evidence: Layers of frozen seawater cap the Arctic Ocean. This sea ice grows dramatically each winter, usually reaching its maximum in March. It melts just as dramatically each summer, reaching its minimum in September. It is a natural fluctuation that has been going on for thousands of years, but winter and summer trends have become negative in recent decades. [...] The new summer lows, combined with poor wintertime recoveries, have fueled a persistent decline in Arctic sea ice. Since 2002, ice extent at the summer minimum has not returned to anything approaching the long-term average. Though winter ice extent has fluctuated, satellite and in situ observations have shown that there is much less multiyear ice and more annual ice. Cycles of natural variability such as the Arctic Oscillation are known to play a role in Arctic sea ice extent, but the sharp decline cannot be explained by natural variability alone. Natural variability and rising global temperatures have worked together to melt greater amounts of Arctic sea ice. Some modelers have forecast an ice-free Arctic for at least part of the year before the end of the 21st century.