The world's coral reefs, weakened by centuries of human exploitation and abuse, may disappear this century, researchers say.
By Alex Kirby
BBC News Online environment correspondent
They believe the recent outbreaks of coral disease and bleaching could prove the final insult.
But another group says some corals are proving more resilient to damage than others.
Although no reef is safe, they argue, many are likely to change rather than simply disappear entirely.
The contrasting views are published in the magazine Science, which also includes research suggesting forest fires pose a growing threat to coral's wellbeing.
The historical review of what humans have done to the reefs comes from a team led by John Pandolfi, of the Smithsonian Institution, Washington DC.
They compiled records reaching back thousands of years, covering the status and trends of seven major groups of reef creatures - carnivores, herbivores and "architectural builders" like seagrasses and corals themselves - on 14 reefs.
They say the destruction began during the hunter-gatherer era of human history, leaving all the Earth's reefs substantially damaged "long before outbreaks of coral disease and bleaching".
Overfishing is one culprit
Most were seriously degraded before 1900, through overfishing abetted, in some cases, by land-based pollution.
The authors conclude: "Regardless of the severity of increasing threats from pollution, disease and coral bleaching, our results demonstrate that coral reef ecosystems will not survive for more than a few decades unless they are promptly and massively protected from human exploitation."
Nerilie Abram, of the Australian National University, Canberra, and colleagues report their study of the 1997 Indonesian wildfires and their apparent link with coral death offshore.
That year stronger than usual south-easterly trade winds caused pronounced upwelling of nutrient-rich deeper water along the south-west coast of Sumatra.
Late in 1997 almost all the coral and fish in the reef ecosystem of the offshore Manawa islands died.
The authors find evidence suggesting they were suffocated by a massive red tide of phytoplankton, tiny aquatic plants, extending for several hundred kilometres.
Fires may have killed reefs
But the fossil record shows even higher upwellings in the past, which were not marked by coral die-off. The authors say the reefs' response in 1997 was highly unusual, but the upwelling itself was not.
The productivity of coastal systems during upwelling is limited by the availability of iron, and the researchers suggest the critical factor in 1997 was the extra iron provided by smoke settling from the wildfires.
A third article examines the resilience of reefs to climate change and human impacts. The team was led by Terry Hughes, of the Centre for Coral Reef Biodiversity, James Cook University, Townsville, Australia.
With an estimated 30% of all reefs already severely damaged, the authors write, and close to 60% liable to be lost by 2030, there are no pristine reefs left.
They say the link between coral bleaching and increased climate change, "considered dubious by many reef researchers only 10 to 20 years ago, is now incontrovertible".
Some reefs may tough it out
Yet the bleaching - which happens when corals stressed by overheating expel the tiny organisms that provide their colour - is "conspicuously patchy".
Bleached and unbleached corals are often found side by side, and the same species seem to develop different tolerances in different parts of their range. Another factor is the very high level of genetic diversity in corals.
So some reefs will prove more resilient than others to the emergent threats they face, the authors believe, with many changing rather than disappearing entirely.
But they write: "We can be certain nonetheless that the projected increases in carbon dioxide and temperature over the next 50 years will substantially and very rapidly exceed the conditions under which coral reefs have flourished over the past half-million years."
They argue for many more marine reserves, or no-take areas( NTAs), to protect reefs against overfishing, which can alter the entire dynamics of a reef.
They say NTAs should cover at least 30% of the Earth's reefs, yet even in rich countries like the US and Australia under 5% of reefs are in NTAs today.
Reef images courtesy of US National Oceanic and Atmospheric Administration.