Few effects from global warming raise more red flags than rising sea levels. The topic has led to a growing pile of conflicting research trying to answer the questions: How fast, and why?
Now, a pair of US scientists conclude that the oceans rose at a global average rate of 1.5 to 2 millimeters a year (6 to 8 inches a century), confirming a hotly debated, decade-old estimate. But their work also points to the key driver of this change: water from melting glaciers and not, as some have argued, a natural swelling of the oceans caused by higher temperatures.
Pinpointing glacial melt as the leading source of the oceans' rise is a new finding that contradicts several past studies.
The work "is a major contribution, and will go a long way toward explaining some of the current enigmas in our understanding of the earth's system," says Mark Meier, a glacier expert at the University of Colorado at Boulder.
"This was a surprise to us," says Laury Miller, an oceanographer with the National Oceanic and Atmospheric Administration's Laboratory for Satellite Altimetry in Silver Spring, Md. "But it just popped out of the data." And, he notes, it dovetails with studies that show oceans now contain more fresh water than 40 years ago.
The difference between these findings and the findings of other scientists who argue for a slower, 4-inch-per-century rise in sea level may seem small, Dr. Miller notes, but it's significant. More than 100 million people worldwide live within a mile of a coastline and would be first affected by any rise.