21. Sea ice extent is defined as the area with at least 15 % ice cover . 22. There's also been very little ice cover on the Great Lakes, resulting in increased evaporation. 23. Referring to a moon of Jupiter, McKay said, " Europa has a big ice cover . 24. Currently, an ice cover exists from April to September; it can reach a thickness of. 25. This spring his institute will publish a striking catalogue of the world's shrinking ice cover . 26. The map of Greenland depicted 15 % less ice cover than in the 1999 edition. 27. The water temperature varies between and and the ice cover on the lake is thick. 28. Previous studies had suggested that the year-round ice cover would last 300 years or so. 29. Of the ice cover still present in 2000, almost 40 percent had disappeared by 2011. 30. The last advance of the ice cover was more limited, affecting only the eastern Burren.
