The downward trends in minimum sea ice area, extent and volume have become stunningly obvious to anyone who sees images such as the top row in this blog's long-term graphs page. But what about trends in annual maximum, or the in-between seasons? Cycle plots (Cox 2006) provide an unorthodox but information-rich visualization that tracks changes in the ice for each month across a series of years. Figure 1 shows a cycle plot of monthly mean Arctic sea ice extent and area, based on NSIDC data from November 1978 through August 2012 -- the full period of their satellite record.
Each squiggly line in Figure 1 follows the mean sea ice extent or area for a particular month, across all years covered by these data. The vertical axis indicates ice cover, from zero to about 16 million km2. In August 2012, area reached the lowest point yet observed, 2.83 million km2, with September (usually the lowest month) yet to come.
Trends in northern sea ice volume (PIOMAS estimates) are even more clear. But for a post later this winter (austral summer) I'll take a cycle-plot look at the unclear patterns of Southern Hemisphere sea ice area and extent.