IT IS

o an observer from space, the growth and retreat of the white ring of sea ice around Antarctica would be one of the clearest signs that Earth’s climate varies on seasonal and longer time scales. However, the record of satellite observations is too short to assess whether there have been cli-mate-related trends in sea ice extent. On page 1203 of this issue, Curran et al. (1) show that a chemical (methanesulfonic acid, MSA) measured in an Antarctic ice core may respond chiefly to sea ice extent in the nearby ocean. They conclude that sea ice in that part of the ocean was stable from 1840 to 1950, but has de