We didn't really know--until now. The ocean, which has risen an average of 8 inches since 1900, should rise another 3 feet or so by 2100, but without an accurate record of where we started, we can't know if the melting starts to speed up or (hopefully) slow down.
On the Climate Central website, Michael D. Lemonick quotes researcher Ian Joughin as saying, "There are lots of processes in play and it remains unclear whether the ice loss will level out or slow down or speed up. We thought we understood ice sheets, but it's clear we don't. We need a lot more observations and a lot better modeling."
NASA checks this with satellites, but the satellite images don't always agree. One problem with information from individual satellites is that they've been in orbit at different times over the decades since observations began, and that they use different techniques for measuring ice loss.
Lemonick quotes researcher Andrew Shepherd as saying, "What we found was that there was a sweet spot in the mid-2000s when 10 satellites were flying at once."
Now a team of 47 scientists from 36 laboratories looked at data from 10 different satellites in order to come up with numbers everyone is in agreement with: between 1992 and 2011, Greenland lost an average of 152 billion metric tons of ice per year, while Antarctica lost 71 billion.
Lemonick quotes Shepherd as saying, "The new estimates end 20 years of uncertainty."
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