Instead
of rising about six inches over the course of the 20th century, as
previous research suggested, the sea actually rose by approximately five
inches, the team from Harvard and Rutgers Universities found. The
difference turns out to be an immense amount of water: on the order of
two quadrillion gallons, or enough to fill three billion Olympic-size
swimming pools.
If the findings stand up to scrutiny by other scientists, they could help resolve a longstanding conundrum
in climate research. For years, when experts added up their best
measurements of melt water from land ice and of other factors causing
the sea to rise, the numbers fell a bit short of the rise that had been
recorded at harbors around the world.
If
the harbor measurements were right and the ocean really had two
quadrillion gallons of extra water, where was it coming from? The
discrepancy set off an intensive search for additional ice that might be
melting from glaciers and ice sheets, or extra heat that might be
causing ocean water to expand, and so on. To some scientists, the
answers that emerged were never entirely satisfactory.
Now, in a paper
published Wednesday by the journal Nature, the Harvard and Rutgers
scientists applied advanced statistical techniques to the measurements
taken at harbors. They found that previous research on that record had
slightly overestimated the amount of sea-level rise in the 20th century.
With their downward revision, the harbor record now matches the other
records rather neatly.
“If
it’s right, it’s pretty important,” said Peter U. Clark, a geologist at
Oregon State University who studies sea level but was not involved in
the new research. John A. Church, a scientist with Australia’s national
scientific organization and a prominent researcher on sea level, echoed
that view, but both men said they would like to see more details of the
research before passing final judgment.
The
main significance of the paper, if it holds up, may be to increase
scientists’ confidence that they understand precisely why the ocean is
rising — and therefore shore up their ability to project future
increases.
A United Nations subcommittee led by Dr. Clark and Dr. Church said last year
that if human emissions of greenhouse gases continued at a high level,
the sea could rise as much as three feet by the end of this century, or
possibly even more in the worst case. The research from Harvard and
Rutgers has already set off efforts to develop new forecasts, with
results due in the coming months.
The new paper was led by two young researchers, Carling C. Hay and Eric Morrow, working with two senior scientists, Robert E. Kopp of Rutgers and Jerry X. Mitrovica of Harvard.
Given that observers have been taking measurements at harbors for centuries, using devices called tide gauges,
it might seem a simple problem to figure out how much the ocean has
been rising. It is anything but simple, though: The tide-gauge record is
plagued by gaps; the land to which many of the gauges are attached has
itself moved over time; and factors like wind and ocean currents can
cause variability of the sea level in particular locales. Moreover, the
early harbor measurements were concentrated in Europe and the United
States, leaving much of the world a blank.
The
Harvard and Rutgers researchers believe their new statistical
techniques do a better job than past research of accounting for the
confounding variables, but they do not claim to have published the last
word on the subject. “One of the main takeaway messages is that sea
level is really complicated,” Dr. Hay said in an interview. “We don’t
have a complete understanding of what’s been happening.”
If
the paper represents an advance in climate science, that silver cloud
comes with a dark lining. The new research confirms numerous previous
estimates that the rate of sea-level rise jumped substantially toward
the end of the 20th century. The ocean now appears to be rising at a
rate of about a foot per century.
That
would be worrisome enough, since mere inches of sea-level rise have
been enough to force governments to spend billions of dollars to fight
coastal erosion. But recent research
suggests that the great ice sheets in Greenland and Antarctica are
starting to break up as a result of global warming, a development that
will send quadrillions of additional gallons of water pouring into the
ocean. Many scientists see the recent jump in the rate as the beginning
of a more troubling long-term acceleration that may ultimately threaten
coastal cities around the world.
Already, tidal flooding is growing worse and causing severe problems in cities like Norfolk and Miami Beach, and in whole countries like Bangladesh.
