Satellite data show that since the late 1970s, September Arctic sea ice extent has decreased by about 12% per decade. What's especially alarming is the decrease in multi-year ice. Sea ice is classified by age, usually as "new ice" or "multi-year" ice (meaning it survived many summer melting seasons). While new ice is very shallow, multi-year ice can grow to be quite thick, typically between 6 and 12 feet, and is very stable. A remarkable study was published in 2007 which measured the amount of multi-year ice in the Arctic. In 1987, 57% of the observed ice pack was at least 5 years old, and around 25% of it was at least 9 years old. When they surveyed the Arctic again in 2007, only 7% of the ice pack was at least 5 years old, and the ice that was at least 9 years old had all but vanished. Likewise, sea ice thickness and volume have decreased markedly since the beginning of the satellite era.