The observed rapid decline in the Arctic perennial ice cover is one of the most remarkable Signals of change in the Arctic region. Updated data now Show an even higher rate of decline of 9.8% decade–1 (1978–2005) than the previous report of 8.9% decade–1 (1978–2000). To gain insights into this decline, the variability of the Second-year ice, which is the relatively thin component of the perennial ice cover, is Studied. The perennial ice cover in the 1990s was observed to be highly variable, leading to relatively high production of Second-year ice that may in part explain the observed ice thinning during the period and have triggered further decline. The microwave Signature of Second-year ice is Shown to be different from that of the older multi-year ice types and, Surprisingly, more Similar to that of first-year ice. This in part explains why previous estimates of the area of multi-year ice during the winter period are considerably lower than the area of the perennial ice cover during the preceding Summer. Analysis of multichannel cluster maps in conjunction with Submarine ice-draft data indicates ability to detect regions covered primarily by Second-year ice and hence to infer ice-thickness information from the microwave data. The periodic increase of Second-year ice in the 1990s was apparently followed by continuous decline due in part to anomolously warm temperatures during the latter period that Shortened the ice Season and kept first-year ice from getting thick enough to Survive the Summer and become Second year ice.