Winter Arctic sea ice thickness, 2004. This image is part of a sequence showing the change in Arctic sea ice thickness from 2003 to 2008. While the extent of sea ice might not change as rapidly, the changes in thickness can be much greater. The thickness scale is shades of blue from zero metres of ice (dark blue) to five metres (white). The image for 2008 shows large thinning of ice over and near the North Pole. In addition, thicker multi-year ice is being replaced by thinner ice that melts and reforms each year. The data for these images were obtained by NASA's Ice, Cloud and land Elevation Satellite (ICESat).