Besides loading the atmosphere with heat-trapping greenhouse gases, human emissions of carbon dioxide have also begun to alter the chemistry of the ocean, according to researcher by scientists at the University of Hawaii.

Their findings were published in the July 4 issue of Science.

In the article, the team of chemical oceanographers, led by Richard Zeebe from the School of Ocean and Earth Science and Technology, conclude that the ecological and economic consequences are difficult to predict but possibly calamitous.

The researchers warn that halting the changes already underway will likely require even steeper cuts in carbon emissions than those currently proposed to curb climate change.

The oceans have absorbed about 40 percent of the carbon dioxide emitted by humans over the past two centuries. This equates to roughly 500 billion metric tons of carbon dioxide, equivalent in weight to about 28 inches of water (~70 cm) across the whole State of Texas.

The research states 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 decline by about 0.1 unit from pre-industrial levels – a 25 percent increase in hydrogen-ion concentration. Small changes in the pH value can make a big difference because pH is measured on a logarithmic scale – analogous to the Richter scale, which measures the strength of earthquakes. For example, a drop by one pH unit means a ten-fold increase in acidity.

Depending on the rate and magnitude of future emissions, the ocean's pH could drop by as much as 0.35 units – more than twice as many hydrogen ions – by the mid-21st century. The increase in the amount of hydrogen ions in the oceans has been labeled "Ocean Acidification" and is cause for concern because it can damage marine organisms.

"If we continue with business as usual and don't cut carbon dioxide emissions, carbonate reefs will ultimately start to dissolve. This is basic chemistry," Zeebe says in the article. "The biology is a bit trickier. Most lab and field experiments show that calcifying organisms struggle under high-CO2 conditions but it's very difficult to predict their long-term reaction, let alone responses of entire marine ecosystems."