Photograph by David Doubilet
A platform of coral surrounding Wreck Island is only one of 2,800 separate reefs that make up the Great Barrier Reef off the eastern coast of Australia.
The Great Barrier Reef is considered the world’s largest living structure; at 1,250 miles (2,000 kilometers) long, it can be seen from space.
But this natural wonder is under threat from climate change and pollution. Reefs around the world, including the Great Barrier, are at risk of dying. A deadly process called bleaching occurs when the acidity of the ocean increases due to absorption of carbon dioxide, killing off microorganisms that make up the reef.
Great Barrier Reef
Photograph by Paul Sutherland
Brightly colored fish swim near sea fans and corals in the Great Barrier Reef. According to the environmental nonprofit WWF, 25 percent of marine life lives on coral reefs, and the Great Barrier Reef is home to thousands of species of coral, fish, mollusks, sea turtles, and mammals.
The United Nations Intergovernmental Panel on Climate Change has estimated that the Great Barrier Reef, and the nearly $5 billion tourist industry built around it, could be “extinct” by 2050.
Chihuahua Desert Landscape
Photograph by George Grall
Joshua trees and barrel cacti grow in the Chihuahuan desert, which straddles the border between the United States and Mexico. It is the largest desert in North America.
Despite its dry climate, the desert relies on the Rio Grande. But the river often runs dry before it hits the Gulf of Mexico due to irrigation withdrawals and recently erratic snowmelts at the river’s source in the Colorado mountains.
According to WWF, the construction of dams, groundwater extraction, deforestation, and agricultural pollution also threaten the Chihuahuan’s water sources.
Photograph by George Grall
An endangered hawksbill sea turtle swims over a reef in the Caribbean Sea. Hawksbill turtles are threatened by destruction of their coral reef and shallow water feeding grounds due to ocean acidification and sea-level rise, which affect the habitats of the invertebrates they feed on. But the most immediate threats to hawksbills are illegal poaching for tortoiseshells—once openly traded in Japan and other countries—and coastal development that destroys habitat.
Rising sea levels and warming temperatures also have an effect on the turtles' breeding habits. More water means smaller beaches where female turtles lay their eggs. And the temperature of the eggs affects the sex of the hatchlings. The WWF estimates there are 8,000 nesting female hawksbill turtles left in the world.
Wild Salmon Run
Photograph by Michael S. Quinton
Alaskan salmon depend on clean, cold water, and face a slew of issues as global temperatures rise. Warmer water tends to fire up a fish’s metabolism so that it needs more food and takes longer to grow.
As less snow falls in the mountains, and melts earlier in the spring, there is less cool water along spawning runs.
By some estimates, nearly 40 percent of the Northwest salmon population could disappear by 2050.
Photograph by Xidong Luo, My Shot
Spring and summer runoff from glaciers in the Himalaya, like this one in Tibet, provide freshwater to hundreds of millions of people downstream through the Ganges, Yangtze, Mekong, and other major Asian rivers.
However, the glaciers are at risk of melting permanently as Earth warms and dramatic weather events become more frequent, potentially causing major spring flooding downriver and then summer droughts. The WWF estimates that some glaciers in the Himalaya have been receding 33 to 49 feet (10 to 15 meters) a year, although some experts say glaciers there aren’t melting at such a fast clip.
Photograph by Andrew Parkinson, Getty Images
A female tiger and her cubs rest in a national park in the Mahdya Pradesh province of India.
The coastal habitat of tigers (and people) in the Sundarbans—part of the largest remaining mangrove forest in the world near the Indian border with Bangladesh—are disappearing as sea levels rise. According to the WWF, there could be as few as 3,200 tigers left in the wild, and 15 percent of the islands that make up the Sundarbans could disappear by 2020.
Tour Boats on Yangtze River
Photograph by Justin Guariglia
Tourist boats wait for passengers in the Yangtze River, China. The Yangtze is longest river in Asia and the third-longest river in the world.
But the river is imperiled because shrinking Tibetan glaciers could cause massive flooding as they melt faster than normal.
The Three Gorges Dam, the world's largest, creates disruptions to wildlife habitat and human settlements.
Alerce Tree, Chile
Photograph by Fabian Gonzales, Getty Images
A hiker admires an alerce tree in the Valdivian rain forest, in southern Chile and Argentina.
Alerce can live to be 3,600 years old, and their rings show climate change over the course of their lives. Tree-ring studies show that rain has been less frequent and drought periods have been increasing, threatening the area with more frequent fires. Despite an international ban on trading its wood, the tree continues to be logged illegally, according to the WWF.
More About Earth Day
See pictures of rallies, concerts, bell-bottoms, and gas masks from the first Earth Day in 1970.
This year a billion are expected to mark Earth Day, which some suspect is anti-capitalist. Go behind the subject of today's Google Doodle.
Auroras, glaciers, and gullies feature among the most stunning pictures of Earth from space, chosen by National Geographic photo editors for Earth Day 2011.
On Earth Day, U.S. leaders reflect on what's at stake.
The National Geographic Society aims to be an international leader for global conservation and environmental sustainability. Learn more about the Society's green philosophy and initiatives.
Beautiful photos highlight the wonders of our world.
The Great Energy Challenge
An initiative to help you understand our current energy situation.
See how you measure up against others, and how changes at home could do tons to protect the planet.
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The World's Water
NG's new Change the Course campaign launches. When individuals pledge to use less water in their own lives, our partners carry out restoration work in the Colorado River Basin.
A special series on how grabbing water from poor people and future generations threatens global food security, environmental sustainability, and local cultures.