The timing of the seasonal freeze-thaw cycle of arctic lakes affects ecological processes and land-atmosphere energy fluxes. We carried out detailed ice-phenology mapping of arctic lakes, based on daily surface-reflectance time series for 2000-2013 from MODIS at 250 m spatial resolution. We used over 13,300 lakes, area >1 km2, in five study areas distributed evenly across the circumpolar Arctic - the first such phenological dataset. All areas showed significant trends towards an earlier break-up, stronger than previously reported. The mean shift in break-up start ranged from -0.10 days/year (Northern Europe) to -1.05 days/year (central Siberia); the shift in break-up end was between -0.14 and -0.72 days/year. Finally, we explored the effect of temperature on break-up timing and compared results among study areas. The 0 °C isotherm shows the strongest relationship (r = 0.56-0.81) in all study areas. If the trend in early break-up continues, rapidly changing ice phenology will likely generate significant, arctic-wide impacts.