Coral reefs have had a tough time in the last two decades as warming temperatures, overfishing, chemical runoff, and disease have sparked massive coral die-offs. But reefs in the Indian Ocean show that substantial recovery is possible, a study released Wednesday found.

The study, published in Scientific Reports, looks at 28 reefs in the remote Chagos Archipelago, an area that lost 90 percent of its corals in 1998 after an unprecedented rise in sea temperatures. Nearly 20 years later, coral reefs there are back to optimum health, demonstrating that reefs can bounce back “rapidly” from major climate-driven disturbances.

Researchers interviewed said the study is another example of how proper management can help corals recover, and also shows the effect human absence can have on corals, since the Chagos Archipelago is mostly deserted of people. Authors note, however, that recovery is nonetheless a fragile process tied to global weather trends. Healthy corals could suffer massive setbacks in the coming years, as man-made climate change increases the likelihood of extreme weather events, as well as sea temperature increases that could mimic conditions that caused the 1998 bleaching event.

Still, “it’s encouraging to know that there are reefs that can have these rates of growth,” said David Kline, associate project scientist at Scripps Institution of Oceanography, who was not involved in the study. “It does tell you that reefs are incredibly resilient.”

Coral reefs are, to a great extent, living ecosystems made up of fragments of corals, coral sands, algae and other organic deposits, as well as solid limestone. Thousands of species of sea animals can live in these environments.

In 1997 though, following one of the strongest El Niño weather phenomena ever recorded, coral reefs in the Indian Ocean and elsewhere suffered mass bleaching damage. About 16 percent of the world’s reefs were seriously damaged or destroyed in 1997 and 1998, studies have found. But while many coral reefs around the world have been unable to heal, the isolated coral reefs of Chagos show so-called positive carbonate budgets, which is a measure of a reef’s health calculated through the amount of calcium carbonate, or skeleton, that corals produce.

The “coral cover was absolutely wonderful,” said Chris Perry, professor of geography at the University of Exeter, and one of the authors of the study. “I mean stunningly beautiful, huge big table corals, branch corals, yeah just fantastic and amazing kinds of fish life. So it was a really, really special place to go.”

Perry attributes the recovery to the healthy fish population in the area, since fish play a key role in maintaining the seaweed and algae cover that coral reefs need to thrive. He notes too that unlike severely damaged coral reefs in areas like the Caribbean, Chagos is spared from chemical urban runoff and overfishing.

In a way, the recovery observed stems from the level of isolation Chagos enjoys, the researcher said, which allows it to be far from human touch and poor land management practices.

“Geographically it is a long way from anywhere,” said Perry, who noted that recovery in Chagos was recorded as early as 2010.

The theory is that “if a reef’s ecosystem is in good shape and it hasn’t been damaged and polluted and degraged, then the natural resilience of the coral reef will help it survive and resurface,” said Philip Renaud, executive director of the Khaled bin Sultan Living Oceans Foundation, who wasn’t involved in the study but whose team was in Chagos early this year observing the same improvements the study recorded.

And yet despite the recovery that Chagos — and other coral reefs — are showing, coral reefs are still at risk, researchers and studies say. In the case of Chagos, Perry said temperatures were unusually warm when the team was recording observations around March and April, which is an alarming trend.

“It was 30 degrees [Celsius] down to about 20 meters, which is a good degree above normal,” said Perry. “And it was pretty prolonged. It was surprising how warm it was actually.”

While coral reefs live in different temperatures around the world, all corals have a narrow temperature tolerance range, similarly to humans’ internal body temperature. For corals that temperature range is about 1°C to 1.5°C, said Perry. So “even fairly small increases above their norm will lead to the corals expelling the photosynthesizing algae … If that’s a very prolonged event, the corals won’t reincorporate their algae and they will die.”

In fact, Renaud said his team saw bleaching happening in Chagos, but as the monsoon season came in bringing cooler temperatures, is likely that the coral recovered.

However, the risk of massive bleaching is high. So far there have been two more large scale coral bleaching happenings since the first massive bleaching ever recorded shocked scientists in 1998. The National Oceanic and Atmospheric Administration announced the latest one in October. Moreover, it is believed that this year’s El Niño, which stems from unusual warm water currents near the Pacific Ocean coast of Peru, could be stronger than the one that caused havoc to corals in 1998.

“The fate of coral reefs is pretty dire, especially with climate change and what might happen if we don’t seriously act,” said Kline, the scientist from Scripps, who however noted that in his research he’s seeing coral reefs recuperating. So “there are signs of hope,” he said.

Perry shared this cautious optimism for the future, noting that the Chagos Archipelago experience is “a very good argument for effective management … to put pressure on people to put the resources into managing these environments.”

Yet local management means little, he said, if global trends in emissions and other factors widely believed to exacerbate climate warming aren’t controlled.

“So the sorts of things that came out of COP 21 are positive,” he said, referring to the successful and historic international climate talks held in Paris earlier this month. “It’s a step in the right direction.”