SINGAPORE, March 9 (Reuters) - Acidifying oceans caused by rising carbon dioxide levels are cutting the shell weights of tiny marine animals in a process that could accelerate global warming, a scientist said on Monday.

William Howard of the University of Tasmania in Australia described the findings as an early-warning signal, adding the research was the first direct field evidence of marine life being affected by rising acidity of the oceans.

Oceans absorb large amounts of CO2 emitted by mankind through the burning of fossil fuels. The Southern Ocean between Australia and Antarctica is the largest of the ocean carbon sinks.

But scientists say the world's oceans are becoming more acidic as they absorb more planet-warming CO2, disrupting the process of calcification used by sea creatures to build shells as well as coral reefs.

Laboratory experiments had earlier predicted these impacts.

Howard and co-author Andrew Moy, also of the University of Tasmania, studied the shells of tiny amoeba-like animals called foraminifera in the Southern Ocean and compared the shell weights to data from sediment core records dating back 50,000 years.

Their findings, which appear in the latest issue of Nature Geoscience, show shell weights of modern-day foraminifera falling between 30 and 35 percent.

"The big challenge will be how do we scale up this kind of change to understand what it means for the ecosystem. And to be honest, we don't know yet," he told Reuters.

The implications for climate change were clearer, he said.

CARBON TRANSFER

Foraminifera, which live on the ocean's surface, play a major role in trapping CO2 and transporting it to the ocean depths where it can be locked away for decades or more.

Disrupting this process could accelerate climate change.

Foraminifera, he said, comprise a significant proportion of all the carbonate shell material produced in the ocean.

"Their presence and production helps facilitate the sinking of organic matter from the surface layers of the ocean into the deep ocean," said Howard, project leader of the ocean acidification team at the Antarctic Climate and Ecosystems Cooperative Research Centre.

"That translates into the transfer of carbon from the atmosphere into the deep ocean. If these organisms are not calcifying as much it may translate into a reduction in the carbon transfer from the atmosphere."

Oceans are alkaline and Howard said that a century ago, oceans had a pH of 8.2, with a pH of 0 being battery acid and 13 being household bleach.

Oceans were now just under 8.1, he said.

"We've already changed the pH of the ocean by about 0.1. At these levels this represents about a 30 percent increase in the acidity of the oceans," Howard said.

"Anything that makes a shell is going to have a hard time making that shell."

The big challenge was understanding the ocean's response to climate change and what happens to ecosystems. The Southern Ocean was one of the first areas scientists will see this kind of shift, Howard said, in part because it is a major carbon sink.

"There's no question acidification is going to affect every part of the ocean because every part of the ocean is taking up CO2 from man-made emissions," he said. (Editing by Paul Tait)