Major changes are occurring across the North Atlantic climate system: in oceanic and atmospheric temperatures and circulation, in sea ice thickness and extent, and in key atmospheric constituents such as ozone, methane and particles known as aerosols. Many observed changes are unprecedented in instrumental records. Changes in the North Atlantic directly affect the UK's climate, weather and air quality, with major economic impacts on agriculture, fisheries, water, energy, transport and health. The North Atlantic also has global importance, since changes here drive changes in climate, hazardous weather and air quality further afield, such as in North America, Africa, and Asia. The ACSIS extension is a 1 year continuation of an ongoing 5-year strategic research programme called ACSIS: the North Atlantic Climate System Integrated Study. ACSIS brings together and exploits a wide range of capabilities and expertise in the UK environmental science community. Its goal is to enhance the UK's capability to detect, attribute (i.e. explain the causes of) and predict changes in the North Atlantic Climate System. ACSIS is delivering new understanding of the North Atlantic climate system by integrating new and old observations of atmospheric physics and chemistry, of the ocean state and of Arctic ice, complemented by detailed data analysis and state-of-the-art computer simulations. Observations are obtained from diverse sources including NERC's observational sites in the North Atlantic, satellite remote sensing and the NCAS FAAM aeroplane. The computer modelling component is providing simulations of the atmosphere, ocean, and sea ice with unprecedented spatial detail. The ACSIS extension will exploit advances made during the past 5 years to address specific new research questions which have arisen recently. It will investigate exciting evidence that changes in the climate of the North Atlantic/European region are much more predictable than was previously thought and will start to assess the impact of North Atlantic changes on the UK environment.