A major new breakthrough in Antarctic indicates that a melting glacier there that contributes more to sea-level rice than any other ice source on Earth may have started melting in the 1940s, and not more recently. Scientists say in a new study that the Pine Island Glacier started its decline when warm ocean water first got underneath the glacier, which happened in the middle of the 20th century.

That’s a big deal because it indicates that the glacier has had an “erratic” past, retreating and then growing again over the decades, which suggests that we shouldn’t expect these giant glaciers to behave in a steady way in the future. It’s an indication that scientists must continuously monitor such glaciers in order to truly understand what is happening to them, according to a Lawrence Livermore National Laboratory statement.

The glacier is currently thinning quite fast, losting 2 meters in elevation every single year. Computer models suggests it could suddenly collapse, and the glacier by itself could add 10 millimeters to sea levels of the coming decades.

“Our results suggest that, even when climate forcing (such as El Niños, which create warmer water) weakened, ice-sheet retreat continued,” said James Smith of the British Antarctic Survey and lead author of an article appearing in the Nov. 23 issue of the journal, Nature.

“This finding provided the first hint that the recent retreat could be part of a longer-term process that started decades or even centuries before satellite observations became available,” Smith said.