Arctic sea ice extent decreased considerably along with the ice cover becoming thinner and more seasonal during the last decades. These observed changes have a strong impact on interactions between atmosphere and ocean and thus play a major role in Earth’s climate system. Until now, it is not possible to quantify shortwave energy fluxes through sea ice sufficiently well over large regions and during different seasons. In order to obtain Arctic-wide estimates of solar radiation under sea ice, new methods are necessary. In this thesis, an upscaling method combining a newly developed parameterization of light transmittance and remote sensing and reanalysis data is presented. The main result suggests that 96% of the total annual solar heat input under Arctic sea ice occurs in the time from May to August, hence in the course of only four months of the year. Sensitivity studies indicate that once the melt season begins two weeks earlier, an increase by 20% of the total annual solar heat input through sea ice is shown. Therefore, the transition period from spring to summer, particularly the timing of the melt season, substantially affects the light availability under ice. Furthermore, a more seasonal ice cover and a higher melt pond coverage lead to higher fraction of solar radiation being transmitted through the sea ice in summer. This positive correlation between enhanced melting and increasing transmittance can be described as ’transmittance-melt feedback’. Assuming an ongoing ice thinning, the transmittance-melt feedback results in a further increase in transmitted and absorbed heat fluxes. Changes in timing and amount of light penetrating through Arctic sea ice might also influence biological and geochemical processes as well as basal and internal melt and freeze rates. These positive feedbacks affect the mass and energy budget of sea ice and alter crucially the interaction of atmosphere and the upper ocean.