Modern analog technique (MAT) applied to Antarctic diatoms is a new approach for quantitative sea-ice paleoreconstructions in the Southern Ocean. In a first step we show that MAT is a better approach than the Imbrie and Kipp Method to reconstruct the modern sea-ice pattern. We then use this approach to reconstruct sea-ice presence in number of months per year during the last glacial maximum (LGM). At this time, sea-ice presence was greater than today, leading to a shorter diatom growing season. The maximum sea-ice extent, inferred from quantitative values of sea-ice presence, was located 5--8¿ north of its actual position, leading to double the surface of modern winter sea ice. This greater sea-ice extent may have played a significant role on atmospheric and surface oceanic circulations and therefore on southern mid-latitude and high-latitude climates. It may also have reduced the amount of heat, moisture, and CO2 from the ocean to the atmosphere, thus participating in the lowering of atmospheric CO2 during the LGM. ¿ 1998 American Geophysical Union