April
2001
Time
to Evolve Webbed Feet? Global Warming and Rising Sea Levels
The Satya Interview with
Lisa Mastny
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Lisa
Mastny is a Staff Researcher at the Worldwatch Institute, where she
writes on a wide range of environmental and developmental issues. Her
most recent project examined global trends in debt, aviation, and polio
incidence. Lately, Mastny has been collecting data on the effects of
global warming on rising sea levels and the disappearance of coral reefs.
Here, she shares some of the more eye-opening statistics and explains
what they can mean to the Earth and to us.
What’s the relationship between global warming and the rise
of ocean levels?
Global warming raises atmospheric temperatures which, in turn,
warms the world’s oceans. Heat makes water molecules expand—called
thermal expansion—causing sea levels to rise. Over the past three
decades, the world’s oceans have warmed by .3 degrees C, on average.
Tropical waters in the northern hemisphere have expanded even faster,
warming by .5 degrees per decade, five times the global rate.
As glaciers and other ice features are particularly sensitive to temperature
shifts, the warming of the oceans and atmosphere also causes the melting
of the permanent global ice cover. Melting ice from mountain glaciers
and from large ice sheets like Antarctica and Greenland leads to an
influx of freshwater into the ocean, which also causes sea levels to
rise.
What percentage of the rise
in sea levels is due to glacial melting?
Melting of ice caps and mountain glaciers has contributed on average
about one-fifth of the estimated global sea level rise over the past
century. The rest (the bulk) is caused by thermal expansion of the oceans.
But ice melt’s share in sea level rise is thought to be increasing.
Greenland now loses some 150 billion tons of freshwater a year—enough
to sustain all U.S. households for almost five months. If the larger
ice sheets crumble, the contribution of ice melt to sea rise will increase
even more.
Antarctica alone is home to some 70 percent of the planet’s fresh
water. The West Antarctic Ice Sheet (WAIS—an ice mass the size
of Mexico), contains enough ice to raise sea levels by an estimated
six meters. Melting of both Antarctic ice sheets would raise them nearly
70 meters.
Roughly, how many inches/centimeters per year is the sea level rising?
Over the past century, global sea levels have risen an estimated
10 to 25 centimeters (4 to 10 inches). The U.S. Geologic Survey says
that seas are rising nearly one tenth of an inch each year. A recent
Inter-governmental Panel on Climate Change (IPCC) report describes various
scenarios by which average sea level could rise anywhere from .09 to
.88 meters by 2100. The IPCC report also says that gradual melting of
the WAIS and Greenland Ice Sheet will each raise sea levels up to three
meters over the next 1,000 years.
Is the rise in sea level related to a rise in river flooding?
Water from melting glaciers can cause rivers to flood as it makes
its way to empty into the sea. This is expected to be a serious problem
in northern India, both in terms of flooding and consequences for water
supply. Half a billion people depend on the tributaries of the Ganges
and Indus for drinking water and irrigation, but as the mountain ice
of the Himalayas melts, these rivers are expected to initially swell
and then fall to dangerously low levels, particularly in summer. This
could also have consequences for hydropower. Ironically, many developing
countries may have to replace hydropower sources with ones that burn
fossil fuels, which will worsen the problem of global warming and rising
sea levels.
But there is also an effect on rivers at the other end. As seas rise,
salt water creeps up rivers, causing salination of formerly freshwater
areas. This not only changes the balance of fragile estuarine ecosystems,
but can also mean salt intrusion into aquifers, endangering water supply.
This could be a big problem in coastal cities, where rising water could
flood canals and sewer systems. To manage flooding, cities would need
to build up flood control systems and storm drains in the future. In
Bangkok, sea level rise is expected to cost an additional $20 million
per year in pumping costs alone, not to mention costs of relocating
people who live along canals and other waterways.
At the current rate of warming, how much might sea levels rise and
what effects would this have?
Scientists are predicting nearly a meter rise by 2100, but even
a half meter would be devastating. Currently about half the world’s
people live in coastal areas and roughly one billion live at sea level
or just a few meters above. A rise in sea levels could mean profound
social upheaval, resulting in the loss of millions of hectares of land
currently devoted to export crops, as well as the displacement of millions
of people. Many areas will become uninhabitable well before actually
disappearing underwater, as salt intrusion would ruin drinking water
and severe storms would batter settlements.
Effects would be most severe in the tropics and warm temperate regions,
where coastlines are heavily populated. The most vulnerable regions
are: South Asia, all coastal states in Southeast Asia, low-lying coral
atolls in the Pacific and Indian Oceans, the southern coast of the Mediterranean,
and the west coast of Africa. Unfortunately, all these regions are among
the poorest and most heavily populated in the world, together home to
just over two billion people. The Chinese and Southeast Asian coastlines
are among the most crowded, with on average more than 2,000 people per
square kilometer.
Are rising sea levels affecting the U.S.?
Even in the U.S., relative sea level has been rising at a rate
of about .2 meters per century along the East and Gulf coasts. Already
we’re feeling some effects, such as increased beach erosion and
the changing of the fresh/salt water balance, as well as flooded wetlands.
But the most rapid rise has been in the Mississippi Delta and in east
Texas, where sea levels are rising by about one meter per century.
This
higher rate of sea rise is due primarily to geological changes in the
land, which is sinking at a faster rate than elsewhere.
In general, the U.S./North America is better off than elsewhere in
the world, simply because it is better prepared to cope with the changes.
But this by no means suggests that we will be free from the effects
of rising levels. In a recent report, the Pew Center for Global Climate
Change highlighted coastal Louisiana/east Texas and the mid-Atlantic
region as sea level rise “hotspots.” The report also said
that major cities like New Orleans, Tampa, Miami, Baltimore, Philadelphia,
New York, Boston, and Washington, DC will likely have to upgrade flood
defenses and drainage systems.
In the U.S., what would happen if levels rise dramatically?
A half meter rise would flood up to 1.9 million hectares of dry
land along the eastern seaboard and Gulf Coast of the U.S. If areas
that are currently developed are protected against this, only 1.6 million
hectares would be flooded. A one meter rise would flood an additional
1.8 million hectares. Rising sea levels would also push insurance costs
up beyond many peoples’ means because the risk of damage to structures
along coasts would be so much higher.
How is thermal warming affecting ocean eco-systems, like coral reefs?
Global warming is now the greatest threat to coral reefs today.
Reefs live at the upper edge of their temperature tolerance and warming
of as little as one degree Celsius above normal can stress the microscopic
plants that inhabit the tissue of coral animals and provide them with
food and color. If the stress endures, the coral expels the plants and
turns a chalky white and often dies. This is known as coral bleaching.
Though bleached reefs can rebound, they are less likely to do so if
temperatures continue to rise. Reefs are thus good indicators of climate
change, because their reaction to warming is swift and visible.
Sort of like canaries in a coal mine…at what rate are coral
reefs dying?
As of late 2000, an estimated 27 percent of the world’s coral
reefs were severely damaged, according to the Global Coral Reef Monitoring
Network. This compares to only 10 percent in 1992, so the health of
reefs is deteriorating quickly. The greatest losses have occurred in
the Indian Ocean, in the Arabian Sea and Persian Gulf, and in Southeast
Asia. Eleven percent of reefs are considered completely lost, damaged
by human activities like fishing and coral mining, coastal development,
waste dumping, oil spills, and inland deforestation and farming. But
16 percent of the damage (and virtually all the loss since 1992) is
due to coral bleaching.
In 1997–98, a combination of El Niño/La Niña–related
climatic changes and record-high tropical sea surface temperatures caused
the worst bleaching episode on record, affecting some 16 percent of
the world’s reefs, in at least 60 countries. In some areas, 1,000-year
old coral reefs died and losses neared 90 percent, at depths of up
to
40 meters.
An estimated third of the bleached reefs show early signs of recovery,
having retained or recruited enough living coral to survive. Roughly
half could rebound in the next 20–50 years—if ocean temperatures
remain steady and human pressures are low. But if the warming continues,
scientists predict that as many as 60 percent of all reefs could be
lost by 2030.
What’s the importance of coral reefs? Why should people care?
One level of importance is the economic value of reefs. More than
100 countries—many of them small islands—rely on coral reefs
for essential goods and services, like tourism.
Reefs are among Earth’s most complex and productive ecosystems,
with unique assemblages of tiny coral animals and symbiotic plants.
They provide habitat for as many as one million species—including
more than a quarter of known marine fish species. Reef-derived molecules
have also been used to develop many medicines, including antibiotics,
cancer treatments, and HIV drugs
Are rising sea levels affecting other eco-systems?
Rising sea levels can flood out and change the composition of rich
coastal wetlands like mangroves, marshes, salt ponds, and intertidal
areas. The Hadley Centre for Climate Prediction and Research in the
UK predicts that 40 to 50 percent of the world’s remaining coastal
wetlands will be lost by 2080, due to effects like drainage and urban
sprawl as well as to sea level rise. If there is no remediation, this
could be as high as 75 percent. This damage would most likely affect
the Atlantic coasts of North and Central America, the U.S. Gulf Coast,
and the Mediterranean and Baltic Seas.
What about animals?
In Asia, one of the most vulnerable wetlands is the Sundarbans,
the largest contiguous mangrove forest in the world. A one meter rise
could mean extinction for many local creatures, including the 350 remaining
Sunderbans tigers.
Meanwhile, at the poles, ice melting is affecting the ecosystems of
marine mammals, seabirds, and other creatures that depend on food found
at the retreating ice edge. In northern Canada, reports of hunger and
weight loss among polar bears have been correlated with changes in the
ice cover. And in Antarctica, loss of the sea ice, together with rising
air temperatures and increased precipitation, is altering the habitats
as well as the feeding and breeding patterns of penguins and seals.
How can we stop rising sea levels? Can it be reversed?
We basically have two choices: reactive or proactive measures. Reactive
measures, such as sea walls, moving inland, slowing coastal erosion,
etc., would mitigate the effects of rising sea levels, but do nothing
to stop them. They are impractical in the long run, particularly for
countries that do not have the money for these expensive measures.
What we really have to do is be proactive. Sea level rise is not likely
to be reversed unless global temperatures drop. What we can do is try
to stabilize climate change and prevent Earth from heating at such an
accelerating rate. This would require genuine efforts by governments
and industries, as well as consumers, to make drastic cuts in their
emissions of carbon dioxide and other climate-altering gases.
What’s your response to people who say that global warming
is just a theory and needs further study?
This is unfortunate. Really, nothing in science has ever been studied
to the point at which no new information can be discovered. There’s
always going to be more out there to find out. The real issue is how
much studying actually needs to be done before scientists can reach
agreement that something is going on. I believe we have already reached
that point. It is time to act on the evidence we’re seeing—from
ice melting to coral reef bleaching. If not, it may be too late. It’s
folly to gamble with something that may have profound and potentially
irreversible consequences for humanity.
For more information visit www.worldwatch.org
or call (202) 452-1999.
