For example satellite records have revealed a significant decrease in sea ice extent in all months, especially in summer. There has been a reduction in summer sea ice extent from about 7 million square kilometres in the late 1970s to around 3.4 million square kilometres in 2012; a reduction of over 50%. Time-series analysis of submarine records concluded that the mean thickness has declined by more than 40%, and taken together that these changes have led to a fundamental shift in the ice regime in the Arctic. The Arctic is no longer a region dominated by a thick multi-year ice (MYI), but it is a regime controlled by thinner, more dynamic, first year ice (FYI). In fact complex computer models predict that the Arctic Ocean is on track to become mostly ice free in summer within a few decades, if not earlier. Our understanding of the functioning of the Arctic marine ecosystem has been overwhelmingly derived from a MYI setting, rather than the FYI dominated Arctic of recent years. As a result, our current state of knowledge of these processes and the validity of many of the parameterisations presently embedded in computer models becomes more questionable. For example, recent studies revealed that that the transition from MYI to FYI summer ice cover corresponds to an increase of 200% in light transmittance into the upper ocean. Light is one of the critical ingredients that controls primary production under sea ice, and in addition recent modelling studies have shown that the sea-ice algal blooms are triggered by light availability. Therefore if we are to understand and predict ecosystem function in this ‘new Arctic’, we must understand and correctly parameterise the light climate under this new FYI environment we find ourselves in today, and for the foreseeable future. To do this correctly we need a holistic approach that seamlessly brings biology, optics, sea ice and ocean physics, together with satellite remote sensing and cutting-edge modelling. This programme, Eco-Light, does this by utilising the modelling and observational expertise of the United Kingdom and Germany to better understand the influence of changing Arctic sea ice on ecosystem function. This is achieved by observationally constraining and improving the parameterisations of processes of light transmission in this ‘new’ Arctic, and by doing so we will better understand its influence on ecosystem function. This information will be used to resolve euphotic depths for primary productivity, and to predict the effects of changing snow-ice-ocean constituents on the underwater light climate. Eco-Light will undoubtedly produce a strong legacy for UK and Germany leadership in key aspects of Arctic observational science, ecosystem dynamics, and predictive modelling.