Trajectory of the Arctic as an integrated system 1

The Arctic has experienced unprecedented changes in recent decades. Warming has been particularly strong in the Arctic since about 1980. Permafrost is warming, glaciers are melting and retreating, hydrological processes are being altered, and biological and social systems are changing in response. Knowing how the structure and function of Arctic ecosystems respond to climate change is important to understanding the future state of the Earth system and how humans must adapt. A range of biophysical states and processes, influenced by threshold and phase changes at the freezing point, are being altered. The components of the Arctic system are interdependently linked to all others and interconnected with the broader global system. However, unlike most other regions, winter conditions dominate in the Arctic for most or all of the year. When thawing of the snow, ice, and near-surface soil occurs, dramatic changes occur through threshold shifts in surface energy balance, glaciological and hydrological processes, and ecosystem dynamics. Such significant changes reverberate throughout the system, particularly if these parts of the system do not usually experience phase change or if the thawed period is lengthened. Also playing roles in the broad-scale changes across the entire system are the closely coupled feedbacks among various system processes. This Invited Feature, Trajectory of the Arctic as an Integrated System, takes a step in improving our understanding of the role of the Arctic in the global climate system by taking an integrated view of the terrestrial, aquatic, oceanic, and atmospheric processes and their linkages. The papers in this Invited Feature focus on the complex interplay of the components and processes that comprise the Arctic as a system. The approach taken in this Invited Feature is to provide syntheses of changes in the Arctic that involve (1) physical changes in the Arctic Ocean (Polyakov et al., Ivanov and Watanabe) and the terrestrial surface of the Arctic (Saito et al.), (2) the responses of growing season exchange of CO2 across Alaska (Ueyama et al.) and of the loading of dissolved organic carbon across the Arctic to changes in environmental conditions (Kicklighter et al.), (3) key component processes and pathways as the building blocks of feedback processes that can amplify or attenuate Arctic change (Hinzman et al.), and (4) an initial assessment of lost climate-regulation services due to changes in the Arctic (Euskirchen et al.). Polyakov et al. show that, while there has been multi-decadal-scale frequency variation in Arctic Ocean freshwater content, intermediate Atlantic water, and fast-ice thickness, fundamental changes in the high-latitude oceans have recently occurred that suggest that retreating sea ice may be playing a role in ventilating the ocean’s interior. Ivanov and Watanabe conduct a modeling study which indicates that the loss of summer sea ice is accompanied by an increase in seasonal ice volume and associated salt rejection. The enhanced salt flux from newly formed sea ice creates locally dense water, which sinks and increases the shelf-slope volume flux below the warm core of Atlantic water, more than offsetting the reduction of deep water formation (due to increased near-surface stratification) in the Greenland, Iceland, and Norwegian seas. Such a mechanism may be important in preventing a catastrophic shutdown of the global thermohaline circulation. The synthesis conducted by Saito et al. indicates that the terrestrial Arctic in recent decades has experienced shortened snow cover duration, increased winter precipitation, increased river discharge, increased soil temperature, and a deepening active layer. Furthermore, Saito et al. argue that the feedbacks associated with terrestrial changes in the Arctic can have consequences for regions outside the Arctic. The changes that have occurred in Alaska in recent decades appear to be responsible for a net uptake of CO2 by Arctic tundra and boreal forests in the state based on the synthesis of eddy covariance data by Ueyama et al. The analysis by Kicklighter et al. suggests that terrestrial ecosystems in the Arctic are losing carbon via increased loading of dissolved organic carbon into freshwater aquatic