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.
"The estimate for the past 100 years forms the basis for future predictions" of the impact of changing climate on the world's oceans, he says. Miller and Bruce Douglas of Florida International University conducted the study, which appears in Thursday's edition of Nature.
The debate over the rate of the oceans' rise emerged in the mid- to late 1990s, researchers say. The Intergovernmental Panel on Climate Change (IPCC), which distills the latest research and lays out related policy issues for government leaders, noted in 1995 that the best evidence argued for an increase of 6 to 8 inches a century. The gains were measured at tide gauges worldwide.
Improved measurements of global ocean heat, however, indicated that the ocean had not warmed nearly enough to rise at the rate the tide gauges indicated. And glacial runoff was still thought to be a tiny contributor to ocean mass.
The situation grew muddier with a study published in 2002 by French researchers who compared satellite data with a new catalog of ocean temperature and salinity estimates and concluded that tide gauges overestimate the rate of sea-level rise.
Miller and Douglas, by contrast, used the tide gauges and raw oceanographic measurements of temperature, salinity, and density to reach their conclusions.
The tide gauges do not appear to be unduly influenced by local conditions, and the combined sources of direct data argue for the higher rate of increase, they say, and glacial melt as the leading cause.