Arctic sea ice volume meltdown has come 1% of the way from Winter Maximum to a Blue Ocean Event per April 7th. The lowest volume year ever, 2017, by the same date, was at 0%.
Q: Where’s the data source for this plot?
A: Here: http://psc.apl.uw.edu/research/projects/arctic-sea-ice-volume-anomaly/data/
Q: How can anyone know if there is a Blue Ocean Event?
A: Easy. Use a computer. If you have less than 1000 km³ sea ice volume in the Arctic, then you have a Blue Ocean Event.
Q: Why would I even do that?
A: Let’s say you want to know when the Arctic Ocean goes ice–free. Basing this on satellite measurements instead of gut feeling makes your conversations more interesting.