Chemical oceanographers have warned that human emissions of carbon dioxide have also begun to alter the chemistry of the ocean, which will likely require even steeper cuts in carbon emissions than those currently proposed to curb climate change.
Ken Caldeira of the Carnegie Institution's Department of Global Ecology, writing with lead author Richard Zeebe of the University of Hawaii and two co-authors, note that the oceans have absorbed about 40% of the carbon dioxide (CO2) emitted by humans over the past two centuries.
This has slowed global warming, but at a serious cost: the extra carbon dioxide has caused the ocean's average surface pH (a measure of water's acidity) to shift by about 0.1 unit from pre-industrial levels.
Depending on the rate and magnitude of future emissions, the ocean's pH could drop by as much as 0.35 units by the mid-21st century.
This acidification can damage marine organisms.
Experiments have shown that changes of as little as 0.2-0.3 units can hamper the ability of key marine organisms such as corals and some plankton to calcify their skeletons, which are built from pH-sensitive carbonate minerals.
Large areas of the ocean are in danger of exceeding these levels of pH change by mid-century, including reef habitats such as Australia's Great Barrier Reef.
Most marine organisms live in the ocean's sunlit surface waters, which are also the waters most vulnerable to CO2-induced acidification over the next century as emissions continue.
To prevent the pH of surface waters from declining more than 0.2 units, the current limit set by the US Environmental Protection Agency in 1976, carbon dioxide emissions would have to be reduced immediately.
According to researchers, though most of the scientific and public focus has been on the climate impacts of human carbon emissions, ocean acidification is as imminent and potentially severe a crisis.
"We know that ocean acidification will damage corals and other organisms, but there's just no experimental data on how most species might be affected," said Caldeira.
We need to consider ocean chemistry effects, and not just the climate effects, of CO2 emissions. That means we need to work much harder to decrease CO2 emissions," he added.