Stack two dimes on top of each other. Their height is a tiny fraction
less than global sea level is rising each year. The increase looks
small, but the consequences are potentially huge. Rising sea level
threatens to inundate low-lying regions, such as the Chesapeake, and
dramatically increase coastal and beach erosion around the world.
While tide gauges have been used to determine sea level for hundreds
of years, the most complete global measurements now come from space.
"Tide gauges can’t detect an increase in the rate of sea level rise
soon enough to be useful for detecting climate change," says Bruce
Douglas, a senior researcher at Florida International University,
Miami, Fla. "Tide gauges can’t measure everywhere. They’re on
practically every rock in the ocean, the problem is there just aren’t
enough rocks."
In contrast, the Topex/Poseidon satellite observes the entire ocean
and has been making precise measurements of global sea level since it
was launched in 1992. Its successor, Jason, is now continuing the same
ocean observations.
"Right now Topex/Poseidon has been seeing an average yearly increase
of 2.8 millimeters (0.11 inches) in global sea level," says University
of Colorado engineering professor Dr. Steve Nerem, a member of the
Topex/Poseidon and Jason 1 science team.
Global sea level is the average of all local rates. If global sea
level is rising by 2.8 millimeters a year, the local rate in some
areas is much higher, as much as 5 millimeters (0.2 inches) or more
over long periods. In some areas it is less.
One of the big questions facing scientists and the public, especially
the more than two billion of us who live within 100 kilometers (62
miles) of a coast, about the rise in sea level is "why?"
"We don’t know yet exactly what is causing it," says Nerem. "The jury
is still out." The current rise in global sea level could be part of
some natural, yet unidentified, decades-long climate pattern, Nerem
says. "During 1997-1998 El NiÒo, the global average went up 15
millimeters (0.6 inches) as a result of increased ocean temperatures
and then went down again."
However, sea level is a barometer of climate change, and the rise
could be a result of a warming Earth. "While the rate of increase we
see is consistent with climate change models, we can’t say for sure if
that is the cause," Nerem says. "We’re just starting to ask those
questions."
"It looks like sea level rise as we now observe it began in the middle
of the 19th century," says Douglas, an expert on the history of sea
level rise and its consequences. "We have a preponderance of evidence
that the current rate is considerably faster than for the previous
several thousand years, although there is still some disagreement
among scientists about this."
The two major factors that determine sea level are temperature and
ocean mass. Warm water expands and raises sea level. Water added to
the ocean from melting glaciers or ice sheets also causes sea level to
go up. Figuring out just how much of the current sea level rise is due
to each of these factors is difficult.
"Our best guess is that thermal expansion accounts for about 0.5
millimeters (.02 inches) per year rise in sea level or five
centimeters (2 inches) per 100 years," says Douglas. "If global sea
level is rising at more than 20 centimeters (8 inches) per hundred
years, then where is the water coming from? Mountain glaciers could
account for three or four centimeters (1.2 to 1.6 inches), so that
leaves Earth’s great ice sheets in Greenland and Antarctica. Are they
losing or gaining? That’s a controversial question."
Scientists expect to have some answers soon. NASA’s new Grace mission
will be able to calculate the ocean’s mass, helping pinpoint whether
rising sea level is a result of more water in the ocean or expansion
due to warming waters. A new generation of tide gauges and monitoring
devices provide details on sea level changes in specific locations.
Meanwhile, Jason continues the global sea level measurements begun by
Topex/Poseidon more than 11 years ago, building up a record of sea
level change that may help explain the past and predict the future.
Ironically, Topex/Poseidon was never expected to be able to make
precise enough measurements to monitor something as small as
millimeter changes in global sea level. "It’s a 100 times more
accurate than we expected it to be before launch," says Douglas. Jason
1 may improve on these measurements even more.
