Scientists Predict 20-Foot Sea Level Rise


Published Jul 12, 2015 7:25 PM by The Maritime Executive

When past temperatures were similar to or slightly higher than the present global average, sea levels rose at least 20 feet (six meters), suggesting a similar outcome could be in store if current climate trends continue, states a team of scientists after conducting a study led by the U.S.

The new estimate far exceeds some, such as those made by the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report, which predict a rise of over three feet (one meter) by 2100.
The U.S. findings, published in the journal Science, cite that in the past sea levels rose in response to melting ice sheets in Greenland and Antarctica. Lead author of the study, Andrea Dutton, a University of Florida geochemist, says, “This evidence leads us to conclude that the polar ice sheets are out of equilibrium with the present climate.”

Warming temperatures contribute to sea level rise by expanding ocean water, melting mountain glaciers and ice caps and causing portions of the Greenland and Antarctic ice sheets to melt or flow into the ocean. 

Dutton and an international team of scientists assessed evidence of higher sea levels during several periods to understand how polar ice sheets respond to warming. Combining computer models and observations from the geologic record, they found that during past periods with average temperatures 1 to 3°C (1.8 to 5.4°F) warmer than preindustrial levels, sea level peaked at least 20 feet higher than today.

“As the planet warms, the poles warm even faster, raising important questions about how ice sheets in Greenland and Antarctica will respond,” she said. “While this amount of sea-level rise will not happen overnight, it is sobering to realize how sensitive the polar ice sheets are to temperatures that we are on path to reach within decades.”

The researchers concluded that sea levels rose 20 to 30 feet higher than present about 125,000 years ago, when global average temperature was 1°C higher than preindustrial levels (similar to today’s average). Sea level peaked somewhere between 20 and 40 feet above present during an earlier warm period about 400,000 years ago, when global average temperatures are less certain, but estimated to be about 1 to 2°C warmer than the preindustrial average. 

During those times, atmospheric carbon dioxide levels peaked around 280 parts per million, but today’s levels are around 400 ppm and rising. The team of researchers looked at the last time period when carbon dioxide was this high – about three million years ago – but couldn’t get a confident estimate on sea-level rise, in part due to land motion that has distorted the position of past shorelines.

Climate Central estimates that a 20-foot rise would mean that Florida would lose land that houses more than nine million people, followed by New York, California, Louisiana, Virginia, and New Jersey, each with more than a million people in threatened areas. 

According to the U.S. EPA, since 1870, the global sea level has risen by about eight inches. Estimates of future sea level rise vary for different regions, but global sea level for the next century is expected to rise at a greater rate than during the past 50 years.

Local factors influence future relative sea level rises. The EPA states that, assuming that these historical geological forces continue, a two-foot rise in global sea level by 2100 would result in the following relative sea level rise: 

2.3 feet at New York City
2.9 feet at Hampton Roads, Virginia
3.5 feet at Galveston, Texas
one foot at Neah Bay in Washington State.