A new analysis of the geological record of the Earth’s sea level, carried out by scientists at Harvard and Princeton University’s Woodrow Wilson School of Public and International Affairs, employs a novel statistical approach that reveals the
planet’s polar ice sheets are vulnerable to large-scale melting even under
moderate global warming scenarios. Such melting would lead to a large and
relatively rapid rise in global sea level.

The analysis indicates an additional 2 degrees of global warming could commit the planet to 20 to 30 feet of long-term sea level rise. This rise would inundate low-lying
coastal areas where hundreds of millions of people now reside. It would
permanently submerge New Orleans and other parts of southern Louisiana, much of
southern Florida and other parts of the U.S. East Coast, much of Bangladesh,
and most of the Netherlands, unless unprecedented and expensive coastal
protection were undertaken. And while the researchers’ findings indicate that
such a rise would likely take centuries to complete, if emissions of greenhouse
gase
s are not abated, the planet could be committed during this century to a
level of warming sufficient to trigger this outcome.

The study, “Probabilistic Assessment of Sea Level During the Last Interglacial Stage,” is published in today’s edition of the journal Nature. The researchers include Jerry X. Mitrovica, a professor of geophysics in Harvard’s Department of Earth and Planetary Sciences; Robert Kopp, who conducted the work as a postdoctoral researcher in Princeton’s
Department of Geosciences and the Woodrow Wilson School; Frederik Simons, a Princeton assistant professor of geosciences; Adam Maloof , an
assistant professor of geosciences at Princeton; and Michael Oppenheimer, a
professor of geosciences and international affairs in the Woodrow
Wilson School.

As part of the study, the researchers compiled an extensive database of geological sea level indicators
for a period known as the last interglacial stage about 125,000 years ago.
Polar temperatures during this stage were likely 5 to 9
degrees Fahrenheit warmer than today, as is expected to occur in the future if
temperatures reach about about 4 to 6 degrees
Fahrenheit above pre-industrial levels.

“The last interglacial stage provides a historical analog for futures with a fairly moderate amount of
warming; the high sea levels during the stage suggest that significant chunks
of major ice sheets could disappear over a period of centuries in such
futures,” Kopp said. “Yet if the global economy continues to depend
heavily on fossil fuels, we’re on track to have significantly warming by the end of century than occurred during the
last interglacial. I find this somewhat worrisome.”

Oppenheimer added, “Despite the uncertainties inherent in such a study, these findings should send a strong
message to the governments negotiating in Copenhagen that the time to avoid
disastrous outcomes may run out sooner than expected.”

The researchers determined through
their analysis that there is a 95 percent probability that, during the last
interglacial stage, global sea level peaked more than 22 feet
above its present level. They further found that it is unlikely (with a 33
percent probability) that global sea level during this period exceeded 31 feet.

Sea levels during the last interglacial stage are of interest to scientists and important to policymakers
for several reasons. Most notably, the last interglacial stage is relatively
recent by geological standards, making it feasible for climate scientists to
develop a credible sea level record for the period, and is the most recent time
period when average global temperatures and polar temperatures were somewhat
higher than today. Because it was slightly warmer, the period can help
scientists understand the stability of polar ice sheets and the future rate of
sea level rise under low to moderate global warming scenarios.

The findings indicate that sea level during the last interglacial stage rose for centuries at least two to
three times faster than the recent rate, and that both the Greenland and West
Antarctic ice sheet likely shrank significantly and made important
contributions to sea level rise. However, the relative timing of temperature
change and sea level change during the last interglacial stage is fairly
uncertain, so it is not possible to infer from the analysis how long an
exposure to peak temperatures during this stage was needed to commit the planet
to peak sea levels.

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