Who can forget Al Gore’s 20-foot wall of water inundating the world’s coastal cities in his Oscar-winning film, An Inconvenient Truth? In the book version, Gore warned that, “If Greenland melted or broke up and slipped into the sea — or if half of Greenland and half of Antarctica melted or broke up and slipped into the sea — sea levels worldwide would increase by 18 to 20 feet.” He estimated that more than 100 million people living in Beijing, Shanghai, Calcutta, and Bangladesh would be “displaced,” “forced to move,” or “have to be evacuated.” The World Trade Center Memorial would be “under water.”
Well, yes, if half of Greenland Ice Sheet (GIS) and half the West Antarctic Ice Sheet (WAIS) suddenly let go, these dreadful things would happen. But there’s no evidence anything of the sort is likely to occur.
The WAIS is actually more stable than scientists once believed, as Science magazine reported last year. Scientists using radar imaging discovered a “miles-long pile of sediments as thick as 100 feet deposited beneath the Ross Ice Shelf over the last 1,000 years.” (The Ross Ice Shelf, the southern flank of the WAIS, is the largest ice shelf in Antarctica.) Previous research suggested that sea level rise of a few meters might float the ice shelf off its moorings, hastening its breakup and demise. Thanks to the stabilizing sedimentary deposits, the researchers now estimate sea levels would have to rise by 35 feet to float the Ross Ice Shelf. In other words, more than half the WAIS would have to fall into the sea to raise sea levels enough to cause half of it to fall into the sea.
Gore also warned that “the ocean flows underneath large sections of this ice shelf, and as the ocean has warmed, scientists have documented significant and alarming structural changes on the underside of the ice shelf” (AIT, p. 190). Gore provided no specifics and no references to the scientific literature, so it’s anybody’s guess what “significant” and “alarming” structural changes he had in mind. In any event, Nature magazine (subscription required) recently reported that the deep waters off Antarctic warmed about 0.1 degrees C during 1989 to 2005, but now appear to be cooling.
So how much sea level rise can we reasonably expect in the 21st century? The IPCC guesstimates a range of 0.18 to 0.59 meters, or 7 to 23 inches (see p. 8 of the Summary for Policymakers). The mean forecast is 14 inches — a far cry from Gore’s 18-20 feet. But even that projection may be overblown.
The IPCC forecast is based, in part, on estimates of past sea level rise. The IPCC believes that sea levels rose 1-2 mm/yr during the 20th century. However, a new study in Global and Planetary Change by Indian researchers finds that sea levels in the Indian Ocean rose between 1.06– 1.75 mm/yr, with an average of 1.29 mm/yr. “Imagine that,” comments World Climate Report, “once someone collects data in their part of the world, they seem to conclude that sea level is rising at a rate slower than the rate reported by the IPCC.”
Another study in Global and Planetary Change, also reviewed by World Climate Report, finds that global sea levels rose 1.48 mm/yr from 1955 to 2003, with no observable acceleration through those five decades. My friends, 1.48 mm/yr translates into 14.8 centimeters in a century, which equals 5.8 inches. Now is certainly not the time to sell that beachfront property.