One contributor is an increase in the amount of heat flowing from the North Atlantic into the Arctic ocean. Another factor is that the volume of old, thick ice

One contributor is an increase in the amount of heat flowing from the North Atlantic into the Arctic ocean. Another factor is that the volume of old, thick ice has been declining as well, so the ice present in 2015 is younger than what was present in 2000. This ice is less likely to persist through the summer melt season.