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Coral reefs are one of the most threatened ecosystems on our planet, and in the past two decades alone, half of the coral in Florida has died off completely. Global warming is known to be a deadly factor, but rising ocean temperatures are only part of the story.

Thirty years of research in the Looe Key Sanctuary Preservation Area (LKSPA) on the southern tip of the Florida Keys has now revealed the cost of a devastating threat to coral that rivals even climate change: direct human pollution.

For years, agricultural run-off and improperly treated sewage have flowed into Florida's ocean waters from the northern Everglades, elevating the sanctuary's nitrogen levels and lowering the reef's temperature threshold for bleaching, researchers say.

As a result of this deadly combination, coral cover in the region has declined from nearly 33 percent in 1984 to less than 6 percent in 2008.

In their analysis, the authors found that three mass bleaching events that occurred during these years only happened after heavy rainfall and increased land-based runoffs. In other words, if we can reduce the amount of local pollution that makes its way into our oceans, we might be able to reduce the worst of the damage.

"Citing climate change as the exclusive cause of coral reef demise worldwide misses the critical point that water quality plays a role, too," says ecologist James Porter from the University of Georgia.

"While there is little that communities living near coral reefs can do to stop global warming, there is a lot they can do to reduce nitrogen runoff. Our study shows that the fight to preserve coral reefs requires local, not just global, action."

Elevated nitrogen levels are known to cause corals metabolic stress, increasing their susceptibility to disease and boosting algal blooms that reduce light and accelerate coral reef decline. Nevertheless, scientists are still not sure how these changes relate to the growing problem of mass coral bleaching, disease, and mortality.

Previous studies have shown that between 1992 and 1996 - when Florida's freshwater flows were directed south against scientific advice - there was a 404 percent increase in coral diseases throughout the Florida Keys National Marine Sanctuary (FKNMS), which, like the LKSPA, also sits downstream from the Everglades.

Some have suggested that these changes are linked to the 1997/1998 El Niño years, but the timing is slightly off and relatively little attention has focused on what role river runoff might have played.

In fact, a local review of nutrient pollution in the FKNMS made absolutely no mention of these river runoffs, even though in places like Australia and the Caribbean, these pollutants have long been regarded as a threat to coral reef health.

The new study represents the longest record of reactive nutrients and algae concentrations for coral reefs anywhere in the world, and it suggests that coral reefs were dying off long before they were impacted by rising water temperatures.

"This analysis indicates that anthropogenic nutrient loading from local sources in the Florida Keys and regionally from the greater Everglades ecosystem is interacting with a changing climate to create conditions unfavourable for living coral at LKSPA," the authors conclude.

"Local sewage and stormwater discharges from residential development have increasingly impacted nearshore waters of the Florida Keys for decades..."

Unless we take action against both of these threats, things are only going to get worse. Around the world, nitrogen pollution flowing to the coast is predicted to increase by 19 percent, and that's simply as a result of changes in rainfall due to climate change.

If the world is serious about saving coral reefs, the authors say we must improve sewage treatment, reduce fertiliser inputs, and increase the storage and treatment of stormwater on the mainland.

The research has been published in Marine Biology.

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We've Just Found a Huge Contributor to Coral Reef Deaths, And It's Easier to Fix Than Climate Change

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Coral reefs are one of the most threatened ecosystems on our planet, and in the past two decades alone, half of the coral in Florida has died off completely. Global warming is known to be a deadly factor, but rising ocean temperatures are only part o

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Although they cover less than 0.1 percent of the earth's surface, coral reefs are the most biodiverse marine ecosystems in the world. They are also among the most threatened. Today, coral reefs face multiple stressors at different scales. When global threats like warming waters combine with direct threats like overfishing and water pollution, it severely compromises the ability of corals to grow, reproduce and thrive. As much as one-third of all reef-building corals are at risk of extinction. Scientists predict that all corals will be threatened by 2050, with 75 percent facing high to critical threat levels.

However, there is a reason for hope. There are many clear-cut actions that can help coral reef ecosystems. The Coral Reef Alliance (CORAL) works at multiple scales from local to global to address reef threats. At a local level, when we reduce direct threats to reefs-such as pollution, overfishing or unsustainable tourism-reefs are healthier and more capable of withstanding the effects of climate change, like bleaching and ocean acidification. At a regional and global level, we help reefs by establishing Adaptive Reefscapes - networks of healthy reefs that enable corals to adapt to climate change because they are diverse, connected and large.

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Although they cover less than 0.1 percent of the earth's surface, coral reefs are the most biodiverse marine ecosystems in the world. They are also among the most threatened. Today, coral reefs face multiple stressors at different scales.

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The world's coral reefs, from the Great Barrier Reef off Australia to the Seychelles off East Africa, are in grave danger of dying out completely by mid-century unless carbon emissions are reduced enough to slow ocean warming, a new UNESCO study says.

And consequences could be severe for millions of people.

The decline of coral reefs has been well documented, reef by reef. But the new study is the first global examination of the vulnerability of the entire planet's reef systems, and it paints an especially grim picture. Of the 29 World Heritage reef areas, at least 25 of them will experience twice-per-decade severe bleaching events by 2040-a frequency that will "rapidly kill most corals present and prevent successful reproduction necessary for recovery of corals," the United Nations Educational, Scientific and Cultural Organization concluded. In some areas, that's happening already.

"These are spectacular places, many of which I've visited. Seeing the damage being wrought has just been heartbreaking," says Mark Eakin, a reef expert with the National Oceanic and Atmospheric Administration, and a lead author of the new report. "We're to the point now where caction is essential. It's urgent."

Mass Bleachings

By 2100, most reef systems will die, unless carbon emissions are reduced. Many others will be gone even sooner. "Warming is projected to exceed the ability of reefs to survive within one to three decades for the majority of the World Heritage sites containing corals reefs," the report says. ( UN announces new biosphere reserves, while U.S. removes some.)

Reefs, often referred to as the rainforests of the oceans, occupy less than one percent of the ocean floor, but provide habitat for a million species, including a fourth of the world's fish. They also protect coastlines against erosion from tropical storms and act as a barrier to sea-level rise.

"It is terrifying to think of the repercussions of the global and large scale loss of reefs," says Ruth Gates, director of the Hawaii Institute of Marine Biology in Kaneohe, Hawaii. "The reduction in food supplies, the lack of coastal protection as the reef collapses and subsequent land erosion will make some places unlivable and people will have to move. And that's not even mentioning the collapse of reef-related tourism."

In the past three years, 25 reefs-which comprise three-fourths of the world's reef systems- experienced severe bleaching events in what scientists concluded was the worst-ever sequence of bleachings to date. The Great Barrier Reef was especially hard hit. Other reefs that experienced severe bleaching include the Seychelles, New Caledonia, 750 miles (1,210 kilometers) east of Australia, and the United States, off Hawaii and Florida.

"The last three years have been extremely depressing for me," Eakin, with NOAA, says. "We're seeing truly catastrophic damage to many reef systems around the world. The Great Barrier Reef damage we've seen is greater than anything we've seen in the past 20 years."

The consequences are already being felt by some people, and will quickly grow more severe, says Eakin's NOAA colleague and co-author Scott F. Heron. Low-lying islands such as Kiribati, a string of 33 coral atolls in the central Pacific Ocean, already see saltwater inundating freshwater drinking sources. Higher tides and crumbling reefs are causing more storm surges. Soon, loss of coral, especially when combined with global overfishing, will translate to fewer fish-and local protein shortages.

"These are real things that real people are experiencing," Heron says. "I've met these people. They've been to my house. This is happening."

Heron also notes that despite skepticism in some corners about climate change, even the crudest of models from two decades ago predicted just the type of reef damage seen today.

"If what the models projected back then has started to come true, even with all of their issues, then we should have good faith in the science of the current projections," Heron says. "And those projections say if we don't act there will be many, many serious impacts."

Most World Heritage sites are managed locally to control pollutants from farm runoff or overfishing. Now, the "ubiquitous global threat" to reef systems has become so great, Eakin and Heron say, that local protections are not enough. They hope their new, bleak assessment will help the world's nations to realize that unless they move faster to reduce greenhouse gas emissions, these special places-and the people who rely on them-will suffer greatly, far sooner than expected.

"When someone needs help, the overwhelming majority of us will stretch ourselves to help out-it's a human trait. It's what makes us people," Heron adds. "That the people most impacted by these changes are not necessarily people we encounter in our day-to-day lives does not remove our responsibility to help them."

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The world's coral reefs, from the Great Barrier Reef off Australia to the Seychelles off East Africa, are in grave danger of dying out completely by mid-century unless carbon emissions are reduced enough to slow ocean warming, a new UNESCO study says. And consequences could be severe for millions of people.