A major new scientific study has concluded humans are on the verge of causing unprecedented damage to the oceans and the animals living in them. The ecologist Douglas McCauley, who wrote the study, said, “We may be sitting on a precipice of a major extinction event.” The report said coral reefs have declined by 40 percent worldwide, and carbon emissions are altering the chemistry of seawater, making it more acidic. The study appears in the new issue of Science.