Groton - Peter Auster explored the coral reefs off Bonaire island in the Netherlands Antilles for the first time in 1982, when he was in his mid-20s and at the start of his career as a marine scientist.

He's been returning with his scuba gear periodically ever since, both for his ongoing research and on his own time during vacations from his post at the University of Connecticut's Avery Point campus.

A quarter-century is a significant span in a person's career, but not in the gradual time frames in which complex organisms like corals and reef fishes have evolved and changed - at least it's not supposed to be. But over those years, the 52-year-old associate professor, whose research focuses on reef fishes, fish behavior and fisheries management and related areas, has witnessed a disturbing transformation of the Bonaire reefs.

”Last June when I went there, it was mostly dead coral, about 80 percent,” said Auster, showing photographs on his computer comparing the reef today with the one 25 years ago. “In 1982, there was 90 percent coral cover.”

In the earlier photo, the underwater world is lush with staghorn corals. The recent one shows a sea floor mostly barren except for a few pieces of brain coral. Various localized forces are likely contributing culprits in the dramatic change, from nearby coastal development and pollution to hurricanes and damage from fishing vessels. But increasingly at this reef and others in seas both tropical and temperate, a global phenomenon is also exacting its toll: climate change.

”Will they recover?” Auster asked, referring to the dead and degraded coral reefs worldwide, which are vital to the health of fish populations and other marine life.

Climate change, he said, “hasn't made the other problems I work with go away,” but over the last five years the effects of climate change are becoming more pronounced in the marine environments he studies.

”It's one big uncontrolled experiment,” he said.

The effects of climate change, caused mainly by carbon dioxide emissions from the burning of fossil fuels and other industrial activities, are evident in oceans and marine ecosystems worldwide in measurable ways that can be more obvious than changes on land.

In a 2008 report on the state of coral reefs, the National Oceanic and Atmospheric Administration notes that the poor health of many reefs is tied to the larger changes in the ocean and coastal environments from global warming.

In the warming planet, sea levels and water temperatures are rising, as glaciers melt and warmer water expands. Corals, notes Auster, live at the edge of their tolerance levels for water depth - they need to be close enough to the surface for sufficient sunlight to penetrate - and temperature. When conditions aren't right, corals are more susceptible to diseases such as coral bleaching, which threatens reef survival.

”Bleaching events have become more frequent and longer,” said Auster.

As the NOAA report notes, the very chemistry of the world's oceans has been altered, and how the marine life that depends on the sea is being affected isn't fully known. Some creatures will thrive in the new environment, but many more, particularly more complex species, may not be able to adapt quickly enough.

Much of the carbon dioxide released into the air since the start of the Industrial Revolution has ended up in the sea. There, it mixes with water and forms carbonic acid. Today ocean surface water is estimated to be 30 percent more acidic than 250 years ago, according to a November report by Oceana, an international ocean conservation group, and is expected to be 100 percent more acidic by the end of the century if current trends continue.

This, in turn, threatens coral growth.

”Corals…” the NOAA report notes, “are able to calcify their skeletal structures from sea water because of particular chemical properties. Continued increases in CO2 … may prevent coral reef growth altogether.” Acidification is expected to have a similar effect on shellfish.

The Intergovernmental Panel on Climate Change, a U.N. scientists' group that synthesizes research from throughout the world, in a 2007 report called coral reefs among the “most vulnerable” of all the world's ecosystems due to global warming. Salt marshes and mangroves, also vital to fish and other marine life, are others.

”The oceans are acidifying faster than we had predicted, and there are widespread effects in the marine environment,” said Auster.

When he researches issues specific to fish and fisheries, he is also seeing the effects of climate change in combination with other factors like overfishing and pollution.

Just offshore from the Avery Point campus in Long Island Sound, for example, data from trawl surveys shows the mix of fish species is changing, Auster notes. Spotted hake and other species more prevalent in southern waters are increasing, while bluefish and others that favor colder waters are declining.

The 2007 report from the I.P.C.C., the group that won the Nobel Peace Prize that year for its climate change research, said, “local extinctions of particular fish are expected …” particularly in species like salmon and sturgeon that spend parts of their lives in fresh and salt water. Both are found in the Sound.

Auster, who grew up in Middletown and now lives in Chester, took up scuba diving as a teenager. For a time he thought about becoming an astronaut, but instead settled on a career as a marine scientist.

”I wanted to study life,” he said, “and there's life all around you in the ocean.”

The condition of the ecosystems he has spent his career studying does get discouraging at times, he admits, and the threats posed by climate change at times seem unstoppable.

”But it's not hopeless,” he said.

His work through Avery Point and the other marine organizations he is part of may increase understanding of how corals and fishes are being affected by climate change. But ultimately, he said, scientists won't be the ones driving any response. Auster is a member of international fisheries management groups, is on the advisory council of the Stellwagen Bank National Marine Sanctuary off the Massachusetts coast and in the midst of a three-year research project there, and is the research director of the National Undersea Research Center at Avery Point.

”It's easy to just say we need more studies,” he said. “But at this point, we know we're in trouble. We know enough” to know what's needed: prompt and widespread actions to reduce carbon emissions and mitigate the effects of climate change already being seen.

”Our response will be based on our values and ethics and the desire for the future we want to see,” he said. “We need the political and social will to do it. We know the direction we need to move.”