Preface "The changing Arctic and Subarctic environment: proxy- and model-based reconstructions of Holocene climate variability in the northern North Atlantic"

International audienceThis special issue originates from the EU FP7-Marie Curie initiative "CASE" (The Changing Arctic and Subarctic Environment), an Initial Training Network (ITN) on marine biotic indicators of recent climate changes in the high latitudes of the North Atlantic (http://caseitn.epoc.u-bordeaux1.fr/).This four-year project (April 2010-March 2014) implemented multidisciplinary research initiatives aiming at assembling palaeoclimate data from the Holocene through recent field programmes in the Nordic Seas, and at integrating palaeoclimate information with modern biological and climate modelling data. Associated objectives were to recruit and train a new generation of European polar scientists with expertise on the Nordic Seas, and to develop a network of European experts in polar research to build structures focused on long-term collaboration in Arctic science.Together with the Arctic Ocean, the Nordic Seas and the Barents Sea have shown unprecedented changes in physical and chemical conditions in recent decades, which directly influence the ecosystem structure and processes. The extreme sensitivity of the northern North Atlantic to climate changes is related to the intricate connection, within this oceanic realm, of cryospheric (ice sheets and sea ice), atmospheric (winds related to strong gradients in sea-level pressures), and oceanic processes (through the opposing poleward flow of Atlantic water and southward flow of polar waters). The recent decline in seasonal sea ice extent is of particular concern as it modulates the reflection of incoming solar radiation, and influences the exchanges of heat and moisture between the surface ocean and the atmosphere. Moreover, sea ice also plays a central role in the efficiency of shelf (brines) and deep ocean convection processes, thus influencing the climate on a global scale.Decadal to millennial scale reconstructions of Holocene climate and environments from archives, such as marine sediment cores, and from palaeoclimate modelling hold keys to a better evaluation of the magnitude and implications of the ongoing warming with regard to the full range natural variability of the climate system. This special issue addresses some of the key questions raised by the CASE project and related to the present polar amplification of climate change: -Is the present global warming and its amplification in the Arctic and Subarctic domains a unique event on the scale of the Earth recent history (last 10 000 years)? -How do past decadal-to centennial-scale natural climate changes, as recorded in marine sediments and ice cores, stand in the context of the present human-induced modulation of climate? -How did Holocene variability in key physical elements affect the structure and diversity of the planktonic ecosystem in the Arctic and Subarctic domains