By: Lykke E. Andersen*
The 21st of September marks the beginning of spring in the southern hemisphere. In Bolivia, it is also the day of Love, Peace, Students, Doctors and Photographers (!!). Most relevant for this article, however, today is usually the day of the year that the extent of sea ice in the northern hemisphere reaches rock bottom, much to the concern of polar bears (or at least people concerned about polar bears). In the southern hemisphere, on the other hand, sea ice usually reaches its maximum extent on this day (possibly to the delight of penguins).
In this satellite era, the extent of sea ice is an easy-to-measure indicator about the state of the global climate (warmer climate -> less ice), but the extent of sea-ice itself also affects the global climate (less ice cover -> lower albedo (reflectance) -> warmer climate). Thus, many people watch this indicator closely, and the availability of several independent web-sites with daily updated data on sea-ice extent makes this possible, and even delightful (at least for data-freaks like me).