It used to persist through the sunny summers, allowing several years of ice growth to accumulate. (sciencemag.org)
It's not just that we're seeing slower ice growth, but the high temperatures are actually melting old, thick ice as well. (slate.com)
The main cause is simply global warming: as the climate has warmed there has been less ice growth during the winter and more ice melt during the summer. (theguardian.com)