The world is on the brink of a massive extinction event, according to the United Nations.
Rapid releases of greenhouse gas emissions are changing habitats at a rate faster than many of the world's species can tolerate.
"Indeed the world is currently facing a sixth wave of extinctions, mainly as a result of human impacts," said Achim Steiner, executive director of the U.N. Environment Programme in a statement.
A study earlier this year in the Proceedings of the National Academies of Science said the current extinction period, known as the Holocene extinction event, may be the greatest event in the Earth's history and the first due to human actions. Unlike previous events, however, extinctions are happening over the course of decades rather than centuries. Recent studies suggest that a quarter of the world's species may go extinct by 2050.
The UN warning accompanies an increasingly frequent round of sobering news about ecosystem failures.
The latest global coral reef assessment estimates that 19 percent of the world's coral reefs are dead. Their major threats include warming sea-surface temperatures and expanding seawater acidification.
Zooxanthellae, the tiny organisms that give coral reefs their vibrant colors, are emigrating from their hosts in massive numbers as oceans heat up, killing themselves and the coral they leave behind - a process known as coral bleaching.
The report, released by the Global Coral Reef Monitoring Network Wednesday at the international climate change negotiations in Poznań, Poland, predicts that many of the remaining reefs may disappear within the next 40 years if current emission trends continue.
"If nothing is done to substantially cut emissions, we could effectively lose coral reefs as we know them, with major coral extinctions," said Clive Wilkinson, the network's coordinator, in a press release.
Overfishing, pollution, and invasive species continue to be risks as well, according to the International Union for the Conservation of Nature (IUCN).
The IUCN declared in October that 38 percent of the 44,838 species it studied across the world are threatened with extinction. Its Red List of Threatened Species considers 22 percent of the world's mammals, 31 percent of amphibians, and 14 percent of birds threatened or extinct.
Steiner's warnings of mass extinction came last week as the U.N. Convention on the Conservation of Migratory Species of Wild Animals added 21 migratory species to its protection list. Migratory species are among the most at-risk to climate change, according to a UNEP report released last year [PDF].
To its list of protected animals, which include the cheetah and Egyptian vulture, the convention added six dolphin species. Nearly one-quarter of the world's dolphin species are threatened with extinction, mostly due to habitat loss and live capture, according to IUCN.
The demise of coral reefs, however, affects the entire ocean ecosystem - a quarter of all marine fish species reside in the reefs, according to The Nature Conservancy. In addition, IUCN estimates that 500 million people depend on coral reefs for their livelihoods.
The coral reef assessment found that 45 percent of the world's reefs are healthy - providing hope that some species may be able to endure the changes expected from global warming. Marine biologists are now attempting to understand how certain coral reef species can survive warmer, more acidic ocean waters when others are less fortunate.
Click to view image: 'Coral Reef'
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