Also, carbon dioxide dissolves into ocean waters, turning them more acidic, which makes it harder for corals, shellfish, and other animals to grow their protective skeletons or shells.
Remote, ice-bound areas in Antarctica and the Arctic are feeling the least impact, the study found, mostly because few people live there and those regions are hard to reach.
But global warming is still affecting those places. (Read related story: "Antarctica Ice Loss Faster Than Ten Years Ago" [January 14, 2008].)
The impacts of warming will continue to get worse, the study notes.
"Projections of future polar ice loss suggest that the impact on these regions will increase substantially," the study authors write.
The second biggest factor affecting marine life is fishing, they add.
Trawl-fishing for animals on the ocean floor, such as groundfish and shrimp, is especially damaging because the rest of the seafloor habitat is destroyed in the process, Halpern says.
The habitats that are suffering the worst impacts, rated "very high" in the study, are continental shelves, the shallow areas off the coasts of continents that are 200 to 750 feet (60 to 200 meters) deep.
Other areas with very high impacts include the northeastern U.S., where pollution, commercial shipping, and fishing are the major causes of harm.
The North Sea and Chinese coasts are hit by almost every kind of impact, Halpern said.
"It's a perfect storm."
Elizabeth Babcock is a marine biologist at the University of Miami who was not involved in the new study.
"The most useful thing about this [study] is the ability to look at the big picture and to pick out areas that are particularly pristine that would be good places for having marine parks," she said.
The research could also be used to find "areas that are a lot more impacted than anyone thinks, that really need some conservation attention," she added.
"The map is useful for international groups to prioritize where they spend money for mitigation [of these problems] and conservation."
John Pandolfi of the University of Queensland in Brisbane, Australia, said the study "highlights the fact that ecosystems in the sea know no political boundaries. Hence an international, cooperative approach is the only way forward."
The team did not measure the effects of certain practices, such as illegal fishing and aquaculture, the farming of aquatic plants and animals.
"This makes their estimates of habitat decline conservative, and things are probably worse than they outline," Pandolfi said.
Including more local impacts would also paint a bleaker picture, he added.
"A closer look at the nearshore human footprints will probably show a greater degree of degradation," he said.
Yet Halpern remains optimistic.
"My hope is that our results serve as a wake-up call to better manage and protect our oceans rather than a reason to give up," he said.
"Our goal, and really our necessity, is to do this in a sustainable way so that our oceans remain in a healthy state and continue to provide us the resources we need and want."
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