Watching Our Seas Rise – Part 2
High and dry
Scientists have long known that sea level changes over time. Paul Hearty, a geologist at the University of North
Carolina at Wilmington, has found boulders covered in barnacle shells some 30 meters (100 feet) above sea level.
Those high and dry barnacles are several million years old. They serve as evidence that sea level was once much
higher. Scientists have also found dead coral reefs buried 150 meters beneath the sea. When that coral was alive, it
grew just below the water’s surface. Today, those coral skeletons provide evidence that sea level was once much
lower, too.
Sea level has risen and fallen in sync with
the ice ages, over hundreds of thousands of
years. During past ice ages, oceans were
lower because more water was tied up in
glaciers on land. But between ice ages, sea
level sometimes rose higher than it is today,
as melting glaciers sweated their water into
the ocean. Some 125,000 years ago, just
before the last ice age began, sea level was
a whopping 5 to 8 meters (16 to 26 feet)
higher than it is today.
The big challenge for scientists has been
how to measure changes to sea level
throughout the past 50 to 100 years. Bruce
Douglas, a retired scientist who worked for
20 years at the National Oceanic and
Atmospheric Administration, or NOAA, in
Rockville, Md., spent years working on this. During the 1980s and 1990s he measured sea level rise by studying
records from tide gauges. Harbor operators have relied on these devices for more than 200 years to monitor the water
level in coastal areas in order to alert ships at risk of running aground. But the gauges gave a limited picture: They
measured the level of the world’s oceans, which cover 360 million square kilometers (139 million square miles), in only
20 or 30 places!
The bumpy ocean
Scientists have gradually solved that problem as satellites have become accurate enough to monitor sea level.
Several satellites, including Jason-2, have now been shot into orbit for this purpose. These satellites let scientists do
something that they could never do before: measure sea level not just in a few places along the coast, but also across
the entire length and breadth of the world’s oceans.
These fuller measurements revealed
something amazing: Unlike the water in your
bathtub, the sea is actually bumpy!
Sea level varies from one part of the ocean to
another by a meter or more. And how quickly
sea level is rising also varies from place to
place. For example, sea level in the western
Pacific Ocean, near the island nations of
Indonesia and the Philippines, is rising three
times faster than the global average.
The ocean’s warming causes much of this
bumpiness. During so-called El Niño years,
when waters of the eastern Pacific Ocean
warm to about 1°C (1.8 °F) higher than usual,
the sea level off of California’s coast can rise
by 10 or 20 centimeters (4 to 8 inches). This is
caused by the expansion of the warming
seawater. But the world’s oceans aren’t warmer
overall during an El Niño period. Heat has just
shifted from one place to another: As the
eastern Pacific warms, the western Pacific
cools down, causing sea level there to temporarily drop.
So sea level operates a bit like a water bed: Push down on it in one place and it will rise in another. By averaging out
all of those bumps detected by Jason-2 and other satellites, scientists have calculated that the world’s oceans are
rising by about 2.4 millimeters per year.
The big question for scientists like NASA’s Willis is: Has sea level been rising that fast for millennia? Or did this rate of
sea level rise begin much more recently? In the past century or two, humans began spewing more carbon dioxide into
the air through the burning of fossil fuels such as coal, oil and natural gas. And as a type of greenhouse gas, carbon
dioxide helps warm Earth’s atmosphere — and its seas. So it would make sense that the seas started rising more
quickly in the last few hundred years.
Power Words
barnacle A small sea animal that grows in colonies and filters the water for food. The hard shells of barnacles stick to
rocks, the sides of ships and other objects at the water’s surface.
coral Small ocean animals that form colonies. Many build large, rocklike reefs out of a mineral known as calcium
carbonate.
El Niño A change in ocean circulation that happens about once every five years, causing the surface waters of the
eastern Pacific Ocean to warm by about half a degree Celsius.
Meltwater Pulse 1A A sudden warming and melting of glacial ice during the close of the last ice age, 14,600 years
ago. The episode caused global sea level to jump by 10 meters (32 feet) or more.
tide gauge A device used to measure the water level and tides in harbors. It is used to predict tide patterns. This
allows harbormasters to alert ships before they risk running aground in shallow water.
Water Systems Reading Assignment
Watching Our Seas Rise
Part 2 :
5. Where do corals grow? What does the location of ancient coral remains found by scientists tell us about sea
level in the past?
6. Why is sea level lower during an ice age than in between ice ages?
7. How did scientist Bruce Douglas measure sea level change in the 1980s and 1990s? What was the weakness
of this measurement method?
8. Is sea level change the same across the oceans, or does it vary place to place?
9. What is El Niño? How does it affect sea level?
10. What is the “big question” for scientists who study sea level?
Water Systems Reading Assignment
Watching Our Seas Rise
Part 2 :
5. Where do corals grow? What does the location of ancient coral remains found by scientists tell us about sea
level in the past?
6. Why is sea level lower during an ice age than in between ice ages?
7. How did scientist Bruce Douglas measure sea level change in the 1980s and 1990s? What was the weakness
of this measurement method?
8. Is sea level change the same across the oceans, or does it vary place to place?
9. What is El Niño? How does it affect sea level?
10. What is the “big question” for scientists who study sea level?