Visualizing Environmental Science

1920 1940 1960

Year

1980 2000 2011

Mean temperature change (°C)

–0.3

–0.6

0.0

0.3

0.6

Peter Scoones / Science Source

Adapted from National Assessment Synthesis Team,

Climate

Change Impacts on the United States: The Potential Conse-quences of Climate Variability and Change

(Report for the

U.S. Global Change Research Program). Cambridge, UK:Cambridge University Press (2001) and based on data from theNational Oceanic and Atmospheric Administration.

a. Bleached coral off the Maldive Islands, in the Indian Ocean.

Scientists have linked coral bleaching to ocean warming. Warmer

than usual temperatures stress the coral animals, causing them

to lose their zooxanthellae. Without their algae, the corals can’t

get enough food, and they die.

b. This 1920–2011 time series of annual global mean

temperature changes of the ocean surface indicates that the

ocean has warmed, particularly during the past 25 years. Most

warming has occurred in shallow waters where corals live. Mean

temperature anomaly data relative to 1901–2000.

Human Impacts on the Ocean 289

degree above average can lead to bleaching). Although

many coral reefs have not recovered from bleaching,

some have. Another threat of warmer ocean tempera-

tures is the recent and rapid widespread acidification

of ocean water, caused by excessive amounts of climate-

warming CO 2 dissolving in the ocean and forming a di-

lute acid. Acidified seawater might cause the calcium

carbonate skeletons of coral animals (and shells of crabs,

oysters, clams, and many other marine species) to thin or,

in extreme cases, dissolve completely away.

Offshore Extraction of Mineral

and Energy Resources

Large deposits of minerals lie on the ocean floor. Man-

ganese nodules—small rocks the size of potatoes that

contain manganese and other minerals, such as copper,

cobalt, and nickel—are widespread on the ocean floor,

Asia and the Indian Ocean are at the greatest risk, with

54 percent of the reefs lost or critically threatened and an

additional 25 percent at moderate risk.

How do we harm coral reefs? In some areas, silt wash-

ing downstream from clear-cut inland forests has smoth-

ered reefs. Overfishing (particularly the removal of top

predators), damage by scuba divers and snorkelers, pol-

lution from ocean dumping and coastal runoff, oil spills,

boat groundings, anchor draggings, fishing with dyna-

mite or cyanide, hurricane damage, disease, reclamation,

tourism, warming ocean temperatures, and the mining

of corals for building material take a heavy toll.

Since the late 1980s, corals in the tropical Atlantic

and Pacific have suffered extensive bleaching (see What a

Scientist Sees), in which stressed corals expel their zooxan-

thellae becoming pale or white in color. The most likely

environmental stressor is warmer seawater temperatures

attributed to global climate change (water only about 1

WHAT A SCIENTIST SEES

Ocean Warming and Coral Bleaching

Interpreting Data

If this warming trend continues, what will the

mean temperature change be by 2050?