For some reason, BuzzFeed on Monday seized on a National Geographic interactive from last month showing what the continents would look like if the oceans rose 216 feet. That figure, credited to the U. S. Geological Survey, doesn't appear online. And, in fact — it's probably too low.

Here's how the magazine describes the rationale for its analysis of what a drowning world will look like.

According to the U.S. Geological Survey, sea level on an iceless Earth would be as much as 216 feet higher than it is today. It might take thousands of years and more than a thousand parts per million to create such a world—but if we burn all the fossil fuels, we will get there.

That "thousands of years" caveat is an important one; the maps suggest what the world would look like if all of the glaciers and ice caps on Earth melted under warmer temperatures. That's not going to happen in your lifetime. It may not happen in the United States' lifetime.

The problem is that we can't find that "216 feet" figure anywhere — and the only numbers from the USGS say the sea level rise will be higher. In 2004, USA Today used a figure of 215 feet, crediting the USGS, but that isn't anywhere to be found, either.

The agency's sea level rise information page puts the number at 80 meters — some 262 feet. It's broken down by melt location.

  • If all of Alaska's glaciers melted, sea level would rise ~ 0.05 meters (about 0.16 feet).
  • If all of Earth's temperate glaciers melted, sea level would rise ~ 0.3 meters (about one foot).
  • If all of Greenland's glaciers melted, sea level would rise ~ 6 meters (about 19.7 feet).
  • If all of Antarctica's glaciers melted, sea level would rise ~ 73 meters (about 240 feet).

That appears to be based on this 2000 study, which puts the exact figure at 80.32. One USGS report (that isn't dated) puts the figure higher still: "If all the present glacial ice were to melt from Antarctica and Greenland, the oceans would rise another 300 feet (90 meters) and inundate most of the coastal cities of the world."

We've reached out to the USGS for clarification. But it seems likely that the base figure is somewhat higher than the National Geographic projections.

More importantly, the raw figure of ice melt — including only the amount of water contained in the ice itself — vastly underestimates how much sea levels will rise. The problem is that as fluids and gasses get warmer, they increase in volume. Warmer water takes up more physical space than colder water, because the molecules have more energy. According to the UN's Intergovernmental Panel on Climate Change, which produces regular reports on the effects of global warming, the link between warmer water and increased volume is substantial.

"On a multi-millennial time scale," its most recent draft report reads, "the range from Earth System Models of Intermediate complexity suggests that thermal expansion contributes between 0.20–0.63 m per °C of global mean temperature increase." In other words, for each degree that the temperature increases, the seas will rise between 0.2 and 0.63 meters. The temperature climbs, the ice melts — and the oceans expand.

Again, you're not going to see this happen. What you'll see will be more subtle: wetter storms, hotter and drier summers, more flooding as we saw from Hurricane Sandy. But somewhere down the line, assuming our use of fossil fuels isn't checked, we could very well see ocean levels that are 216 feet higher than today. And that may just be a way station before even higher increases to come.

Update, Tuesday: The USGS responded with its most current data.

  • Greenland: 7 meters
  • West Antarctic ice sheet: 3.3 meters
  • East Antarctic ice sheet: 52 meters
  • All other glaciers and small ice caps: 0.5 meters

That's 62.8 meters, total — 206 feet in increase. But that doesn't include the sea level expansion.

Update, Wednesday: Jason Treat from National Geographic wrote in.

[A]fter speaking with a number of experts, we were led to Phillipe Huybrechts at Vrije Universiteit Brussel, who most of those we spoke with considered an authority on the subject (including some at USGS). He quoted us these numbers:

  • 7.5 m from Greenland
  • 0.5 m from glaciers and ice caps
  • 58 m from Antarctica (latest 2013 numbers)
  • 66 m in total