1 melted Greenland ice sheet = 23-foot rise in global sea level

That’s the rough equation behind a frequently cited stat in news coverage of climate change. According to a transcript, Anderson Cooper said in a CNN special report this week, “If the entire ice sheet dissolved, sea levels would rise by 23 feet — spurning a global catastrophe that would flood coastal cities and displace tens of millions of people. Scientists don’t think the entire ice sheet can melt any time soon, but every inch of sea level rise counts. Millions live near coastlines less than three feet above sea level.” The dire 23-feet number also appeared in a CNN.com article this week, as well as in the Washington Post. (Numbers Guy reader Brian Sivy suggested I examine the estimate.)


The math behind the assumption is straightforward. To begin, you’d need to know the quantity of ice on Greenland. In 2001, scientists measured a total volume of 2.93 million cubic kilometers, using ice-penetrating radar similar to the system described here. Should all that ice melt, it would take up less space because its density is about 11% lower than seawater’s, according to an estimate used by a working group of the Intergovernmental Panel on Climate Change, a United Nations research body. That translates to 2.61 million cubic kilometers of seawater. The IPCC group’s calculation, shown in Table 4.1 of this report, assumes the area of the world’s oceans is 362 million square kilometers.

Distribute all that additional seawater equally across the world, convert to the English measurement system (used only in the U.S., Liberia and Myanmar), subtract a bit to account for the water that will be needed to cover the parts of Greenland below sea level, and you get about 23 feet.