage and decline across thousands of square kilometres. This loss of biomass and vegetation greenness is termed ‘Arctic browning’. Extreme events which drive Arctic browning are already occurring more frequently and with greater severity: a trend predicted to continue as climate change progresses. However the effects of these events on high latitude ecosystems are not well understood. In particular, their impacts on ecosystem CO2 balance are almost unknown. Furthermore, methods to upscale impacts across Arctic regions, or to assess the regional importance of extreme event-driven browning, do not yet exist. As the Arctic plays an important role in regulating global climate, there is an urgent need to address these uncertainties and to understand the role of extreme events in determining vegetation change and carbon balance at high latitudes. Therefore this thesis assesses the consequences of extreme events and subsequent browning for key ecosystem CO2 fluxes. How impacts vary with event type, across the growing season, and when associated with different browning responses is quantified. In all cases, major reductions in ecosystem CO2 uptake are found. Mechanistic insight into these changes is provided through additional field and remotely sensed data. Finally, the interacting climatic drivers underlying extreme event driven browning are analysed and upscaled. This represents the most comprehensive assessment to date of the causes and consequences of extreme winter events which drive Arctic browning.