Complex sentence: Jakobshavn Isbræ is a major contributor to the mass balance of the Greenland ice sheet, producing some 10% of all Greenland icebergs some 35 billion tonnes of icebergs calved off and passing out of the fjord every year.
Split into simpler sentences: Jakobshavn Isbræ is a major contributor to the mass balance of the Greenland ice sheet, producing some 10% of all Greenland icebergs. Some 35 billion tonnes of icebergs calve off and pass out of the fjord every year.