One of the most fundamental changes in the Arctic Ocean sea ice over the last decade has been a decrease in the amount of older ice. Ice that has survived at least one melt season, called multi-year ice, becomes thicker each year as new ice forms underneath it. In contrast, first-year ice, which develops through the winter and then melts completely during summer is relatively thin, making it more sensitive to changes in winds and air temperature.
There was 42% less multi-year ice at the end of the 2008 summer melt season than there was in 2005. The volume of multi-year ice in winter shrank between 2005 and 2008, while the first-year ice gained volume, almost compensating for the loss of multi-year ice. First-year ice covered more than two-thirds of the Arctic Ocean in 2007. These trends set the stage for further rapid declines in sea ice in future summers.