The consequences of global warming could be twice as bad as previously suggested, according to a new report published last week in the Proceedings of the National Academy of Sciences.
Using new mathematical prediction models, the report said global oceans could rise by about six feet by the year 2100, about double previous estimates.
The new estimates call into question the findings of the fifth climate assessment report by the Intergovernmental Panel on Climate Change, published in 2013, which showed that without major reductions in emissions of greenhouse gases, oceans could rise by between 20 and 38 inches.
That would be enough to flood, for example Miami, Lower Manhattan, and Shanghai. Those estimates may have been conservative, however, based on estimates that the climate could get warmer by 2 degrees Celsius (3.6 degrees Fahrenheit), a number previously seen as the worst case scenario.
While the 2013 study considered the most likely outcomes—those with a probability ranging from 17 percent to 83 percent—the new one studies a wider range of possibilities, ranging from the 5th to the 95th percentile in outcomes. While the worst-case scenario with a 17 percent likelihood showed 3 degrees Celsius (5.4-degrees Fahrenheit) of warming, the 5th percentile ranges to 5 degrees (about 9 degrees Fahrenheit), with catastrophic additional effects on sea-level rise.
Greater sea-level rise would devour more of those cities, along with others such as Rio de Janeiro and Bangkok—and in the U.S., significant portions of Florida and New Jersey. It would add up to more than 690,000 square miles, or an area larger than California, and displace 187 million people, and more than 2.5 percent of the world's population, the report says.
"It really is pretty grim. Two meters is not a good scenario." the study's lead author, Jonathan Bamber, a professor of Physical Geography at the University of Bristol in England told CNN.
The study asked 22 ice-sheet scientists to make predictions about how global warming might affect zones in Greenland and Antarctica, using new data tools geared toward the study of such continents where excess water is currently trapped in melting ice sheets. The scientists say their observations show glaciers in these regions melting much faster than expected.
Bamber explained it this way: "For 2100, the ice sheet contribution is very likely in the range of 7 to 178 centimeters [2.75 inches to 5.8 feet] but once you add in glaciers and ice caps outside the ice sheets and thermal expansion of the seas, you tip well over two meters [6.6 feet.]"
The scientists acknowledge that the study of how global warming may affect ice sheets and sea-level rise is uncertain, but new tools give better predictions of less-likely outcomes than previous studies could represent.