A new University of Colorado study debunks the scientific speculation that global warming will cause seas to rise 20 feet or more by the end of the century.

In fact, the study says, global sea rise exceeding 6 feet looks to be a physical impossibility.

Tad Pfeffer, a fellow of CU’s Institute of Arctic and Alpine Research, and his colleagues made calculations using conservative, medium and extreme assumptions for sea rise expected from Greenland, Antarctica and the world’s smaller glaciers and ice caps. The team concluded the most plausible scenario, when factoring in thermal expansion due to warming waters, will be a total sea level rise of roughly 3 to 6 feet by 2100.

Pfeffer said the research calling for 20 to 30 feet of sea rise is not backed up by solid glaciological evidence.

Still, the team’s estimate of seas rising roughly 3 to 6 feet would be potentially devastating to low-lying coastal areas, he said.

“The gist of the study is that very simple, physical considerations show that some of the very large predictions of sea level rise are unlikely because there is simply no way to move the ice or the water into the ocean that fast,” Pfeffer said.

A paper on the subject will be published in today’s issue of Science, according to CU.

Pfeffer expects the findings will create a stir among scientists.

“There are a lot of people in the scientific community that think we are going to have these enormous sea level rises,” he said. “This is a bit of a reality check.”

Accurate sea level predictions are crucial, Pfeffer said, so policymakers can plan effectively to prepare cities and countries around the world.

“If we plan for 6 feet and only get 2 feet, for example, or vice versa, we could spend billions of dollars of resources solving the wrong problems,” he said.

Secondary problems that could be tied to sea-level rising, such as salt water moving into rivers and disrupting the ecology, could be overlooked if policymakers focus too heavily on building levees and sea walls, preparing for more drastic scenarios, Pfeffer said.

Gerald Meehl, a senior scientist at the National Center for Atmospheric Research, helped lead research on climate change caused by humans. As a co-author for the “Global Climate Projections” chapter of the pivotal report from the Intergovernmental Panel on Climate Change, Meehl said scientists grappled with a large uncertainty: the stability of Greenland and west Antarctic ice sheets.

The report estimated that the global sea level would rise less than a meter by 2100. Researchers found themselves criticized by others who accused them of being too conservative, Meehl said. Typically, he said, climate scientists are criticized for being alarmists.

A March 2006 study that involved researchers from NCAR and the University of Arizona said the Earth’s warming temperatures are on track to melt the Greenland and Antarctic ice sheets sooner than previously thought and that they ultimately could lead to a global sea level rise of at least 20 feet.

Co-authors of the latest study are Joel Harper of the University of Montana and Shad O’Neel of the Scripps Institute of Oceanography at the University of California, San Diego.

The study was funded by the National Science Foundation and a CU faculty fellowship.

The team researched the three primary contributors to sea rise and discovered the following:

Greenland: Researchers assumed future sea level rise at about 2 meters — or about 6.6 feet — by 2100 produced only by Greenland. Since rapid, unstable ice discharge into the ocean is restricted to Greenland glacier beds below sea level, they identified and mapped all of the so-called outlet glacier “gates” on Greenland’s perimeter — bedrock bottlenecks most tightly constraining ice and leaking water.

“For Greenland alone to raise sea level by two meters by 2100, all of the outlet glaciers involved would need to move more than three times faster than the fastest outlet glaciers ever observed, or more than 70 times faster than they presently move,” Pfeffer said. “And they would have to start moving that fast today, not 10 years from now. It is a simple argument with no fancy physics.”

Antarctica: The majority of ice entering the ocean comes from the Antarctic Peninsula and the Pine Island and Thwaites glaciers, Pfeffer said. Most of the marine-based ice in west Antarctica is held behind the Ross and Filcher-Ronne ice shelves, which Pfeffer’s team believes are unlikely to melt by climate or oceanographic changes during the next century.

Small glacier and ice cap contributions: These formations contribute about 60 percent of the world’s ice to oceans at present, a percentage that is accelerating.